Citrulline and nitrogen homeostasis: an overview

Regulatory mechanisms of nitrogen homeostasis in insect growth and development

Nitrogen is an essential element for the synthesis of proteins, nucleic acids, and various other critical biological molecules in insects. The maintenance of nitrogen homeostasis in insects is achieved through a balance of dietary intake, metabolic conversion, and excretion. Insects primarily acquire nitrogen from their diet, which is subsequently metabolized into amino acids, proteins, and other vital biomolecules following digestion and absorption. Excess nitrogen is excreted in forms such as uric acid, allantoin, allantoic acid, urea, and ammonia. Disruptions in nitrogen regulation can result in ammonia toxicity and abnormal production or excretion of nitrogenous metabolites, including uric acid, ultimately impairing insect development and survival. This review examines the mechanisms underlying nitrogen homeostasis in insects, with a focus on the intricate regulatory roles of carbohydrate metabolism, amino acid metabolism, uric acid metabolism, urea and polyamine metabolism, ammonia transport pathways, and symbiotic interactions. By elucidating these processes, this review aims to enhance our understanding of insect nutritional metabolism and developmental biology, while offering novel perspectives for the development of more effective pest management strategies.

The roles of arginases and arginine in immunity

Arginase activity and arginine metabolism in immune cells have important consequences for health and disease. Their dysregulation is commonly observed in cancer, autoimmune disorders and infectious diseases. Following the initial description of a role for arginase in the dysfunction of T cells mounting an antitumour response, numerous studies have broadened our understanding of the regulation and expression of arginases and their integration with other metabolic pathways. Here, we highlight the differences in arginase compartmentalization and storage between humans and rodents that should be taken into consideration when assessing the effects of arginase activity. We detail the roles of arginases, arginine and its metabolites in immune cells and their effects in the context of cancer, autoimmunity and infectious disease. Finally, we explore potential therapeutic strategies targeting arginases and arginine.

Nutritional L-Citrulline and Tetrahydrobiopterin in Peripheral Artery Disease: A Phase II Randomized Trial (CIPER Study)

BACKGROUND

Peripheral artery disease (PAD) is a major public health concern due to its high prevalence, severe impact on individuals' health and quality of life, and substantial economic burden. Pharmacological interventions are still limited with numbers needed-to-treat ranging from 6 (cilostazol) to 50 (aspirin, statins, and vorapaxar).

OBJECTIVES

This randomized, placebo-controlled, double-blinded crossover interventional trial aims to measure the effect of L-citrulline and tetrahydrobiopterin (H4Bip) on walking distance in patients with PAD, stratified by plasma levels of asymmetric dimethyl L-arginine (ADMA), the endogenous inhibitor of endothelial nitric oxide (NO) synthase.

METHODS

We measured preinterventional ADMA levels in 51 patients with PAD in Australia and Germany with mean changes in absolute claudication distance (dACD) as the primary outcome upon orally supplementing the L-arginine precursor, L-citrulline (3 g) twice daily for 12 weeks, and, in one arm, additionally H4Bip (0.45 g) once per day for a further 2 weeks.

RESULTS

Preinterventional ADMA levels were pathological (>0.4 μM) in 34 patients. Supplementation with L-citrulline significantly increased the mean plasma levels of both L-citrulline and L-arginine, from 41.8 ± 2.7 μmol/l to 246.3 ± 67.3 μmol/l (P = 0.004) and from 75.2 ± 4.2 μmol/l to 119.2 ± 6.9 μmol/l (P < 0.0001) respectively, when compared with placebo. dACD in % of control was significantly improved by L-citrulline vs placebo (20.11% ± 4.50% vs 5.73% ± 2.74%, respectively; P = 0.011). Further addition of H4Bip increased the mean percentage dACD to 28.15% ± 6.84% (P = 0.021), but only in patients with preinterventional pathological ADMA levels.

CONCLUSIONS

L-citrulline and, when ADMA levels are pathological, H4Bip are effective nutritional interventions in patients with PAD warranting further confirmatory trials.

Overview of mechanisms related to citrulline malate supplementation and different methods of high-intensity interval training on sports performance: A narrative review

Regular exercise is a practical non-pharmacological approach to maintaining physical and mental health through rehabilitation and prevention of chronic diseases due to its cardiovascular, cardiorespiratory, neurological, and neuromuscular benefits. Despite awareness of the benefits of exercise, a "lack of time" has proven to be the most common impediment to regular activity. Thus, a time-efficient and potentially enjoyable training modality with growing popularity is high-intensity interval training (HIIT). HIIT incorporates intermittent bouts of work and recovery intervals performed at an intensity close to maximal oxygen consumption (VO2max). HIIT is considered to have equivalent or superior benefits compared to moderate-intensity continuous training (MICT). This narrative review focuses on the mechanisms of Citrulline Malate (CM) supplementation and various modes of HIIT on exercise performance. CM serves as a nitric-oxide enhancer leading to improved aerobic and anaerobic exercise performance by increasing muscle adenosine triphosphate (ATP) production, vasodilation, and blood flow to the active musculature and boosting work capacity. This article reviews the mechanisms related to CM supplementation and different modes of HIIT on exercise performance. Even though a single, acute 8 g dose of CM has been recommended, its mechanism of action remains to be seen due to the synergistic impact of both components (citrulline and malate). Moreover, the limited evidence for the standard level of supplement use and source of purchase results in athletes' self-prescription of supplements. Therefore, to reduce the risk of accidental doping or toxicity, further studies should continue to investigate the optimal dose, timing, mechanism of action, as well as reliable sources of purchase for CM consumption.

Impact of L-Citrulline Supplementation and HIIT on Lipid Profile, Arterial Stiffness, and Fat Mass in Obese Adolescents with Metabolic-Dysfunction-Associated Fatty Liver Disease: A Randomized Clinical Trial

BACKGROUND

Metabolic-dysfunction-associated steatotic liver disease (MASLD) and obesity contribute to vascular dysfunction through oxidative stress, heightening cardiovascular risk. Oral supplementation with L-citrulline (L-cit), a precursor of L-arginine (L-arg) and nitric oxide, and high-intensity interval training (HIIT) may improve vascular function and cardiometabolic health.

OBJECTIVES

This study aimed to evaluate the combined effects of L-cit supplementation and HIIT on arterial stiffness, body composition, glucose metabolism, lipid profile, and blood pressure (BP) in adolescents with MASLD and obesity.

METHODS

In this double-blind, placebo-controlled, randomized clinical trial (ClinicalTrials.gov (NCT05778266), 44 adolescents (15-19 years) with MASLD and obesity were assigned to HIIT + L-cit (n = 14), HIIT + placebo (n = 14), or L-cit (n = 15) for 12 weeks. HIIT sessions (85% and 60% peak heart rate during intense and recovery periods) occurred thrice weekly. Training volume progressively increased, and participants performed 20 min of HITT per session in the last 8 weeks.

RESULTS

Outcomes included pulse wave velocity (PWV), augmentation index (Aix75), VO2peak, body composition, BP, glucose and lipid profiles, and hepatic steatosis. Compared to L-cit, HIIT + L-cit improved non-high-density lipoprotein cholesterol (p = 0.04), very-low-density lipoprotein cholesterol (p = 0.01), triglycerides (p = 0.02), and VO2peak (p = 0.001). No significant between-group changes were found in PWV, AIx75, hepatic steatosis, and body composition. HIIT + placebo improved VO2peak (p = 0.002), and L-cit decreased the degree of steatosis (p = 0.038).

CONCLUSIONS

HIIT + L-cit supplementation enhanced lipid profile and cardiorespiratory fitness, while HIIT + placebo improved cardiorespiratory capacity, and L-cit alone decreased hepatic steatosis. Thus, L-cit could be an adjuvant strategy to manage obesity-related MASLD in adolescents.

Promotion of nitric oxide production: mechanisms, strategies, and possibilities

The discovery of nitric oxide (NO) and the role of endothelial cells (ECs) in its production has revolutionized medicine. NO can be produced by isoforms of NO synthases (NOS), including the neuronal (nNOS), inducible (iNOS), and endothelial isoforms (eNOS), and via the non-classical nitrate-nitrite-NO pathway. In particular, endothelium-derived NO, produced by eNOS, is essential for cardiovascular health. Endothelium-derived NO activates soluble guanylate cyclase (sGC) in vascular smooth muscle cells (VSMCs), elevating cyclic GMP (cGMP), causing vasodilation. Over the past four decades, the importance of this pathway in cardiovascular health has fueled the search for strategies to enhance NO bioavailability and/or preserve the outcomes of NO's actions. Currently approved approaches operate in three directions: 1) providing exogenous NO, 2) promoting sGC activity, and 3) preventing degradation of cGMP by inhibiting phosphodiesterase 5 activity. Despite clear benefits, these approaches face challenges such as the development of nitrate tolerance and endothelial dysfunction. This highlights the need for sustainable options that promote endogenous NO production. This review will focus on strategies to promote endogenous NO production. A detailed review of the mechanisms regulating eNOS activity will be first provided, followed by a review of strategies to promote endogenous NO production based on the levels of available preclinical and clinical evidence, and perspectives on future possibilities.

A Study on the Function of Arginine in the Growth, Immunity, Antioxidant Activity, and Oxygen Carrying-Capacity of Juvenile Gibel Carp (Carassius auratus gibelio)

An eight-week trial was designed to study the effects of arginine (Arg) supplemented diets on the growth, immunity, antioxidant activity, and oxygen-carrying capacity of juvenile Gibel carp (Carassius auratus gibelio). A total of 300 fish (27.53 ± 0.03 g) were randomized into 15 equal groups and fed on diets with graded Arg levels: 0 (control), 0.2%, 0.4%, 0.6%, and 0.8% (w/w). The results showed that final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) all increased and then declined with increasing levels of Arg supplementation, while feed conversion ratio (FCR) showed the opposite trend. In addition, the fish's whole-body crude protein and ash content had no remarkable difference at different levels of Arg addition (p > 0.05). Supplementation with 0.6% and 0.8% Arg significantly increased plasma alanine transaminase (ALT) activity (p < 0.05). The malondialdehyde (MDA) levels and superoxide dismutase (SOD) activities of the liver were not significantly different between the different levels of Arg supplementation (p > 0.05), while catalase (CAT) activity was significantly increased with 0.4% Arg supplementation levels (p < 0.05). The 0.8% Arg supplementation greatly increased the expression of hepatic-related genes to the Nrf2 signaling pathway, including sod and gpx (p < 0.05). However, the 0.8% Arg supplementation did not significantly increase the relative expression of genes related to the NF-κB signaling pathway, including il-1β, il-8, and tnf-α (p > 0.05). Similarly, the relative expression of hif-1 signaling pathway-related genes at 0.8% Arg supplementation was significantly elevated, including hif-1α, epo, and vegf (p < 0.05). Hence, Arg supplementation could promote growth and improve immune, antioxidant, and oxygen-carrying capacity in juvenile Gibel carp.

Comparative Analysis of Serum Amino Acid Profiles in Patients with Myasthenia Gravis and Multiple Sclerosis

Background: Multiple sclerosis (MS) and myasthenia gravis (MG) are autoimmune diseases that attack the central nervous system (CNS) and the neuromuscular junction, respectively. As the common pathogenesis of both diseases is associated with an autoimmune background and the involvement of T and B lymphocytes, the overlapping of selected clinical symptoms may cause difficulties in the differential diagnosis of both diseases. Methods: The aim of the study was to use Liquid Chromatography-Electrospray Ionization-Mass Spectrometry (LC-ESI-MS/MS) in conjunction with multivariate statistical analyses to examine the changes in amino acid metabolic profiles between patients with MG, MS, and a control group. Results: Comparative analysis of amino acids (AA) between patients with MG, MS, and within the control group allowed for the identification of statistically significant differences in the amino acid profile. Comparing the patients (patients with MS and MG) with the control group, and after taking the results of multiple tests into account, it was observed that amino acids such as ARG, PRO, TRP, CIT were significantly different between the groups. When considering the comparison between the AA concentrations in MS and MG patients, we found three AAs that were significantly different in the MS and MG groups, after correcting for multiple testing (CIT, GABA, and AAA). Higher concentrations of amino acids that showed significant differences were observed in patients with myasthenia gravis. Conclusions: Our results have indicated AAs that may prove valuable for improving the diagnostics of MS and MG patients. To better assess the potential utility of these markers, their performance requires further validation in a larger study group and limitation of possible confounding factors, e.g., medications and diet.

Efficacy of amino acids in sports nutrition- review of clinical evidences

The efficacy of amino acids as popular sports supplements has triggered debates, with their impact on athletic performance varying across sports disciplines due to diversity and heterogeneity in clinical trials. This review evaluates the ergogenic potential of amino acids, by critical appraisal of results of clinical trials of Branched chain amino acids (BCAAs), arginine, glutamine, citrulline, β-alanine, and taurine, performed on elite sportsmen from various land and water sports. Clinical trials reviewed here confirm notable physiological benefits thereby supporting the claim that BCAA, citrulline and arginine in various doses can have positive effects on endurance and overall performance in sportsperson. Furthermore, results of clinical trials and metabolomic studies indicate that in future it would be more beneficial to design precise formulations to target the requirement of specific sports. For instance, some combinations of amino acids may be more suitable for long term endurance and some others may be suitable for short burst of excessive energy. The most important insights from this review are the identification of three key areas where research is urgently needed: a) Biomarkers that can identify the physiological end points and to distinguish the specific role of amino acid as anti-fatigue or reducing muscle soreness or enhancing energy b) In-depth sports-wise clinical trials on elite sportsperson to understand the ergogenic needs for the particular sports c) Design of precision formula for similar types of sports instead of common supplements.

The Effects of Combined Exercise with Citrulline Supplementation on Body Composition and Lower Limb Function of Overweight Older Adults: A Systematic Review and Meta-Analysis

The combined exercise with citrulline (CIT) supplementation is a potential adjuvant treatment approach to address the declining body composition and lower limb function of overweight older adults. However, research on this approach is limited. Thus, this study performed a meta-analysis review to explore the effects of combined exercise with CIT supplementation on body composition and lower limb function among overweight older adults. The search strategy and manuscript development of this study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. Eligible studies were first searched through four databases (Web of Science, Scopus, PubMed, and EBSCO) from January 2003 until April 2023, followed by screening. The main inclusion criteria for the article selection are as follows: 1) Randomized Controlled Trial studies; 2) Participants aged over 55; 3) Studies involved exercise with CIT supplementation for the experimental group and exercise with Placebo (PLA) supplementation for the control group; 4) Body composition and lower limb function were measured at pre- and post-intervention. Subsequently, the Cochrane risk of bias assessment tool was utilized to evaluate the selected studies' quality. The Standardized Mean Difference (SMD) was chosen as the suitable effect scale index, and the mean differences of the data from the selected articles were analyzed using Revman 5.4 software with a 95% Confidence Interval (CI). A total of seven studies fulfilled the inclusion criteria and were selected for the meta-analysis. The included studies involved 105 males and 198 females, where 157 belonged to the PLA group and 146 from the CIT group. Significant improvements were observed among overweight older adults with CIT supplementation in 6-Minute Walking Test (6MWT) (P = 0.04, I2 = 4%), SMD (95% CI) = -0.28 (-0.54, -0.01), and Lower Limb Strength (LLS) (P < 0.01, I2 = 30%), SMD (95% CI) = -0.38 (-0.65, -0.12) compared to those with PLA supplementation. Combined exercise with CIT supplementation could be an effective non-pharmaceutical intervention to improve the physical function of overweight older adults by increasing their muscle strength.

Glutamine and amino acid metabolism as a prognostic signature and therapeutic target in endometrial cancer

INTRODUCTION

Endometrial cancer (EC) is the most common female reproductive system cancer in developed countries with growing incidence and associated mortality, which may be due to the growing prevalence of obesity. Metabolism reprogramming including glucose, amino acid, and lipid remodeling is a hallmark of tumors. Glutamine metabolism has been reported to participate in tumor proliferation and development. This study aimed to develop a glutamine metabolism-related prognostic model for EC and explore potential targets for cancer treatment.

METHOD

Transcriptomic data and survival outcome of EC were retrieved from The Cancer Genome Atlas (TCGA). Differentially expressed genes related to glutamine metabolism were recognized and utilized to build a prognostic model by univariate and multivariate Cox regressions. The model was confirmed in the training, testing, and the entire cohort. A nomogram combing prognostic model and clinicopathologic features was established and tested. Moreover, we explored the effect of a key metabolic enzyme, PHGDH, on the biological behavior of EC cell lines and xenograft model.

RESULTS

Five glutamine metabolism-related genes, including PHGDH, OTC, ASRGL1, ASNS, and NR1H4, were involved in prognostic model construction. Kaplan-Meier curve suggested that patients recognized as high risk underwent inferior outcomes. The receiver operating characteristic (ROC) curve showed the model was sufficient to predict survival. Enrichment analysis recognized DNA replication and repair dysfunction in high-risk patients whereas immune relevance analysis revealed low immune scores in the high-risk group. Finally, a nomogram integrating the prognostic model and clinical factors was created and verified. Further, knockdown of PHGDH showed cell growth inhibition, increasing apoptosis, and reduced migration. Promisingly, NCT-503, a PHGDH inhibitor, significantly repressed tumor growth in vivo (p = 0.0002).

CONCLUSION

Our work established and validated a glutamine metabolism-related prognostic model that favorably evaluates the prognosis of EC patients. DNA replication and repair may be the crucial point that linked glutamine metabolism, amino acid metabolism, and EC progression. High-risk patients stratified by the model may not be sufficient for immune therapy. PHGDH might be a crucial target that links serine metabolism, glutamine metabolism as well as EC progression.

Effects of supplementation with different concentrations of L-citrulline on the plasma amino acid concentration, reproductive hormone concentrations, antioxidant capacity, and reproductive performance of Hu ewes

Context L-citrulline (L-Cit) does not degrade in the rumen and has the ability to form peptide bonds in the body; however, it does not participate in protein synthesis. Aims This study aimed to evaluate the effects of L-Cit on the reproductive performance of Hu ewes. Methods In total, 30 ewes were randomly categorised into five groups. The control group was fed with a basic diet, whereas the Experimental Groups I, II, III, and IV were provided feed supplemented with 5, 10, 15, and 20 g/day of L-Cit respectively. Blood samples of ewes were collected 4 h after feeding on Day 21 of the experiment and before feeding on Day 30. The optimal supplementary feeding dose was selected on the basis of blood biochemical indexes. Overall, ninety 2-year-old ewes were classified into two groups. The control group was fed with a basic diet and the experimental group was fed with a diet supplemented with 10 g/day of L-Cit. After 30 days of supplementary feeding, reproductive performance of ewes was determined. Key results The plasma concentrations of Cit, ornithine, and arginine in ewes increased linearly with an increase in the level of L-Cit supplementation. The plasma concentrations of gonadotropin-releasing hormone, luteinising hormone, and follicle-stimulating hormone in the experimental group increased significantly compared with those in the control group. The plasma total antioxidant capacity and catalase, superoxide dismutase, and glutathione peroxidase in the experimental group were significantly higher than those in the control group, whereas the concentrations of malondialdehyde in all experimental groups were significantly lower than those in the control group. The conception, lambing, and double lambing rates of the experimental group were increased by 28.76%, 15.90%, and 40.21% respectively. Conclusions Supplementation with different doses of L-Cit can improve the concentrations of some plasma amino acids and reproductive hormones as well as antioxidant capacity of ewes. Supplementary feeding with 10 g/day of L-Cit could increase the lambing and double lambing rates of ewes. Implication L-Cit can improve the reproductive performance of ewes.

Serum Citrulline and Ornithine: Potential Markers of Coeliac Disease Activity

INTRODUCTION

To date, there is not generally accepted and universal indicator of activity, and functional integrity of the small intestine in patients with coeliac disease. The aim of our study was to investigate whether serum concentrations of the non-essential amino acids citrulline and ornithine might have this function.

METHODS

We examined serum citrulline and ornithine concentrations in a subgroup of patients with proven coeliac disease and healthy controls (blood donors).

RESULTS

A total of 94 patients with coeliac disease (29 men, mean age 53 ± 18 years; 65 women, mean age 44 ± 14 years) and 35 healthy controls (blood donors) in whom coeliac disease was serologically excluded (10 men, mean age 51 ± 14 years; 25 women, mean age 46 ± 12 years) were included in the study. Significantly lower concentrations of serum ornithine were found in patients with coeliac disease (mean 65 ± 3 μmol/L; median 63 μmol/L, IQR 34 μmol/L, p < 0.001). No statistically nor clinically significant differences were found in the citrulline concentrations between the study and control group.

CONCLUSIONS

Serum ornithine (but not citrulline) may be useful for assessing the functional status of the small intestine in uncomplicated coeliac disease. Further studies involving more detailed analysis of dietary and metabolic changes in patients will be needed to reach definitive conclusions.

Effects of L-Citrulline Supplementation on Endothelial Function and Blood Pressure in Hypertensive Postmenopausal Women

Aging and menopause are associated with decreased nitric oxide bioavailability due to reduced L-arginine (L-ARG) levels contributing to endothelial dysfunction (ED). ED precedes arterial stiffness and hypertension development, a major risk factor for cardiovascular disease. This study investigated the effects of L-citrulline (L-CIT) on endothelial function, aortic stiffness, and resting brachial and aortic blood pressures (BP) in hypertensive postmenopausal women. Twenty-five postmenopausal women were randomized to 4 weeks of L-CIT (10 g) or placebo (PL). Serum L-ARG, brachial artery flow-mediated dilation (FMD), aortic stiffness (carotid-femoral pulse wave velocity, cfPWV), and resting brachial and aortic BP were assessed at 0 and 4 weeks. L-CIT supplementation increased L-ARG levels (Δ13 ± 2 vs. Δ−2 ± 2 µmol/L, p < 0.01) and FMD (Δ1.4 ± 2.0% vs. Δ−0.5 ± 1.7%, p = 0.03) compared to PL. Resting aortic diastolic BP (Δ−2 ± 4 vs. Δ2 ± 5 mmHg, p = 0.01) and mean arterial pressure (Δ−2 ± 4 vs. Δ2 ± 6 mmHg, p = 0.04) were significantly decreased after 4 weeks of L-CIT compared to PL. Although not statistically significant (p = 0.07), cfPWV decreased after L-CIT supplementation by ~0.66 m/s. These findings suggest that L-CIT supplementation improves endothelial function and aortic BP via increased L-ARG availability.

Effects of Citrulline Supplementation on Different Aerobic Exercise Performance Outcomes: A Systematic Review and Meta-Analysis

Supplementation with Citrulline (Cit) has been shown to have a positive impact on aerobic exercise performance and related outcomes such as lactate, oxygen uptake (VO₂) kinetics, and the rate of perceived exertion (RPE), probably due to its relationship to endogenous nitric oxide production. However, current research has shown this to be controversial. The main objective of this systematic review and meta-analysis was to analyze and assess the effects of Cit supplementation on aerobic exercise performance and related outcomes, as well as to show the most suitable doses and timing of ingestion. A structured literature search was carried out by the PRISMA^® (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) and PICOS guidelines in the following databases: Pubmed/Medline, Scopus, and Web of Science (WOS). A total of 10 studies were included in the analysis, all of which exclusively compared the effects of Cit supplementation with those of a placebo group on aerobic performance, lactate, VO₂, and the RPE. Those articles that used other supplements and measured other outcomes were excluded. The meta-analysis was carried out using Hedges’ g random effects model and pooled standardized mean differences (SMD). The results showed no positive effects of Cit supplementation on aerobic performance (pooled SMD = 0.15; 95% CI (−0.02 to 0.32); I², 0%; p = 0.08), the RPE (pooled SMD = −0.03; 95% CI (−0.43 to 0.38); I², 49%; p = 0.9), VO₂ kinetics (pooled SMD = 0.01; 95% CI (−0.16 to 0.17); I², 0%; p = 0.94), and lactate (pooled SMD = 0.25; 95% CI (−0.10 to 0.59); I², 0%; p = 0.16). In conclusion, Cit supplementation did not prove to have any benefits for aerobic exercise performance and related outcomes. Where chronic protocols seemed to show a positive tendency, more studies in the field are needed to better understand the effects.

Current Evidence of Watermelon (Citrullus lanatus) Ingestion on Vascular Health: A Food Science and Technology Perspective

The amino acid L-arginine is crucial for nitric oxide (NO) synthesis, an important molecule regulating vascular tone. Considering that vascular dysfunction precedes cardiovascular disease, supplementation with precursors of NO synthesis (e.g., L-arginine) is warranted. However, supplementation of L-citrulline is recommended instead of L-arginine since most L-arginine is catabolized during its course to the endothelium. Given that L-citrulline, found mainly in watermelon, can be converted to L-arginine, watermelon supplementation seems to be effective in increasing plasma L-arginine and improving vascular function. Nonetheless, there are divergent findings when investigating the effect of watermelon supplementation on vascular function, which may be explained by the L-citrulline dose in watermelon products. In some instances, offering a sufficient amount of L-citrulline can be impaired by the greater volume (>700 mL) of watermelon needed to reach a proper dose of L-citrulline. Thus, food technology can be applied to reduce the watermelon volume and make supplementation more convenient. Therefore, this narrative review aims to discuss the current evidence showing the effects of watermelon ingestion on vascular health parameters, exploring the critical relevance of food technology for acceptable L-citrulline content in these products. Watermelon-derived L-citrulline appears as a supplementation that can improve vascular function, including arterial stiffness and blood pressure. Applying food technologies to concentrate bioactive compounds in a reduced volume is warranted so that its ingestion can be more convenient, improving the adherence of those who want to ingest watermelon products daily.

Protein and Sport: Alternative Sources and Strategies for Bioactive and Sustainable Sports Nutrition

Nutrition and sport play an important role in achieving a healthy lifestyle. In addition to the intake of nutrients derived from the normal diet, some sport disciplines require the consumption of supplements that contribute positively to improved athletic performance. Protein intake is important for many aspects related to health, and current evidence suggests that some athletes require increased amounts of this nutrient. On the other hand, society's demand for more environmentally friendly products, focus on the search for alternative food sources more sustainable. This review aims to summarize the latest research on novel strategies and sources for greener and functional supplementation in sport nutrition. Alternative protein sources such as insects, plants or mycoproteins have proven to be an interesting substrate due to their high added value in terms of bioactivity and sustainability. Protein hydrolysis has proven to be a very useful technology to revalue by-products, such as collagen, by producing bioactive peptides beneficial on athletes performance and sport-related complications. In addition, it has been observed that certain amino acids from plant sources, as citrulline or theanine, can have an ergogenic effect for this target population. Finally, the future perspectives of protein supplementation in sports nutrition are discussed. In summary, protein supplementation in sports nutrition is a very promising field of research, whose future perspective lies with the search for alternatives with greater bioactive potential and more sustainable than conventional sources.

Citrulline protects human retinal pigment epithelium from hydrogen peroxide and iron/ascorbate induced damages

Oxidative stress plays an important role in the ageing of the retina and in the pathogenesis of retinal diseases such as age‐related macular degeneration (ARMD). Hydrogen peroxide is a reactive oxygen species generated by the photo‐excited lipofuscin that accumulates during ageing in the retinal pigment epithelium (RPE), and the age‐related accumulation of lipofuscin is associated with ARMD. Iron also accumulates with age in the RPE that may contribute to ARMD as an important source of oxidative stress. The aim of this work was to investigate the effects of L‐Citrulline (CIT), a naturally occurring amino acid with known antioxidant properties, on oxidative stressed cultured RPE cells. Human RPE (ARPE‐19) cells were exposed to hydrogen peroxide (H₂O₂) or iron/ascorbate (I/A) for 4 h, either in the presence of CIT or after 24 h of pretreatment. Here, we show that supplementation with CIT protects ARPE‐19 cells against H₂O₂ and I/A. CIT improves cell metabolic activity, decreases ROS production, limits lipid peroxidation, reduces cell death and attenuates IL‐8 secretion. Our study evidences that CIT is able to protect human RPE cells from oxidative damage and suggests potential protective effect for the treatment of retinal diseases associated with oxidative stress.

Ornipural® Mitigates Malathion-Induced Hepato-Renal Damage in Rats via Amelioration of Oxidative Stress Biomarkers, Restoration of Antioxidant Activity, and Attenuation of Inflammatory Response

The current study was instigated by investigating the ameliorative potential of Ornipural^® solution against the hepato-renal toxicity of malathion. A total number of 35 male Wistar albino rats were divided equally into five groups. Group 1 served as control and received normal saline intraperitoneally. Group 2, the sham group, were administered only corn oil (vehicle of malathion) orally. Group 3 was orally intoxicated by malathion in corn oil at a dose of 135 mg/kg BW via intra-gastric gavage. Group 4 received malathion orally concomitantly with Ornipural^® intraperitoneally. Group 5 was given Ornipural^® solution in saline via intraperitoneal injection at a dose of (1 mL/kg BW). Animals received the treatment regime for 30 days. Histopathological examination revealed the harmful effect of malathion on hepatic and renal tissue. The results showed that malathion induced a significant decrease in body weight and marked elevation in the activity of liver enzymes, LDH, and ACP. In contrast, the activity of AchE and Paraoxonase was markedly decreased. Moreover, there was a significant increase in the serum content of bilirubin, cholesterol, and kidney injury markers. A significant elevation in malondialdehyde, nitric oxide (nitrite), and 8-hydroxy-2-deoxyguanosine was observed, along with a substantial reduction in antioxidant activity. Furthermore, malathion increased tumor necrosis factor-alpha, the upregulation of IL-1B, BAX, and IFN-β genes, and the downregulation of Nrf2, Bcl2, and HO-1 genes. Concurrent administration of Ornipural^® with malathion attenuated the detrimental impact of malathion through ameliorating metabolic biomarkers, restoring antioxidant activity, reducing the inflammatory response, and improving pathologic microscopic alterations. It could be concluded that Ornipural^® solution demonstrates hepatorenal defensive impacts against malathion toxicity at biochemical, antioxidants, molecular, and cellular levels.

Functional roles of taurine, L-theanine, L-citrulline, and betaine during heat stress in poultry

Heat stress (HS) is an important environmental stress factor affecting poultry production on a global scale. With the rise in ambient temperature and increasing effects of global warming, it becomes pertinent to understand the effects of HS on poultry production and the strategies that can be adopted to mitigate its detrimental impacts on the performance, health, welfare, immunity, and survival of birds. Amino acids (AAs) have been increasingly adopted as nutritional modifiers in animals to ameliorate the adverse effects of HS. They are essential for protein synthesis, growth, maintenance, reproduction, immunity, stress response, and whole-body homeostasis. However, HS tends to adversely affect the availability, transport, absorption, and utilization of these AAs. Studies have investigated the provision of these AAs to poultry during HS conditions, and variable findings have been reported. Taurine, L-theanine, and L-citrulline are non-essential amino acids that are increasingly gaining attention as nutritional supplements in HS animals. Similarly, betaine is an amino acid derivative that possesses favorable biological properties which contributes to its role as a functional additive during HS. Of particular note, taurine is negligible in plants, while betaine, L-theanine, and L-citrulline can be found in selected plants. These nutrients are barely found in feed ingredients, but their supply has been shown to elicit important physiological roles including anti-stress effects, anti-oxidative, anti-inflammatory, gut promoting, and immunomodulatory functions. The present review provides information on the use of these nutritionally and physiologically beneficial nutrients as functional additives to poultry diets during HS conditions. Presently, although several studies have reported on the positive effects of these additives in human and murine studies, however, there is limited information regarding their utilization during heat stress in poultry nutrition. Therefore, this review aims to expound on the functional properties of these nutrients, their potentials for HS alleviation, and to stimulate further researches on their biological roles in poultry nutrition.

The Forgotten Brother: The Innate-like B1 Cell in Multiple Sclerosis

Multiple sclerosis (MS) is a neurodegenerative disease of the central nervous system (CNS), traditionally considered a chronic autoimmune attack against the insulating myelin sheaths around axons. However, the exact etiology has not been identified and is likely multi-factorial. Recently, evidence has been accumulating that implies that autoimmune processes underlying MS may, in fact, be triggered by pathological processes initiated within the CNS. This review focuses on a relatively unexplored immune cell-the "innate-like" B1 lymphocyte. The B1 cell is a primary-natural-antibody- and anti-inflammatory-cytokine-producing cell present in the healthy brain. It has been recently shown that its frequency and function may differ between MS patients and healthy controls, but its exact involvement in the MS pathogenic process remains obscure. In this review, we propose that this enigmatic cell may play a more prominent role in MS pathology than ever imagined. We aim to shed light on the human B1 cell in health and disease, and how dysregulation in its delicate homeostatic role could impact MS. Furthermore, novel therapeutic avenues to restore B1 cells' beneficial functions will be proposed.

Investigating Phragmites australis response to copper exposure using physiologic, Fourier Transform Infrared and metabolomic approaches

Phragmites australis (Cav.) Trin. ex Steud is a landscape plant with resistance to heavy metals that has significance in phytoremediation. However, little is known about the metabolomic background of the heavy metal resistance mechanisms of Phragmites . We studied copper stress on Phragmites and monitored physiological indicators such as malondialdehyde (MDA) and electrolyte leakage (EL). In addition, Fourier Transform Infrared (FTIR) was used to study the related chemical composition in the roots, stems, and leaves under copper stress. Furthermore, LC-MS technology was used to analyse the plants metabolic profile. Results showed that increased copper concentration in Phragmites led to the accumulation of MDA and EL. FTIR spectrum detected the presence of O-H and C=O stretching. O-H stretching was related to the presence of flavonoids, while C=O stretching reflected the presence of protein amide I. The latter was related to the change of amino acid composition. Both flavonoids and amino acids are regarded as contributors to the antioxidant of Phragmites under copper stress. Metabolomics analysis revealed that arginine and ayarin were accumulated and Phragmites leaves responded to copper stress with changes in the pool size of arginine and ayarin. It is speculated that they could improve resistance. Arginine is accumulated through two pathways: the citrulline decomposition and conversion pathway; and the circular pathway composed of ornithine, citrulline, l -argininosuccinate and arginine. Ayarin is synthesised through the quercetin methylation pathway. This study elucidates the antioxidant mechanisms for enhancing its resistance to heavy metal stress, thus improving of phytoremediation efficiency.

L-Citrulline supplementation attenuates aortic pulse pressure and wave reflection responses to cold stress in older adults

BACKGROUND AND AIMS

Augmented aortic systolic blood pressure (SBP) and wave reflection via sympathetic-mediated vasoconstriction elevates the risk for adverse cardiovascular events in older adults. L-citrulline (L-CIT) supplementation has shown to reduce aortic SBP and pulse pressure (PP) responses to cold pressor test (CPT) induced sympathoactivation in young men. The aim of this study was to elucidate the efficacy of L-CIT supplementation to attenuate aortic hemodynamic responses to CPT in older adults.

METHODS AND RESULTS

Sixteen older adults were randomly assigned to placebo or L-CIT (6 g/day) for 14-days in a crossover, double-blind, placebo-controlled design. Brachial SBP and aortic SBP, PP, augmented pressure (AP), augmentation index standardized at 75 bpm (AIx@75), and pressure of the forward (Pf) and reflected (Pb) waves were evaluated at rest and during CPT pre- and post-intervention. Although no hemodynamic changes at rest, brachial SBP (Δ-12 ± 18 vs. Δ4 ± 14 mmHg; P = 0.008) and aortic SBP (Δ-10 ± 14 vs. Δ4 ± 12 mmHg; P = 0.005), PP (Δ-10 ± 12 vs. Δ4 ± 11 mmHg; P = 0.002), AP (Δ-4 ± 4 vs. Δ2 ± 7 mmHg; P = 0.004), AIx@75 (Δ-3.2 ± 7.2 vs. Δ2.2 ± 6.9%; P = 0.038), Pf (Δ-6 ± 10 vs. Δ3 ± 9 mmHg; P = 0.019), and Pb (Δ-4 ± 6 vs. Δ2 ± 6 mmHg; P = 0.008) responses to the CPT were significantly attenuated following L-CIT supplementation vs. placebo.

CONCLUSIONS

L-CIT supplementation attenuated aortic pulsatile pressure and pressure wave reflection responses to CPT in older adults, providing possible cardioprotection during cold-induced sympathoactivation in older adults.

Regulation of citrulline synthesis in human enterocytes: Role of hypoxia and inflammation

Intensive care unit patients and chronic airway inflammatory disease are characterized by chronic systemic hypoxia and inflammation inducing a decrease in nitric oxide release due to impaired l-arginine (ARG) homeostasis. As ARG is synthesized from circulating l-citrulline (CIT), an alteration of CIT production in small intestine by ornithine carbamoyltransferase could be involved. Here, we posit that hypoxia and/or inflammation has effects on ornithine carbamoyltransferase regulation in enterocytes. A duodenal explant incubation model was used. Biopsy specimens taken from 25 selected patients were incubated for 6 h in 4 groups: control, inflammation, hypoxia, and hypoxia + inflammation. At the end of the incubation period, we measured CIT concentration in culture media, ornithine carbamoyltransferase activity, ornithine carbamoyltransferase protein and gene expression, protein expression of enzymes involved in the CIT production pathway, and expression of energy status proteins. Inflammation and/or hypoxia conditions did not affect CIT production. Ornithine carbamoyltransferase activity was increased in hypoxia conditions (p = 0.023). Expression of enzymes implicated in the CIT crossroads pathway and enzymes reflecting energy status variation was not affected by inflammation and hypoxia. Data sets were pooled to evaluate the variability of the four quartiles for each parameter. CIT production was found to increase over the quartiles whereas other parameters remained stable. Our results showed that intestinal CIT production is preserved during inflammation and/or hypoxia, thus confirming the significance of this metabolic pathway. This suggests that the CIT deficiency observed in clinical hypercatabolic states could be a consequence of high utilization for ARG synthesis.

Lactational performance of dairy cows in response to supplementing N-acetyl-l-methionine as source of rumen-protected methionine

The objective of this experiment was to evaluate the effects of supplementing a rumen-protected source of Met, N-acetyl-l-methionine (NALM), on lactational performance and nitrogen metabolism in early- to mid-lactation dairy cows. Sixty multiparous Holstein dairy cows in early lactation (27 ± 4.3 d in milk, SD) were assigned to 4 treatments in a randomized complete block design. Cows were blocked by actual milk yield. Treatments were as follows: (1) no NALM (control); (2) 15 g/d of NALM (NALM15); (3) 30 g/d of NALM (NALM30); and (4) 45 g/d of NALM (NALM45). Diets were formulated using a Cornell Net Carbohydrate and Protein System (CNCPS) v.6.5 model software to meet or exceed nutritional requirements of lactating dairy cows producing 42 kg/d of milk and to undersupply metabolizable Met (control) or supply incremental amounts of NALM. The digestible Met (dMet) supply for control, NALM15, NALM30, and NALM45 were 54.7, 59.8, 64.7, and 72.2 g/d, respectively. The supply of dMet was 88, 94, 104, and 115% of dMet requirement for control, NALM15, NALM30, and NALM45, respectively. Milk yield data were collected, dry matter intake (DMI) was measured daily, and milk samples were collected twice per week for 22 wk. Blood, ruminal fluid, urine, and fecal samples were collected during the covariate period and during wk 4, 8, and 16. Data were analyzed using the GLIMMIX procedure of SAS (SAS Institute) using covariates in the model for all variables except body weight. Linear, quadratic, and cubic contrasts were also tested. Treatments did not affect DMI, milk yield, and milk component concentration and yield; however, feed efficiency expressed as milk yield per DMI and 3.5% fat-corrected milk per DMI were quadratically affected, with greater response observed for NALM15 and NALM30 compared with control. Acetate proportion linearly increased, whereas propionate proportion linearly decreased with NALM supplementation. Blood urea nitrogen linearly decreased with NALM supplementation. Total plasma essential AA concentrations were quadratically affected, as greater values were observed for control and NALM45 than other treatments. Plasma Met concentration was quadratically affected as lower levels were observed with NALM15, whereas Met concentrations increased with NALM45 compared with control. Nitrogen utilization efficiency and apparent total-tract nutrient digestibility were not affected by treatment. Supplementation of NALM at 15 or 30 g/head per day resulted in the greatest improvements in feed efficiency without affecting N metabolism of early- to mid-lactation dairy cows.

Reconstructing a recycling and nonauxotroph biosynthetic pathway in Escherichia coli toward highly efficient production of L-citrulline

L-citrulline is a high-value amino acid with promising application in medicinal and food industries. Construction of highly efficient microbial cell factories for L-citrulline production is still an open issue due to complex metabolic flux distribution and L-arginine auxotrophy. In this study, we constructed a nonauxotrophic cell factory in Escherichia coli for high-titer L-citrulline production by coupling modular engineering strategies with dynamic pathway regulation. First, the biosynthetic pathway of L-citrulline was enhanced after blockage of the degradation pathway and introduction of heterologous biosynthetic genes from Corynebacterium glutamicum. Specifically, a superior recycling biosynthetic pathway was designed to replace the native linear pathway by deleting native acetylornithine deacetylase. Next, the carbamoyl phosphate and L-glutamate biosynthetic modules, the NADPH generation module, and the efflux module were modified to increase L-citrulline titer further. Finally, a toggle switch that responded to cell density was designed to dynamically control the expression of the argG gene and reconstruct a nonauxotrophic pathway. Without extra supplement of L-arginine during fermentation, the final CIT24 strain produced 82.1 g/L L-citrulline in a 5-L bioreactor with a yield of 0.34 g/g glucose and a productivity of 1.71 g/(L ⋅ h), which were the highest values reported by microbial fermentation. Our study not only demonstrated the successful design of cell factory for high-level L-citrulline production but also provided references of coupling the rational module engineering strategies and dynamic regulation strategies to produce high-value intermediate metabolites.

Potential Implications of Citrulline and Quercetin on Gut Functioning of Monogastric Animals and Humans: A Comprehensive Review

The importance of gut health in animal welfare and wellbeing is undisputable. The intestinal microbiota plays an essential role in the metabolic, nutritional, physiological, and immunological processes of animals. Therefore, the rapid development of dietary supplements to improve gut functions and homeostasis is imminent. Recent studies have uncovered the beneficial effects of dietary supplements on the immune response, microbiota, gut homeostasis, and intestinal health. The application of citrulline (a functional gut biomarker) and quercetin (a known potent flavonoid) to promote gut functions has gained considerable interest as both bioactive substances possess anti-inflammatory, anti-oxidative, and immunomodulatory properties. Research has demonstrated that both citrulline and quercetin can mediate gut activities by combating disruptions to the intestinal integrity and alterations to the gut microbiota. In addition, citrulline and quercetin play crucial roles in maintaining intestinal immune tolerance and gut health. However, the synergistic benefits which these dietary supplements (citrulline and quercetin) may afford to simultaneously promote gut functions remain to be explored. Therefore, this review summarizes the modulatory effects of citrulline and quercetin on the intestinal integrity and gut microbiota, and further expounds on their potential synergistic roles to attenuate intestinal inflammation and promote gut health.

Ornithine Transcarbamylase - From Structure to Metabolism: An Update

Ornithine transcarbamylase (OTC; EC 2.1.3.3) is a ubiquitous enzyme found in almost all organisms, including vertebrates, microorganisms, and plants. Anabolic, mostly trimeric OTCs catalyze the production of L-citrulline from L-ornithine which is a part of the urea cycle. In eukaryotes, such OTC localizes to the mitochondrial matrix, partially bound to the mitochondrial inner membrane and part of channeling multi-enzyme assemblies. In mammals, mainly two organs express OTC: the liver, where it is an integral part of the urea cycle, and the intestine, where it synthesizes citrulline for export and plays a major role in amino acid homeostasis, particularly of L-glutamine and L-arginine. Here, we give an overview on OTC genes and proteins, their tissue distribution, regulation, and physiological function, emphasizing the importance of OTC and urea cycle enzymes for metabolic regulation in human health and disease. Finally, we summarize the current knowledge of OTC deficiency, a rare X-linked human genetic disorder, and its emerging role in various chronic pathologies.

Effects of L-Citrulline Supplementation and Aerobic Training on Vascular Function in Individuals with Obesity across the Lifespan

Children with obesity are at higher risk for developing cardiometabolic diseases that once were considered health conditions of adults. Obesity is commonly associated with cardiometabolic risk factors such as dyslipidemia, hyperglycemia, hyperinsulinemia and hypertension that contribute to the development of endothelial dysfunction. Endothelial dysfunction, characterized by reduced nitric oxide (NO) production, precedes vascular abnormalities including atherosclerosis and arterial stiffness. Thus, early detection and treatment of cardiometabolic risk factors are necessary to prevent deleterious vascular consequences of obesity at an early age. Non-pharmacological interventions including L-Citrulline (L-Cit) supplementation and aerobic training stimulate endothelial NO mediated vasodilation, leading to improvements in organ perfusion, blood pressure, arterial stiffness, atherosclerosis and metabolic health (glucose control and lipid profile). Few studies suggest that the combination of L-Cit supplementation and exercise training can be an effective strategy to counteract the adverse effects of obesity on vascular function in older adults. Therefore, this review examined the efficacy of L-Cit supplementation and aerobic training interventions on vascular and metabolic parameters in obese individuals.

A critical review of citrulline malate supplementation and exercise performance

As a nitric oxide (NO) enhancer, citrulline malate (CM) has recently been touted as a potential ergogenic aid to both resistance and high-intensity exercise performance, as well as the recovery of muscular performance. The mechanism has been associated with enhanced blood flow to active musculature, however, it might be more far-reaching as either ammonia homeostasis could be improved, or ATP production could be increased via greater availability of malate. Moreover, CM might improve muscle recovery via increased nutrient delivery and/or removal of waste products. To date, a single acute 8 g dose of CM on either resistance exercise performance or cycling has been the most common approach, which has produced equivocal results. This makes the effectiveness of CM to improve exercise performance difficult to determine. Reasons for the disparity in conclusions seem to be due to methodological discrepancies such as the testing protocols and the associated test–retest reliability, dosing strategy (i.e., amount and timing), and the recent discovery of quality control issues with some manufacturers stated (i.e., citrulline:malate ratios). Further exploration of the optimal dose is therefore required including quantification of the bioavailability of NO, citrulline, and malate following ingestion of a range of CM doses. Similarly, further well-controlled studies using highly repeatable exercise protocols with a large aerobic component are required to assess the mechanisms associated with this supplement appropriately. Until such studies are completed, the efficacy of CM supplementation to improve exercise performance remains ambiguous.

Effects of Citrulline Malate Supplementation on Muscle Strength in Resistance-Trained Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Although the ergogenic mechanisms of supplementation with citrulline malate are well known, unclear findings regarding variables of muscle strength have been recorded. Such misleading findings in the literature illustrate the need for well-conducted meta-analysis research to elucidate the possible ergogenic impact, which could have major practical consequences for athletes and recreational practitioners seeking to optimize gains in muscle strength. The objective of this systematic review was to summarize the existing literature that evaluated the effects of citrulline malate supplementation on muscle strength outcomes from resistance exercise in resistance-trained individuals. A systematic electronic search in Medline and Scientific Electronic Library Online (SciELO) was completed in August 2020 identifying randomized controlled trials investigating the effect of citrulline malate supplementation on muscle strength in resistance-trained adults. A subsequent meta-analysis was performed. The meta-analysis involved four studies and 138 assessments (69 in citrulline-malate and 69 in placebo groups). We did not observe an overall effect favoring citrulline-malate supplementation (SMD_95% = 0.13 [-0.21; 0.46]). Considering the lower (SMD_95% = 0.06 [-0.47; 0.60]) and upper (SMD_95% = 0.17 [-0.26; 0.60]) limbs, a non-significant overall effect was identified. The mean effects were similar for "limbs" (upper vs lower) [p = 0.763]. Accordingly, our findings suggest that citrulline malate supplementation does not improve muscle strength in healthy and resistance-trained individuals (PROSPERO registration number: CRD42020159338).

Attenuation of doxorubicin-induced oxidative damage in rat brain by regulating amino acid homeostasis with Astragali Radix

The nervous system disorders caused by doxorubicin (DOX) are among the severe adverse effects that dramatically reduce the quality of life of cancer survivors. Astragali Radix (AR), a popular herbal drug and dietary supplement, is believed to help treat brain diseases by reducing oxidative stress and maintaining metabolic homeostasis. Here we show the protective effects of AR against DOX-induced oxidative damage in rat brain via regulating amino acid homeostasis. By constructing a clinically relevant low-dose DOX-induced toxicity rat model, we first performed an untargeted metabolomics analysis to discover specific metabolic features in the brain after DOX treatment and AR co-treatment. It was found that the amino acid (AA) metabolism pathways altered most significantly. To accurately characterize the brain AA profile, we established a sensitive, fast, and reproducible hydrophilic interaction chromatography-tandem mass spectrometry method for the simultaneous quantification of 22 AAs. The targeted analysis further confirmed the changes of AAs between different groups of rat brain. Specifically, the levels of six AAs, including glutamate, glycine, serine, alanine, citrulline, and ornithine, correlated (Pearson |r| >  0.47, p < 0.05) with the brain oxidative damage that was caused by DOX and rescued by AR. These findings present that AAs are among the regulatory targets of DOX-induced brain toxicity, and AR is a promising therapeutic agent for it.

L-Citrulline: A Non-Essential Amino Acid with Important Roles in Human Health

L-Arginine (Arg) has been widely used due to its functional properties as a substrate for nitric oxide (NO) generation. However, L-citrulline (CIT), whose main natural source is watermelon, is a non-essential amino acid but which has important health potential. This review provides a comprehensive approach to different studies of the endogenous synthesis of CIT, metabolism, pharmacokinetics, and pharmacodynamics as well as its ergogenic effect in exercise performance. The novel aspect of this paper focuses on the different effects of CIT, citrulline malate and CIT from natural sources such as watermelon on several topics, including cardiovascular diseases, diabetes, erectile dysfunction, cancer, and exercise performance. CIT from watermelon could be a natural food-sourced substitute for pharmacological products and therefore the consumption of this fruit is promoted.

Identification of biomarkers for physical frailty and sarcopenia through a new multi-marker approach: results from the BIOSPHERE study

Physical frailty and sarcopenia (PF&S) is a prototypical geriatric condition characterized by reduced physical function and low muscle mass. The aim of the present study was to provide an initial selection of biomarkers for PF&S using a novel multivariate analytic strategy. Two-hundred community-dwellers, 100 with PF&S and 100 non-physically frail, non-sarcopenic (nonPF&S) controls aged 70 and older were enrolled as part of the BIOmarkers associated with Sarcopenia and Physical frailty in EldeRly pErsons (BIOSPHERE) study. A panel of 74 serum analytes involved in inflammation, muscle growth and remodeling, neuromuscular junction damage, and amino acid metabolism was assayed. Biomarker selection was accomplished through sequential and orthogonalized covariance selection (SO-CovSel) analysis. Separate SO-CovSel models were constructed for the whole study population and for the two genders. The model with the best prediction ability obtained with the smallest number of variables was built using seven biomolecules. This model allowed correct classification of 80.6 ± 5.3% PF&S participants and 79.9 ± 5.1% nonPF&S controls. The PF&S biomarker profile was characterized by higher serum levels of asparagine, aspartic acid, and citrulline. Higher serum concentrations of platelet-derived growth factor BB, heat shock protein 72 (Hsp72), myeloperoxidase, and α-aminobutyric acid defined the profile of nonPF&S participants. Gender-specific SO-CovSel models identified a "core" biomarker profile of PF&S, characterized by higher serum levels of aspartic acid and Hsp72 and lower concentrations of macrophage inflammatory protein 1β, with peculiar signatures in men and women.SO-CovSel analysis allowed identifying a set of potential biomarkers for PF&S. The adoption of such an innovative multivariate approach could help address the complex pathophysiology of PF&S, translate biomarker discovery from bench to bedside, and unveil novel targets for interventions.

Metabolomic profiling reveals the effects of early-life lactoferrin intervention on protein synthesis, energy production and antioxidative capacity in the liver of suckling piglets

This study aimed to determine the effects of an early-life lactoferrin (LF) intervention on liver metabolism in suckling piglets. Sixty newborn piglets with an average initial body weight (BW) of 1.51 ± 0.05 kg were assigned to a control (CON) group and an LF group. At age 1 to 7 days, the piglets in the LF group were orally administered LF solution (0.5 g per kg BW daily), whereas the piglets in the CON group were orally administered the same dose of physiological saline. Plasma, jejunum and liver samples were collected on days 8 and 21. The LF piglets showed a decreased plasma urea nitrogen level on day 8 and an increased plasma albumin level on day 21. Pathway analysis of the metabolomic profiles showed that the LF treatment affected amino acid metabolism in the liver. In addition, the LF treatment upregulated the gene expression levels of proteolytic enzymes and amino acid transporters (APA, APN, EAAC1, Pept1, CAT1, B0AT1 and ASCT2) in the jejunum, and it enhanced the phosphorylation levels of mTOR and p70S6K in the liver. The LF treatment also upregulated the expression of a β-oxidation-related gene (CPT1) and affected the tricarboxylic acid cycle in the liver on day 21. Furthermore, the LF piglets showed a decreased level of malondialdehyde and increased levels of GSH, GSH-Px and GCLC in the liver mitochondria. Overall, the early-life LF intervention affected the protein synthesis, energy production and antioxidative capacity in the liver of the neonatal piglets.

Rootstock mediates transcriptional regulation of citrulline metabolism in grafted watermelon

Citrulline is a non-essential amino acid, involved in key biological functions in plants and humans. Rootstocks have a major impact on citrulline accumulation in grafted watermelon. Information regarding rootstock induced changes in citrulline metabolism is elusive. To understand the regulatory mechanism, parallel changes in the expression profiles of citrulline metabolic genes and citrulline content of watermelon were monitored during the development of self-rooted watermelon and watermelon grafted onto pumpkin, wild and bottle gourd rootstocks. Results demonstrated that rootstocks regulated the expression profiles in different ways to influence the citrulline content. GAT, NAGPR, ASS3 ASS2 and Asl2 showed the negative correlation with citrulline content in pumpkin grafted watermelon. Pumpkin rootstock promoted the citrulline content by high down-regulation and synergistic effect of ASS2, ASS3, ASL1 and ASl2 genes. In wild grafted watermelon, citrulline was accumulated as a result of down regulation of GAT, NAGS and ASL2 genes, which showed an inverse correlation with citrulline. In gourd grafted watermelon, changes in citrulline content were observed to be linked with lower expressions of GAT, NAGK, ASS2, ASS3, ASL1 and ARG which were negatively correlated with citrulline content. Our study will provide the basis to understand the molecular mechanism of citrulline accumulation in various rootstocks.

Host cell glutamine metabolism as a potential antiviral target

A virus minimally contains a nucleic acid genome packaged by a protein coat. The genome and capsid together are known as the nucleocapsid, which has an envelope containing a lipid bilayer (mainly phospholipids) originating from host cell membranes. The viral envelope has transmembrane proteins that are usually glycoproteins. The proteins in the envelope bind to host cell receptors, promoting membrane fusion and viral entry into the cell. Virus-infected host cells exhibit marked increases in glutamine utilization and metabolism. Glutamine metabolism generates ATP and precursors for the synthesis of macromolecules to assemble progeny viruses. Some compounds derived from glutamine are used in the synthesis of purines and pyrimidines. These latter compounds are precursors for the synthesis of nucleotides. Inhibitors of glutamine transport and metabolism are potential candidate antiviral drugs. Glutamine is also an essential nutrient for the functions of leukocytes (lymphocyte, macrophage, and neutrophil), including those in virus-infected patients. The increased glutamine requirement for immune cell functions occurs concomitantly with the high glutamine utilization by host cells in virus-infected patients. The development of antiviral drugs that target glutamine metabolism must then be specifically directed at virus-infected host cells to avoid negative effects on immune functions. Therefore, the aim of this review was to describe the landscape of cellular glutamine metabolism to search for potential candidates to inhibit glutamine transport or glutamine metabolism.

Dose-dependent beneficial effects of citrulline supplementation in short bowel syndrome in rats

OBJECTIVES

Supplementing diet with citrulline has proved an efficient means of preserving nitrogen balance and improving nutritional status after massive intestinal resection. The aim of this study was to model the action of citrulline in gut-resected rats using a dose-ranging study focused on skeletal muscle nitrogen homeostasis.

METHODS

Forty-six rats were randomly assigned to one of the following groups: citrulline 0.5 g·kg·d^-1 (n = 9), citrulline 1 g·kg·d^-1 (n = 7), citrulline 2.5 g·kg·d^-1 (n = 8), citrulline 5 g·kg·d^-1 (n = 8), control (n = 6), and sham (n = 8). The sham group underwent transection and the other groups underwent resection of 80% of the small intestine. All rats were then fed enteral nutrition (EN; all diets were isocaloric and isonitrogenous). After 10 d, the rats were sacrificed to measure and analyze animal weight; duodenum, jejunum, and ileum weight; and muscle trophicity. Protein fractional synthesis rate (FSR) and mammalian target of rapamycin complex (mTORC)1 activation were measured in the tibialis muscle.

RESULTS

There was a significant dose-dependent association between rat weight and citrulline dose up to 2.5 g·kg·d^-1 (P = 0.004). There was a significant improvement in tibialis weight correlated to plasma citrulline. Net protein FSR in the tibialis tended to be greater after resection and tended to return to baseline after citrulline supplementation. Citrulline supplementation significantly decreased the activated phosphorylated forms of S6 K1 (P = 0.003) and S6 RP (P = 0.003), with a significant positive association between myofibrillar FSR and activation of S6 K1 (r = 0.614; P = 0.02) and S6 RP (r = 0.601; P = 0.023). Jejunum weight was significantly positively correlated with plasma citrulline (r = 0.319; P = 0.0345).

CONCLUSION

Citrulline promotes body weight gain, preserves muscle trophicity, and enhances intestinal adaptation in a dose-dependent manner in a model of resected rats.

The Effect of Citrulline Supplementation on Lactic Acid Blood Level and Oxygen Saturation (SaO2) in Submaximal Physical Activity

An achievement in sport is one of the benchmarks of success of every athlete. One of the causes of the performance degradation is fatigue due to the formation of lactic acid. Supplements are proved to improve the performance in athletes. The purpose of this study was to determine the effect of citrulline supplementation on lactic acid levels and oxygen saturation on submaximal physical activity. Fatigue can be seen from lactic acid levels in subjects who have performed physical activity. This research method used randomized group pretest postest group design. The subjects used were male students of pencak silat airlangga university, and divided into two groups. The first group was given placebo and the second group was given citrulline. Each group was given the same treatment of three minutes of submaximal physical activity using an ergocycle. Before and after doing the activity the blood lactic acid and oxygen saturation of the subject was checked. The result of the data showed that lactic acid and oxygen saturation in both groups had p<0,05. Citrulline supplementation had effect on lactic acid and oxygen saturation. The results of the research showed that the group given with citrulline had significant effect on lactic acid and oxygen saturation before and after treatment.

Metabolic signatures of muscle mass loss in an elderly Taiwanese population

To identify the association between metabolites and muscle mass in 305 elderly Taiwanese subjects, we conducted a multivariate analysis of 153 plasma samples. Based on appendicular skeletal muscle mass index (ASMI) quartiles, female and male participants were divided into four groups. Quartile 4 (Men: 5.67±0.35, Women: 4.70±0.32 Kg/m²) and quartile 1 (Men: 7.60±0.29, Women: 6.56±0.53 Kg/m²) represented low muscle mass and control groups, respectively. After multivariable adjustment, except for physical function, we found that blood urea nitrogen, creatinine, and age were associated with ASMI in men. However, only triglyceride level was related to ASMI in women. The multiple logistic regression models were used to analyze in each baseline characteristic and metabolite concentration. After the adjustment, we identify amino acid-related metabolites and show that glutamate levels in women and alpha-aminoadipate, Dopa, and citrulline/ornithine levels in men are gender-specific metabolic signatures of muscle mass loss.

Synergetic Effect of NO Precursor Supplementation and Exercise Training

INTRODUCTION

Nitric oxide (NO) precursor supplementation has been shown to increase NO bioavailability and can potentially improve vascular function and exercise performance. It remains unclear whether the combination of NO precursor supplementation and exercise training has synergic effects on exercise performance. This study aims to assess the effect of chronic nitrate and citrulline intake on exercise training adaptations in healthy young individuals.

METHODS

In this randomized, double-bind trial, 24 healthy young (12 females) subjects performed vascular function assessment (blood pressure, pulse wave velocity, postischemia vasodilation, and cerebrovascular reactivity) and both local (submaximal isometric unilateral knee extension) and whole-body (incremental cycling) exercise tests to exhaustion before and after a 2-month exercise training program and daily intake of a placebo or a nitrate-rich salad and citrulline (N + C, 520 mg nitrate and 6 g citrulline) drink. Prefrontal cortex and quadriceps oxygenation was monitored continuously during exercise by near-infrared spectroscopy.

RESULTS

N + C supplementation had no effect on vascular function and muscle and cerebral oxygenation during both local and whole-body exercise. N + C supplementation induced a significantly larger increase in maximal knee extensor strength (+5.1 ± 3.5 vs +0.2 ± 5.5 kg, P = 0.008) as well as a trend toward a larger increase in knee extensor endurance (+35.2 ± 26.1 vs +24.0 ± 10.4 contractions, P = 0.092) than placebo, but no effect on exercise training-induced maximal aerobic performance improvement.

CONCLUSION

These results suggest that chronic nitrate and citrulline supplementation enhances the effect of exercise training on quadriceps muscle function in healthy active young individuals, but this does not translate into improved maximal aerobic performances.

A novel multi-marker discovery approach identifies new serum biomarkers for Parkinson's disease in older people: an EXosomes in PArkiNson Disease (EXPAND) ancillary study

Dopaminergic nigrostriatal denervation and widespread intracellular α-synuclein accumulation are neuropathologic hallmarks of Parkinson's disease (PD). A constellation of peripheral processes, including metabolic and inflammatory changes, are thought to contribute to neurodegeneration. In the present study, we sought to obtain insight into the multifaceted pathophysiology of PD through the application of a multi-marker discovery approach. Fifty older adults aged 70+, 20 with PD and 30 age-matched controls were enrolled as part of the EXosomes in PArkiNson Disease (EXPAND) study. A panel of 68 circulating mediators of inflammation, neurogenesis and neural plasticity, and amino acid metabolism was assayed. Biomarker selection was accomplished through sequential and orthogonalized covariance selection (SO-CovSel), a multi-platform regression method developed to handle highly correlated variables organized in multi-block datasets. The SO-CovSel model with the best prediction ability using the smallest number of variables was built with seven biomolecules. The model allowed correct classification of 94.2 ± 3.1% participants with PD and 100% controls. The biomarker profile of older adults with PD was defined by higher circulating levels of interleukin (IL) 8, macrophage inflammatory protein (MIP)-1β, phosphoethanolamine, and proline, and by lower concentrations of citrulline, IL9, and MIP-1α. Our innovative approach allowed identifying and evaluating the classification performance of a set of potential biomarkers for PD in older adults. Future studies are warranted to establish whether these biomolecules could serve as biomarkers for PD as well as unveil new targets for interventions.

Metabolic studies reveal that ruminal microbes of adult steers do not degrade rumen-protected or unprotected L-citrulline

In vitro and in vivo experiments were conducted to determine the metabolism of rumen-protected or unprotected l-citrulline (Cit) plus l-glutamine (Gln) by ruminal microbes. In the in vitro experiment, whole ruminal fluid (3 mL, containing microorganisms) from steers was incubated at 37 ºC with 5 mM Cit plus 6 mM Gln (in a rumen-protected or unprotected form) for 0, 0.5, 2, or 4 h after which times 50 µL samples were collected for AA and ammonia analyses. In the in vivo experiment, at 0.5 h before and 0, 0.5, 1, 2, 4, and 6 h after cannulated adult steers consumed 0.56 kg dried-distillers' grain mixed with 70 g Cit plus 70 g Gln (in a rumen-protected or unprotected form), samples of ruminal fluid and jugular venous blood were obtained for AA analyses. Results from both in vitro and in vivo experiments demonstrated extensive hydrolysis of rumen-unprotected Gln into glutamate, but little degradation of the rumen-protected Gln or rumen-protected and unprotected Cit by ruminal microbes. Concentrations of Cit and arginine in the plasma of steers consuming rumen-protected or unprotected AA increased at 1 and 2 h after the meal, respectively, when compared with values at 0 h. Collectively, these novel findings indicate that ruminal microbes of adult steers do not degrade extracellular Cit in a rumen-protected or unprotected form. Our results refute the view that all dietary AAs are extensively catabolized by ruminal microorganisms and also have important implications for dietary supplementation with Cit to ruminants to enhance the concentration of arginine in their plasma and their productivity.

Acute effects of Nitrosigine® and citrulline malate on vasodilation in young adults

Background

Athletes are increasingly exploring ways to enhance their physical performance. Increasing blood flow to the working tissues through endothelium-dependent vasodilation is one factor athletes use to realize these results. Sports supplements such as pre-workouts tout this benefit; however, many have not been tested under laboratory conditions to examine the effects of commonly used supplements on vasodilation. Two popular supplements are Nitrosigine® and citrulline malate (CM). Thus, the purpose of this experiment was to determine the effects of Nitrosigine and CM on vasodilation using ultrasound and flow mediated dilation (FMD).

Methods

Healthy, normotensive, and physically active male (n = 16) and female (n = 8) young adults participated in the present investigation. We utilized a randomized, double-blind, within-subjects design where participants reported for three trials, each preceded by a 7-day washout period. Baseline FMD measurement was obtained for each visit, followed by consumption of one clinical dose CM (8 g), Nitrosigine (1.5 g), or dextrose placebo (8 g). Following a 60-min digestion period, FMD was repeated. Supplementation order was randomized controlling for potential order effects.

Results

Repeated measures ANOVA yielded a significant supplement (3) x time (2) effect (p < .001), such that Nitrosigine and CM yielded a greater improvement in FMD response than placebo. After supplementation, Nitrosigine and CM increased FMD by 31 and 34%, respectively, compared to a decrease of 2% during the placebo trial. After allometric scaling of the FMD values, supplement x time effect remained significant (p = .001) and changes were similar to non-scaled results. Nitrosigine (23%) and CM (25%) generated significantly greater allometric scaled FMD values when compared to the placebo trial (0.60%).

Discussion

Both Nitrisigine and CM increased endothelial-dependent vasodilation as measured by a change in FMD. Increased vasodilation leads to an increase in skeletal muscle blood flow resulting in potential improvements in exercise performance.

Metabolome profiling in skeletal muscle to characterize metabolic alterations in over-conditioned cows during the periparturient period

The transition from late gestation to early lactation is associated with extensive changes in metabolic, endocrine, and immune functions in dairy cows. Skeletal muscle plays an important role in maintaining the homeorhetic adaptation to the metabolic needs of lactation. The objective of this study was to characterize the skeletal muscle metabolome in the context of the metabolic changes that occur during the transition period in dairy cows with high (HBCS) versus normal body condition (NBCS). Fifteen weeks antepartum, 38 pregnant multiparous Holstein cows were assigned to 1 of 2 groups, which were fed differently to reach the targeted BCS and back fat thickness (BFT) until dry-off at -49 d before calving (HBCS: >3.75 and >1.4 cm; NBCS: <3.5 and <1.2 cm). During the dry period and the subsequent lactation, both groups were fed identical diets. The differences in both BCS and BFT were maintained throughout the study. The metabolome was characterized in skeletal muscle samples (semitendinosus muscle) collected on d -49, 3, 21, and 84 relative to calving using a targeted metabolomics approach (AbsoluteIDQ p180 kit; Biocrates Life Sciences AG, Innsbruck, Austria), which allowed for the quantification of up to 188 metabolites from 6 different compound classes (acylcarnitines, amino acids, biogenic amines, glycerophospholipids, sphingolipids, and hexoses). On d -49, the concentrations of citrulline and hydroxytetradecadienyl-l-carnitine in muscle were higher in HBCS cows than in NBCS cows, but those of carnosine were lower. Over-conditioning did not affect the muscle concentrations of any of the metabolites on d 3. On d 21, the concentrations of phenylethylamine and linoleylcarnitine in muscle were lower in HBCS cows than in NBCS cows, and the opposite was true for lysophosphatidylcholine acyl C20:4. On d 84, the significantly changed metabolites were mainly long-chain (>C32) acyl-alkyl phosphatidylcholine and di-acyl phosphatidylcholine, along with 3 long-chain (>C16) sphingomyelin that were all lower in HBCS cows than in NBCS cows. These data contribute to a better understanding of the metabolic adaptation in skeletal muscle of dairy cows during the transition period, although the physiological significance and underlying molecular mechanisms responsible for the regulation of citrulline, hydroxytetradecadienyl-l-carnitine, carnosine, and phenylethylamine associated with over-conditioning are still elusive and warrant further investigation. The changes observed in muscle lysophosphatidylcholine and phosphatidylcholine concentrations may point to an alteration in phosphatidylcholine metabolism, probably resulting in an increase in membrane stiffness, which may lead to abnormalities in insulin signaling in the muscle of over-conditioned cows.

Plasma biomarkers of small intestine adaptations in obesity-related metabolic alterations

BACKGROUND

Evidence suggests that pathophysiological conditions such as obesity and type 2 diabetes (T2D) are associated with morphologic and metabolic alterations in the small intestinal mucosa. Exploring these alterations generally requires invasive methods, limiting data acquisition to subjects with enteropathies or undergoing bariatric surgery. We aimed to evaluate small intestine epithelial cell homeostasis in a cohort of men covering a wide range of adiposity and glucose homoeostasis statuses.

METHODS

Plasma levels of citrulline, a biomarker of enterocyte mass, and I-FABP, a biomarker of enterocyte death, were measured by UHPLC‑MS and ELISA in 154 nondiabetic men and 67 men with a T2D diagnosis.

RESULTS

Plasma citrulline was significantly reduced in men with insulin resistance and T2D compared to insulin sensitive men. Decreased citrulline levels were, however, not observed in men with uncontrolled metabolic parameters during T2D. Plasma I-FABP was significantly higher in men with T2D, especially in presence of uncontrolled glycemic and lipid profile parameters. Integration of both parameters, which estimate enterocyte turnover, was associated with glucose homeostasis as well as with T2D diagnosis. Differences in biomarkers levels were independent of age and BMI and glucose filtration rates.

CONCLUSIONS

Our study supports a decreased functional enterocyte mass and an increased enterocyte death rate in presence of metabolic alterations but emphasizes that epithelial cell homeostasis is especially altered in presence of severe insulin resistance and T2D. The marked changes in small intestine cellularity observed in obesity and diabetes are thus suggested to be part of gut dysfunctions, mainly at an advanced stage of the disease.