Endogenous glycine and tyrosine production is maintained in adults consuming a marginal-protein diet

Potential New Methods to Analyze Basal and Total Endogenous Protein Losses of Host and Bacterial Origin in Pigs

BACKGROUND

Current systems for assessing protein quality such as the Digestible Indispensable Amino Acid Score correct apparent amino acid (AA) digestibility for basal endogenous protein losses (bEPL), ignoring the potential influence of the diet on these losses. However, the quantification of total endogenous protein losses (tEPL) poses a challenge.

OBJECTIVES

To evaluate different methods for quantifying tEPL and bEPL, and to assess their potential in discriminating between tEPL originating from bacteria and host.

METHODS

Using an incomplete Youden square design, 12 ileal cannulated pigs received 10 different protein sources, and a nitrogen-free (NF) diet. Ileal digesta were collected on days 6 and 7 of each 1-wk feeding period, to quantify endogenous protein losses (EPL) and analyze apparent ileal digestibility. Ileal EPL were estimated based on 1) 16S-+18S gene copy quantitative polymerase chain reaction, 2) diaminopimelic acid (DAPA)+18S, 3) differential AA profiles in digesta, EPL, and bacteria, equaling tEPL, and 4) an NF diet and 5) whey protein isolate (WPI), equaling bEPL.

RESULTS

Ileal bEPL based on the NF and WPI method correlated moderately to highly (r = 0.69, P < 0.05), but the NF method probably underestimated bEPL. In pigs fed the WPI diet, EPL based on the WPI and AA profile method were highly correlated (r = 0.88, P < 0.01). Overall, tEPL based on the AA profile method were moderately correlated with the 16S+18S method (r = 0.58, P < 0.001), and DAPA+18S (r = 0.57, P < 0.001). Low correlations were observed between bacterial tEPL based on the AA profile method and 16S or DAPA. Host tEPL based on the AA profile method and 18S were weakly correlated (r = 0.39, P < 0.001).

CONCLUSIONS

The AA profile method seems the most appropriate method for tEPL quantification, whereas the WPI method is preferred for bEPL quantification. Despite challenges in distinguishing between bacterial and host EPL, it is evident that bacterial proteins substantially (on average 37%-83%, depending on method) contribute to the EPL.

Protein intake affects erythrocyte glutathione synthesis in healthy adults aged ≥60 years in a repeated-measures trial

BACKGROUND

Protein recommendations for older adults are based on nitrogen balance data from young adults. Physiological studies using the indicator amino acid oxidation method suggest they need 30% to 50% more protein than current recommendations. We herein present glutathione (GSH) as a physiological estimate of protein adequacy in older adults.

OBJECTIVES

The objective was to measure GSH kinetics in response to varying protein intakes in a repeated-measures design in healthy adults aged ≥60 y using the precursor-product method.

METHODS

Sixteen healthy older adults (n = 8 male and n = 8 female; body mass index ≤30 kg/m2) were studied. Each received 4 of 6 protein intakes in random order (0.66, 0.8, 0.9, 1.1, 1.3 and 1.5 g⋅kg-1⋅d-1). At each intake level, participants underwent isotope infusion studies of 7 h duration following a 3-d adaptation to the test level of protein. On the fourth day, GSH fractional (FSR) and absolute synthesis (ASR) rates were quantified by measuring the incorporation of U-[13C2-15N]glycine into GSH at isotopic steady state. A mixed-effect change-point regression model was used to determine a breakpoint in FSR and ASR. Secondary outcomes included plasma concentrations of oxidative stress markers, homocysteine, 5-L-oxoproline (5-OP), and urinary sulfate. The effect of secondary outcomes on GSH kinetics was analyzed using a joint linear mixed-effect model and Tukey's post hoc test.

RESULTS

A protein intake of 1.08 g⋅kg-1⋅d-1 (95% confidence interval [CI]: 0.83, 1.32; Rm2 = 0.207; Rc2 = 0.671; P < 0.001) maximized GSH FSR. There was no effect of protein intake on concentrations of erythrocyte GSH, plasma homocysteine, oxidative stress markers, or 5-OP (P > 0.05). Protein intake had a positive effect on urinary sulfate excretion (P < 0.0001).

CONCLUSION

A protein intake of 1.08 g⋅kg-1⋅d-1 from a high-quality protein maximized GSH synthesis in adults ≥60 y. This lends support to data suggesting a requirement higher than the current recommendation. This study was registered at clinicaltrials.gov as NCT02971046.

Protein intake affects erythrocyte glutathione synthesis in young healthy adults in a repeated-measures trial

BACKGROUND

In 2005, the Institute of Medicine advised using methods other than nitrogen balance (NB) for determining protein requirements. Since then, protein requirements using indicator amino acid oxidation (IAAO) have been published and are higher than NB. Glutathione (GSH), a tripeptide of cysteine, glutamate, and glycine, is a principal antioxidant that can be used as a functional indicator of protein adequacy.

OBJECTIVES

The aim of this study was to measure changes in erythrocyte GSH kinetics [fractional synthesis rate (FSR) and absolute synthesis rate (ASR)] in healthy adults following a range of protein intakes at and above the current recommendations.

METHODS

Sixteen healthy adults [8 males and 8 females, aged 25.6 ± 0.9 y (mean ± SEM)] were studied at 4 of 6 protein intakes ranging from 0.6 to 1.5 g⋅kg-1⋅d-1. Erythrocyte GSH kinetics were assessed during a 7-h infusion of [U-13C2-15N]glycine following 2 d of adaptation to each protein intake. Blood and urine tests were performed to measure oxidative stress markers, plasma homocysteine, triglycerides, plasma amino acid concentrations, 5-L-oxoproline (5-OP), and urinary sulfate. The protein intake that maximized GSH synthesis was determined using mixed-effect change-point regression in R. Primary and secondary outcomes were analyzed using linear mixed-effects and repeated-measures analysis of variance with Tukey's post hoc test.

RESULTS

The protein intake that maximized GSH FSR at 78%⋅d-1 was 1.0 g⋅kg-1⋅d-1 (95% confidence interval: 0.63, 1.39). GSH ASR was significantly lower at 0.6 and 0.8 g⋅kg-1⋅d-1 than at 1.5 g⋅kg-1⋅d-1 (2.03 and 2.17, respectively, compared with 3.71 mmol⋅L-1⋅d-1). Increasing the protein intake led to increased urinary sulfate but did not affect erythrocyte GSH concentration, plasma oxidative stress markers, triglycerides, homocysteine, or 5-OP.

CONCLUSIONS

A protein intake of 1.0 g⋅kg-1⋅d-1 maximized GSH synthesis, which is in agreement with earlier IAAO-derived protein requirements of 0.93 to 1.2 g⋅kg-1⋅d-1. These findings suggest that recommendations based on NB (0.66 g⋅kg-1⋅d-1) may underestimate protein needs for adequate health. This trial was registered at clinicaltrials.gov as NCT02971046.

Effects of Glycine Supplementation in Drinking Water on the Growth Performance, Intestinal Development, and Genes Expression in the Jejunum of Chicks

Glycine, the most basic amino acid found in nature, is considered an essential amino acid for chicks. However, the precise understanding of high concentrations of glycine's significance in promoting the growth performance of chicks, as well as its impact on intestinal development, re-mains limited. Consequently, the objective of this study was to investigate the effects of glycine supplementation in drinking water on growth performance, intestine morphology, and development in newly hatched chicks. In this study, 200 newly born chicks were selected and pro-vided with a supplementation of 0.5%, 1%, and 2% glycine in their drinking water during their first week of life. The results revealed that glycine supplementation in drinking water could significantly increase the average daily gain of chicks from days 7 to 14. Furthermore, a significant difference was observed between the group supplemented with 1% glycine and the control group. Concurrently, this glycine supplementation increased the villus height and the ratio of the villus height to crypt depth in jejunum on both day 7 and day 14. Glycine supplementation in drinking water significantly affected the mRNA expression level of the ZO-1, GCLM, and rBAT genes in jejunum, which may have certain effects on the mucosal immune defense, cellular antioxidant stress capacity, and amino acid absorption. Overall, the findings of this study indicate that glycine supplementation in drinking water can enhance the growth performance of chicks and promote their intestine development.

Is It Time to Reconsider the U.S. Recommendations for Dietary Protein and Amino Acid Intake?

Since the U.S. Institute of Medicine's recommendations on protein and amino acid intake in 2005, new information supports the need to re-evaluate these recommendations. New lines of evidence include: (1) re-analysis/re-interpretation of nitrogen balance data; (2) results from indicator amino acid oxidation studies; (3) studies of positive functional outcomes associated with protein intakes higher than recommended; (4) dietary guidance and protein recommendations from some professional nutrition societies; and (5) recognition that the synthesis of certain dispensable amino acids may be insufficient to meet physiological requirements more often than previously understood. The empirical estimates, theoretical calculations and clinical functional outcomes converge on a similar theme, that recommendations for intake of protein and some amino acids may be too low in several populations, including for older adults (≥65 years), pregnant and lactating women, and healthy children older than 3 years. Additional influential factors that should be considered are protein quality that meets operational sufficiency (adequate intake to support healthy functional outcomes), interactions between protein and energy intake, and functional roles of amino acids which could impact the pool of available amino acids for use in protein synthesis. Going forward, the definition of "adequacy" as it pertains to protein and amino acid intake recommendations must take into consideration these critical factors.

Dietary glycine supplementation prevents heat stress-induced impairment of antioxidant status and intestinal barrier function in broilers

This study tested the hypothesis that glycine improves intestinal barrier function through regulating oxidative stress in broilers exposed to heat stress. A total of 300 twenty-one-day-old female Arbor Acres broilers (600 ± 2.5g) was randomly allocated to 5 treatments (6 replicate of 10 birds each). The 5 treatments were as follows: the control group (CON) was kept under thermoneutral condition (24 ± 1°C) and was fed a basal diet. Broilers fed a basal diet and reared under high ambient temperature (HT) were considered as the HT group (34 ± 1°C for 8 h/d). Broilers fed a basal diet supplemented with 0.5%, 1.0%, and 2.0% glycine and exposed to HT were regarded as the HT + glycine treatments. The results exhibited that heat stress reduced growth performance, serum total antioxidant capacity (T-AOC), and glutathione (GSH) concentration (P < 0.05); increased activity of serum catalase (CAT) and the contents of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) (P < 0.05). HT exposure led to downregulating the mRNA expression of NAD(P)H quinone dehydrogenase 1 (NQO1), Occludin, and zonula occludens-1 (ZO-1) (P < 0.05); enhanced the mRNA levels of Kelch-like ECH-associated protein 1 (Keap1), CAT, glutathione synthetase (GSS), and glutamate-cysteine ligase modifier subunit (GCLM) (P < 0.05); impaired the intestinal morphology (P < 0.05); and altered the diversity and community of gut microbiota (P < 0.05). The final body weight (FBW), ADFI, ADG, and gain-to-feed ratio (G: F) increased linearly or quadratically, and the antioxidant capacity was improved (P < 0.05) with glycine supplementation. Glycine treatment increased the villus height (VH), and villus height to crypt depth ratio (V/C) of the duodenum linearly or quadratically, and linearly increased the VH of jejunum and ileum. The mRNA expression of Occludin, and ZO-1 were increased linearly in the ileum mucosa of broilers subjected to HT. Collectively, these results demonstrated that glycine supplementation alleviates heat stress-induced dysfunction of antioxidant status and intestinal barrier in broilers.

De Novo Glycine Synthesis Is Reduced in Adults With Morbid Obesity and Increases Following Bariatric Surgery

BACKGROUND

Glycine is a dietary non-essential amino acid that is low in obesity and increases following bariatric surgery. However, the exact mechanism responsible remains unclear and it is unknown whether hypoglycinemia is a cause or consequence of insulin resistance.

OBJECTIVE

Using multiple isotopically labeled tracers, we aimed to determine the underlying kinetic changes responsible for hypoglycinemia in obesity by: 1) Comparing glycine kinetics between participants with morbid obesity (BMI ≥ 32.5 kg/m²) to those with healthy weight (BMI < 25 kg/m²), and 2) Comparing glycine kinetic changes in participants with morbid obesity after bariatric surgery.

METHODS

[1,2-¹³C₂] glycine, [2,3,3-²H₃] serine, and [²H₅] phenylalanine were infused to compare the glycine kinetic parameters between 21 participants with morbid obesity and 21 controls with healthy weight. Participants with morbid obesity then underwent bariatric surgery and 17 were re-studied 6 months later. Data were analyzed by non-parametric methods and presented as median (interquartile range).

RESULTS

Compared to controls, participants with morbid obesity had significantly lower plasma glycine concentrations at 163 (153-171) vs. 201 (172-227) µmol/L and significantly reduced de novo glycine synthesis rate at 86.2 (64.5-111) vs.124 (103-159) µmol·kg LBM^-1·h¹, p < 0.001. Following surgery, body weight and insulin resistance decreased and this was accompanied by significant increases in plasma glycine concentration to 210 (191-243) µmol/L as well as the de novo glycine synthesis rate to 127 (98.3-133) µmol·kg LBM^-1·h^-1, p < 0.001 vs. baseline.

CONCLUSION

Hypoglycinemia in participants with morbid obesity was associated with impaired de novo glycine synthesis. The increase in plasma glycine concentration and de novo glycine synthesis plus the marked improvement in insulin resistance after bariatric surgery suggest that hypoglycinemia may be secondary to impaired glycine synthesis because of obesity-induced insulin resistance.

CLINICAL TRIAL REGISTRATION

[https://tinyurl.com/6wfj7yss], identifier [NCT04660513].

Glycinergic Signaling in Macrophages and Its Application in Macrophage-Associated Diseases

Accumulating evidences support that amino acids direct the fate decision of immune cells. Glycine is a simple structural amino acid acting as an inhibitory neurotransmitter. Besides, glycine receptors as well as glycine transporters are found in macrophages, indicating that glycine alters the functions of macrophages besides as an inhibitory neurotransmitter. Mechanistically, glycine shapes macrophage polarization via cellular signaling pathways (e.g., NF-κB, NRF2, and Akt) and microRNAs. Moreover, glycine has beneficial effects in preventing and/or treating macrophage-associated diseases such as colitis, NAFLD and ischemia-reperfusion injury. Collectively, this review highlights the conceivable role of glycinergic signaling for macrophage polarization and indicates the potential application of glycine supplementation as an adjuvant therapy in macrophage-associated diseases.

The effect of reduced dietary glycine and serine and supplemental threonine on growth performance, protein deposition in carcass and viscera, and skin collagen abundance of nursery pigs fed low crude protein diets

Thirty five barrows (initial body weight [BW]: 15.1 ± 1.0 kg) were used to determine the effect of partially replacing Gly + Ser with Thr in reduced crude protein (CP) diets on growth performance, protein deposition in carcass and viscera, and skin collagen abundance during the late nursery phase to 25 kg BW. Pigs were individually fed one of five iso-nitrogenous diets (n = 7) for 21 d. The basal diet met estimated essential amino acids (AA) requirements by using all essential AA plus Gly and Ser in free form (CON; 12.1% CP; as-fed, analyzed contents). The remaining four diets were formulated by reducing total Gly and Ser concentrations to 60% or 20% of the CON diet. The N removed with Gly and Ser was replaced with either crystalline Thr or Glu. Total analyzed Thr made up either 1.59% (T1; 12.5% CP) or 2.34% (T2; 12.2% CP) of the Thr-supplemented diets, and total analyzed Glu made up either 3.47% (G1; 12.7% CP) or 4.64% (G2; 12.9% CP) of the Glu-supplemented diets. Pigs were slaughtered on day 21 to determine body composition and skin collagen abundance via bright field microscopy. Overall, average daily gain (ADG) and G:F and final carcass weights were greater for pigs fed diets supplemented with Glu (G1 + G2) vs. those fed diets supplemented with Thr (T1 + T2; P < 0.05, P = 0.060, and P = 0.050 for ADG, G:F, and final carcass weight, respectively); intermediate values were observed for CON. Nitrogen retention in carcass plus viscera and the AA profile of deposited protein in the carcass were not influenced by dietary treatment. Pigs fed the T2 and G2 diets had greater retention of Thr (vs. CON and G2) and Glu (vs. CON and T2) in the viscera protein, respectively (P < 0.05). The apparent utilization efficiency of standardized ileal digestible Thr for protein deposition in carcass plus viscera was less for pigs fed T2 (15.1%) vs. those fed CON (56.7%) or G2 (58.6% ± 2.9%) diets (P < 0.001). Only pigs fed T1 had skin collagen abundance not different from CON; pigs fed G1, G2, and T2 had reduced skin collagen abundance compared with CON and T1 (P < 0.01). Using Glu as an N source when Gly and Ser were reduced to 60% and 20% of CON in reduced CP diets maintained ADG for pigs between 15 and 25 kg BW, whereas supplying Thr as a N source reduced ADG and carcass weight. When dietary Gly and Ser were supplied at 60% of CON, only Thr supplementation rescued skin collagen abundance. Therefore, supplemental Thr at excess levels is not sufficient to replace N from Gly and Ser in reduced CP diets fed to late nursery pigs, despite supporting skin collagen abundance as a secondary indicator of Gly status.

Important roles of dietary taurine, creatine, carnosine, anserine and 4-hydroxyproline in human nutrition and health

Taurine (a sulfur-containing β-amino acid), creatine (a metabolite of arginine, glycine and methionine), carnosine (a dipeptide; β-alanyl-L-histidine), and 4-hydroxyproline (an imino acid; also often referred to as an amino acid) were discovered in cattle, and the discovery of anserine (a methylated product of carnosine; β-alanyl-1-methyl-L-histidine) also originated with cattle. These five nutrients are highly abundant in beef, and have important physiological roles in anti-oxidative and anti-inflammatory reactions, as well as neurological, muscular, retinal, immunological and cardiovascular function. Of particular note, taurine, carnosine, anserine, and creatine are absent from plants, and hydroxyproline is negligible in many plant-source foods. Consumption of 30 g dry beef can fully meet daily physiological needs of the healthy 70-kg adult human for taurine and carnosine, and can also provide large amounts of creatine, anserine and 4-hydroxyproline to improve human nutrition and health, including metabolic, retinal, immunological, muscular, cartilage, neurological, and cardiovascular health. The present review provides the public with the much-needed knowledge of nutritionally and physiologically significant amino acids, dipeptides and creatine in animal-source foods (including beef). Dietary taurine, creatine, carnosine, anserine and 4-hydroxyproline are beneficial for preventing and treating obesity, cardiovascular dysfunction, and ageing-related disorders, as well as inhibiting tumorigenesis, improving skin and bone health, ameliorating neurological abnormalities, and promoting well being in infants, children and adults. Furthermore, these nutrients may promote the immunological defense of humans against infections by bacteria, fungi, parasites, and viruses (including coronavirus) through enhancing the metabolism and functions of monocytes, macrophages, and other cells of the immune system. Red meat (including beef) is a functional food for optimizing human growth, development and health.

The effect of supplementing glycine and serine to a low crude protein diet on growth and skin collagen abundance of nursery pigs1

A total of 96 newly weaned barrows (initial body weight [BW]: 6.3 ± 0.5 kg) were used to determine the effect of a low crude protein (CP) diet supplemented with Gly and Ser on growth and skin collagen abundance. Barrows were assigned to one of three experimental diets in a three-phase feeding program fed for 35 days (n = 8; pen was the experimental unit): 1) corn-soybean meal diet (CON; 20.3% to 23.1% CP; as-fed, analyzed contents); 2) low CP diet (14.8% to 21.4% CP) supplemented with Gly and Ser (G + S) to the same concentrations as CON; 3) low CP diet supplemented with Glu to maintain the same CP concentration as the G + S diet (GLU; 15.0% to 22.1% CP). On days 21 and 35, eight pigs per treatment were euthanized for the determination of physical and chemical body composition and skin collagen abundance. Pigs fed the CON diet had greater overall ADG and final BW compared to pigs fed GLU and G + S (P < 0.01). Over the entire 35-day experimental period, ADFI was not influenced by dietary treatment but G:F tended to be greater for pigs fed CON than G + S (P = 0.084), while intermediate values were observed for GLU. Carcass weights on days 21 and 35 were greater for pigs fed CON than G + S or GLU (P < 0.01). Viscera weights on day 21 were greater for CON than G + S and GLU (P < 0.05) and on day 35 were greater for CON than G + S (P < 0.05) with intermediate values observed for GLU. The N intake (g/d) between days 0 and 35 was greater for CON than G + S or GLU (P < 0.05) and N retention in combined carcass and viscera was greater for CON than G + S (P < 0.01) with intermediate values observed for GLU. No treatment effects were observed for efficiency of N utilization. Between days 0 and 21 however, the efficiency of using dietary N for N retention in carcass and viscera tended to be less for pigs fed CON vs. GLU (73.8% vs. 91.6%), while intermediate values were observed for G + S (84.3%; P = 0.095). Pigs fed CON and G + S diets had greater skin collagen abundance than pigs fed GLU on days 21 and 35 (P < 0.01). Supplementing low CP diets with Glu or with Gly and Ser at the levels used in the current study did not maintain ADG or combined carcass and viscera N retention and only the G + S diet supported skin collagen abundance not different from pigs fed CON. The importance of meeting essential AA requirements for growth are well accepted, but supplementing specific NEAA may be needed when feeding reduced CP diets to newly weaned pigs to support secondary indicators of AA status, such as skin collagen abundance.

High glycine concentration increases collagen synthesis by articular chondrocytes in vitro: acute glycine deficiency could be an important cause of osteoarthritis

Collagen synthesis is severely diminished in osteoarthritis; thus, enhancing it may help the regeneration of cartilage. This requires large amounts of glycine, proline and lysine. Previous works of our group have shown that glycine is an essential amino acid, which must be present in the diet in large amounts to satisfy the demands for collagen synthesis. Other authors have shown that proline is conditionally essential. In this work we studied the effect of these amino acids on type II collagen synthesis. Bovine articular chondrocytes were cultured under a wide range of different concentrations of glycine, proline and lysine. Chondrocytes were characterized by type II collagen immunocytochemistry of confluence monolayer cultures. Cell growth and viability were assayed by trypan blue dye exclusion method. Type II collagen was measured in the monolayer, every 48 h for 15 days by ELISA. Increase in concentrations of proline and lysine in the culture medium enhances the synthesis of type II collagen at low concentrations, but these effects decay before 1.0 mM. Increase of glycine as of 1.0 mM exceeds these effects and this increase continues more persistently by 60–75%. Since the large effects produced by proline and lysine are within the physiological range, while the effect of glycine corresponds to a much higher range, these results demonstrated a severe glycine deficiency for collagen synthesis. Thus, increasing glycine in the diet may well be a strategy for helping cartilage regeneration by enhancing collagen synthesis, which could contribute to the treatment and prevention of osteoarthritis.

Combined Vitamin B-12 and Balanced Protein-Energy Supplementation Affect Homocysteine Remethylation in the Methionine Cycle in Pregnant South Indian Women of Low Vitamin B-12 Status

Background: Low-quality dietary protein intake and vitamin B-12 deficiency could interact to decrease methionine transmethylation and remethylation rates during pregnancy and may affect epigenetic modifications of the fetal genome.Objective: The objective of this randomized, partially open-labeled intervention trial was to examine the effect of supplemental high-quality protein and vitamin B-12 on third-trimester methionine kinetics in pregnant Indian women with a low vitamin B-12 status.Methods: Pregnant women with low serum vitamin B-12 concentrations (<200 pmol/L) were randomly assigned to 1 of 3 groups: the first group received balanced protein-energy supplementation of 500 mL milk/d plus a 10-μg vitamin B-12 tablet/d (M+B-12 group; n = 30), the second group received milk (500 mL/d) plus a placebo tablet (M+P group; n = 30), and the third group received a placebo tablet alone (P group; n = 33). Third-trimester fasting plasma amino acid kinetics were measured by infusing 1-¹³C,methyl-²H₃-methionine, ring-²H₅-phenylalanine, ring-²H₄-tyrosine,1-¹³C-glycine, and 2,3,3-²H₃,¹⁵N-serine in a subset of participants. Placental mRNA expression of genes involved in methionine pathways, placental long interspersed nuclear elements 1 (LINE-1) methylation, and promoter methylation levels of vascular endothelial growth factor (VEGF) were analyzed.Results: Remethylation rates in the M+B-12, M+P, and P groups were 5.1 ± 1.7, 4.1 ± 1.0, and, 5.0 ± 1.4 μmol ⋅ kg^-1 ⋅ h^-1, respectively (P = 0.057), such that the percentage of transmethylation remethylated to methionine tended to be higher in the M+B-12 group (49.5% ± 10.5%) than in the M+P group (42.3% ± 8.4%; P = 0.053) but neither differed from the P group (44.2% ± 8.1%; P > 0.1). Placental mRNA expression, LINE-1, and VEGF promoter methylation did not differ between groups.Conclusions: Combined vitamin B-12 and balanced protein-energy supplementation increased the homocysteine remethylation rate in late pregnancy. Thus, vitamin B-12 along with balanced protein-energy supplementation is critical for optimal functioning of the methionine cycle in the third trimester of pregnancy in Indian women with low serum vitamin B-12 in early pregnancy. This trial was registered at clinicaltrials.gov as CTRI/2016/01/006578.

Effect of multi-nutrient insufficiency on markers of one carbon metabolism in young women: response to a methionine load

BACKGROUND/OBJECTIVES

Multi-nutrient insufficiencies as a consequence of nutritional and economic factors are common in India and other developing countries. We have examined the impact of multi-nutrient insufficiency on markers of one carbon (1C) metabolism in the blood, and response to a methionine load in clinically healthy young women.

SUBJECTS/METHODS

Young women from Pune, India (n=10) and Cleveland, USA (n=13) were studied. Blood samples were obtained in the basal state and following an oral methionine load (50 mg/kg of body weight in orange juice). Plasma concentrations of vitamin B12, folate and B6 were measured in the basal state. The effect of methionine load on the levels of methionine, total homocysteine, cysteine, glutathione and amino acids was examined.

RESULTS

Indian women were significantly shorter and lighter compared with the American women and had lower plasma concentration of vitamins B12, folate and B6, essential amino acids and glutathione, but higher concentration of total homocysteine. The homocysteine response to methionine load was higher in Indian women. The plasma concentrations of glycine and serine increased in the Indian women after methionine (in juice) load. A significant negative correlation between plasma B6 and homocysteine (r= -0.70), and plasma folate and glycine and serine levels were observed in the Indian group (P<0.05) but not in the American group.

CONCLUSIONS

Multi-nutrient insufficiency in the Indian women caused unique changes in markers of whole body protein and 1C metabolism. These data would be useful in developing nutrient intervention strategies.

Glycine Regulates Expression and Distribution of Claudin-7 and ZO-3 Proteins in Intestinal Porcine Epithelial Cells

BACKGROUND

Glycine traditionally is classified as a nutritionally nonessential amino acid in humans and animals. Because of its abundance in the body and its extensive use via multiple pathways, requirements for glycine are particularly high in neonates. Our recent studies show that dietary glycine supplementation is needed for optimal intestinal development in piglets. Importantly, reduced concentrations of glycine in the lumen of the small intestine are associated with gut dysfunction in low-birth-weight piglets. However, the mechanisms responsible for the beneficial effects of glycine on the intestinal mucosal barrier are largely unknown.

OBJECTIVE

This study tested the hypothesis that glycine may regulate the expression and distribution of tight junction (TJ) proteins, thereby contributing to intestinal mucosal barrier function.

METHODS

Enterocytes isolated from the jejunum of a healthy newborn pig were propagated to establish a stable cell line. The cells were cultured with 0.05 mmol glycine/L (control; concentration in the small intestinal lumen of low-birth-weight piglets) or 0.25 or 1.0 mmol glycine/L for the indicated periods of time. Epithelial barrier integrity and expression and localization of TJ proteins were analyzed by using monolayer transepithelial electrical resistance (TEER) and paracellular permeability, Western blot, and immunofluorescence imaging.

RESULTS

Compared with controls, cells cultured with 0.25 or 1.0 mmol glycine/L increased TEER (P < 0.05) by 46-53% and 80-111%, respectively, at 60-72 h. Correspondingly, paracellular permeability was reduced (P < 0.05) by 6-21% and 18-27% for 0.25 or 1.0 mmol glycine/L treatment, respectively, at 36-72 h. Compared with controls, protein abundances for claudin-3, claudin-7, and zonula occludens (ZO) 3 were enhanced (25-33%, P < 0.05) by 0.25 and 1.0 mmol glycine/L at 8 h, whereas those for occludin, claudin-1, claudin-4, and ZO-2 were not affected. Compared with controls, 1.0 mmol glycine/L reduced the protein abundance of ZO-1 by 20% at 8 h (P < 0.05), but 0.25 mmol glycine/L had no effect. A glycine concentration of 0.25 mmol/L sustained the localization of claudin-7 and ZO-3 to the interface between enterocytes. Interestingly, 1 mmol glycine/L promoted the distribution of claudin-4 and claudin-7 to the cytosol and nucleus, and the localization of ZO-3 to the plasma membranes, while decreasing the distribution of ZO-1 at cell-cell contact sites, compared with control cells.

CONCLUSION

Physiologic concentrations of glycine support intestinal mucosal barrier function by regulating the abundance and distribution of claudin-7 and ZO-3 in enterocytes. Supplementation with glycine may provide an effective nutritional strategy to improve intestinal integrity in piglets.

Division of labour: how does folate metabolism partition between one-carbon metabolism and amino acid oxidation?

One-carbon metabolism is usually represented as having three canonical functions: purine synthesis, thymidylate synthesis and methylation reactions. There is however a fourth major function: the metabolism of some amino acids (serine, glycine, tryptophan and histidine), as well as choline. These substrates can provide cells with more one-carbon groups than they need for these three canonical functions. Therefore, there must be mechanisms for the disposal of these one-carbon groups (when in excess) which maintain the complement of these groups required for the canonical functions. The key enzyme for these mechanisms is 10-formyl-THF (tetrahydrofolate) dehydrogenase (both mitochondrial and cytoplasmic isoforms) which oxidizes the formyl group to CO2 with the attendant reduction of NADP(+) to NADPH and release of THF. In addition to oxidizing the excess of these compounds, this process can reduce substantial quantities of NADP(+) to NADPH.

Effects of decreased availability of sulfur amino acids in severe childhood undernutrition

In studies of glutathione (GSH) metabolism in children with severe childhood undernutrition (SCU), slower erythrocyte GSH synthesis in children with edema was associated with lower concentrations of cysteine, the rate-limiting precursor of GSH synthesis. This finding suggested a shortage of cysteine available for GSH synthesis in children with edematous SCU. The plasma concentration of methionine, the sulfur donor for cysteine synthesis, was also lower in children with edematous SCU, suggesting decreased availability of methionine for cysteine synthesis. It is also possible that reduced methionine availability will result in decreased synthesis of S-adenosylmethionine, which could lead to an overall defect in methylation reactions. This review focuses on the relationship between cysteine availability and GSH synthesis in children with SCU. It also examines whether there is an inadequate supply of cysteine in those with edematous SCU and, if so, whether this is due to a shortage of methionine due to a decreased release of methionine from protein breakdown. Finally, the review explores whether a shortage of methionine results in decreased synthesis of S-adenosylmethionine, the universal methyl donor.

Measurement of hepatic protein fractional synthetic rate with stable isotope labeling technique in thapsigargin stressed HepG2 cells

Severe burn-induced liver damage and dysfunction is associated with endoplasmic reticulum (ER) stress. ER stress has been shown to regulate global protein synthesis. In the current study, we induced ER stress in vitro and estimated the effect of ER stress on hepatic protein synthesis. The aim was two-fold: (1) to establish an in vitro model to isotopically measure hepatic protein synthesis and (2) to evaluate protein fractional synthetic rate (FSR) in response to ER stress. Human hepatocellular carcinoma cells (HepG2) were cultured in medium supplemented with stable isotopes 1,2-¹³C₂-glycine and L-[ring-¹³C₆]phenylalanine. ER stress was induced by exposing the cells to 100 nM of thapsigargin (TG). Cell content was collected from day 0 to 14. Alterations in cytosolic calcium were measured by calcium imaging and ER stress markers were confirmed by Western blotting. The precursor and product enrichments were detected by GC-MS analysis for FSR calculation. We found that the hepatic protein FSR were 0.97±0.02 and 0.99±0.05%/hr calculated from 1,2-¹³C₂-glycine and L-[ring-¹³C₆]phenylalanine, respectively. TG depleted ER calcium stores and induced ER stress by upregulating p-IRE-1 and Bip. FSR dramatically decreased to 0.68±0.03 and 0.60±0.06%/hr in the TG treatment group (p<0.05, vs. control). TG-induced ER stress inhibited hepatic protein synthesis. The stable isotope tracer incorporation technique is a useful method for studying the effects of ER stress on hepatic protein synthesis.

Tyrosine requirement during the rapid catch-up growth phase of recovery from severe childhood undernutrition

The requirement for aromatic amino acids during the rapid catch-up in weight phase of recovery from severe childhood undernutrition (SCU) is not clearly established. As a first step, the present study aimed to estimate the tyrosine requirement of children with SCU during the catch-up growth phase of nutritional rehabilitation using a diet enriched in energy and proteins. Tyrosine requirement was calculated from the rate of excretion of 13CO2 (F 13CO2) during [13C]phenylalanine infusion in thirteen children with SCU, five females and eight males, at about 19 d after admission when the subjects were considered to have entered their rapid catch-up growth phase and were consuming 627·3 kJ and about 3·5 g protein/kg per d. Measurements of F 13CO2 during [13C]phenylalanine infusion were made on two separate days with a 1 d interval. Three measurements at tyrosine intakes of 48, 71 and 95 mg/kg per d were performed on experimental day 1 and measurements at tyrosine intakes of 148, 195 and 241 mg/kg per d were performed on experimental day 2. An estimate of the mean requirement was derived by breakpoint analysis with a two-phase linear regression cross-over model. The breakpoint, which represents an estimate of the mean tyrosine requirement, is a value of 99 mg/kg per d when the children were growing at about 15 g/kg per d. The result indicates that the mean requirement for tyrosine during the catch-up growth phase of SCU is about 99 mg/kg per d under similar conditions to the present study.

A weak link in metabolism: the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis

In a previous paper, we pointed out that the capability to synthesize glycine from serine is constrained by the stoichiometry of the glycine hydroxymethyltransferase reaction, which limits the amount of glycine produced to be no more than equimolar with the amount of C 1 units produced. This constraint predicts a shortage of available glycine if there are no adequate compensating processes. Here, we test this prediction by comparing all reported fl uxes for the production and consumption of glycine in a human adult. Detailed assessment of all possible sources of glycine shows that synthesis from serine accounts for more than 85% of the total, and that the amount of glycine available from synthesis, about 3 g/day, together with that available from the diet, in the range 1.5-3.0 g/day, may fall significantly short of the amount needed for all metabolic uses, including collagen synthesis by about 10 g per day for a 70 kg human. This result supports earlier suggestions in the literature that glycine is a semi-essential amino acid and that it should be taken as a nutritional supplement to guarantee a healthy metabolism.

Glycine and urea kinetics in nonalcoholic steatohepatitis in human: effect of intralipid infusion

The rates of oxidation of glycine and ureagenesis were quantified in the basal state and in response to an intravenous infusion of intralipid with heparin (IL) in healthy subjects (n = 8) and in subjects with nonalcoholic steatohepatitis (NASH) (n = 6). During fasting, no significant difference in weight-specific rate of appearance (R(a)) of glycine, glycine oxidation, and urea synthesis was observed. Intralipid infusion resulted in a significant increase in plasma beta-hydroxybutyrate in both groups. The correlation between free fatty acids and beta-hydroxybutyrate concentration in plasma was 0.94 in NASH compared with 0.4 in controls, indicating greater hepatic fatty acid oxidation in NASH. Intralipid infusion resulted in a significant decrease in urea synthesis and glycine R(a) in both groups and did not impact glycine oxidation. The fractional contribution of glycine carbon to serine was lower in subjects with NASH before and after IL infusion. In contrast, the fractional contribution of serine carbon to cystathionine was higher in NASH before and following IL infusion. These results suggest that hepatic fatty acid oxidation is higher in NASH compared with controls and that glycine oxidation and urea synthesis are not altered. An increase in oxidative stress, induced by a higher rate of fatty acid oxidation in NASH, may have caused an increase in the contribution of serine to cystathionine to meet the higher demands for glutathione.

Fed state protein turnover in healthy older persons under a usual protein-rich diet

The objective of this study was to verify the protein turnover rates of healthy older persons under a usual protein-rich diet and to compare values to those described in the literature. This cross-sectional study was conducted at Metabolism Unit, Univ. Hospital of the School of Medicine of Ribeirao Preto, Univ. of Sao Paulo, Brazil. In this study, 7 healthy older persons aged 65.4 +/- 2.8 y, with BMI 22.7 +/- 2.4 kg/m(2) and a mean daily protein intake of 1.34 g of protein/kg were studied. A 9-h whole-body (15)N-glycine single-dose study was performed after an overnight fast. During the study, each subject received 6 isoenergetic, isonitrogenous meals at 2-h intervals based on their average intake. Ammonium, urea, and total nitrogen were quantified and analyzed by mass spectrometry, with the determination of total protein turnover rates by the (15)N-glycine method. The results show that total nitrogen output was 3.2 +/- 0.96 g/N and intake 7.7 +/- 1 g/N, (15)N nitrogen flux was 30.6 +/- 6.3 g/9 h. Endogenous nitrogen balance was positive (4.5 g +/- 1 g/N in 9 h). In conclusion, the protein turnover of healthy older persons under a usual protein-rich diet is positive during the fed state and has synthesis and degradation rates similar to those previously described in studies involving diet adaptation periods.

Glucose kinetics and pregnancy outcome in Indian women with low and normal body mass indices

BACKGROUND/OBJECTIVES

Fetal energy demands are met from the oxidation of maternally supplied glucose and amino acids. During the fasted state, the glucose supply is thought to be met by gluconeogenesis. Underweight women with low body mass index (BMI) might be unable to adequately supply amino acids to satisfy the demands of gluconeogenesis.

SUBJECTS/METHODS

Glucose kinetics were measured during the first and second trimesters of pregnancy in 10 low-BMI and 10 normal-BMI pregnant women at the 12th hour of an overnight fast using a primed 6 h U-(13)C glucose infusion and was correlated to maternal dietary and anthropometric variables and birth weight.

RESULTS

Low-BMI mothers consumed more energy, carbohydrates and protein, had faster glucose production (R (a)) and oxidation rates in the first trimester. In the same trimester, dietary energy and carbohydrate correlated with glucose production, glycogenolysis and glucose oxidation in all women. Both groups had similar rates of gluconeogenesis in the first and second trimesters. Glucose R (a) in the second trimester was weakly correlated with the birth weight (r=0.4, P=0.07).

CONCLUSIONS

Maternal energy and carbohydrate intakes, not BMI, appear to influence glucose R (a) and oxidation in early and mid pregnancy.

Moderate dietary vitamin B-6 restriction raises plasma glycine and cystathionine concentrations while minimally affecting the rates of glycine turnover and glycine cleavage in healthy men and women

Glycine is a precursor of purines, protein, glutathione, and 1-carbon units as 5,10-methylenetetrahydrofolate. Glycine decarboxylation through the glycine cleavage system (GCS) and glycine-serine transformation by serine hydroxymethyltransferase (SHMT) require pyridoxal 5'-phosphate (PLP; active form of vitamin B-6) as a coenzyme. The intake of vitamin B-6 is frequently low in humans. Therefore, we determined the effects of vitamin B-6 restriction on whole-body glycine flux, the rate of glycine decarboxylation, glycine-to-serine conversion, use of glycine carbons in nucleoside synthesis, and other aspects of 1-carbon metabolism. We used a primed, constant infusion of [1,2-(13)C(2)]glycine and [5,5,5-(2)H(3)]leucine to quantify in vivo kinetics in healthy adults (7 males, 6 females; 20-39 y) of normal vitamin B-6 status or marginal vitamin B-6 deficiency. Vitamin B-6 restriction lowered the plasma PLP concentration from 55 +/- 4 nmol/L (mean +/- SEM) to 23 +/- 1 nmol/L (P < 0.0001), which is consistent with marginal deficiency, whereas the plasma glycine concentration increased (P < 0.01). SHMT-mediated conversion of glycine to serine increased from 182 +/- 7 to 205 +/- 9 micromol x kg(-1) x h(-1) (P < 0.05), but serine production using a GCS-derived 1-carbon unit (93 +/- 9 vs. 91 +/- 6 micromol x kg(-1) x h(-1)) and glycine cleavage (163 +/- 11 vs. 151 +/- 8 micromol x kg(-1) x h(-1)) were not changed by vitamin B-6 restriction. The GCS produced 1-carbon units at a rate (approximately 140-170 micromol x kg(-1) x h(-1)) that greatly exceeds the demand for remethylation and transmethylation processes (approximately 4-7 micromol x kg(-1) x h(-1)). We conclude that the in vivo GCS and SHMT reactions are quite resilient to the effects of marginal vitamin B-6 deficiency, presumably through a compensatory effect of increasing substrate concentration.

Arginine flux and intravascular nitric oxide synthesis in severe childhood undernutrition

BACKGROUND

Although nutritionally dispensable amino acids are not essential in the diet, adequate synthesis is necessary for maintenance of good health. Whereas children with edematous severe childhood undernutrition (SCU) can maintain production rates of glycine and serine despite a slower body protein breakdown rate, it is unknown whether the same is true for the semidispensable amino acid arginine.

OBJECTIVE

We aimed to measure arginine flux and intravascular nitric oxide synthesis in children with SCU.

DESIGN

Arginine flux and the fractional and absolute synthesis rates of plasma nitrite plus nitrate were measured postabsorptively by using a 6-h infusion of [(15)N(2)]-arginine in 2 groups of children with edematous (n = 14) or nonedematous (n = 7) SCU when they were infected and malnourished (postadmission day approximately 3; clinical phase 1), when they were no longer infected (postadmission day approximately 15; clinical phase 2), and when they were recovered (postadmission day approximately 55; clinical phase 3).

RESULTS

Arginine flux was slower (P < 0.01) and plasma arginine concentrations were lower in the edematous group than in the nonedematous group at clinical phase 1. At clinical phase 2, flux doubled to a value that was not significantly different from the value at clinical phase 3. There were no significant differences in the plasma concentration or fractional or absolute synthesis rate of plasma nitrite plus nitrate between the groups at any clinical phase and among clinical phases within each group.

CONCLUSION

Whereas children with nonedematous SCU can maintain arginine flux at the same rate as when recovered, children with edematous SCU cannot. The slower arginine flux was not, however, associated with slower nitric oxide synthesis.

Association between glutamine extraction and release of citrulline and glycine by the human small intestine

Although glutamine is an important fuel for the intestinal epithelium, the metabolic fate of glutamine extracted by the human intestine remains unclear. The aim of this study was to investigate the relationship between glutamine extraction and the release of other amino acids by the human intestine. In 21 patients undergoing major abdominal cancer surgery, differences in the plasma concentrations of 22 amino acids including glutamine across the superior or inferior mesenteric vein draining viscera were measured using a high-performance liquid chromatography. Arterial minus venous (A-V) or venous minus arterial (V-A) balances of the amino acids were calculated, and then the correlations between A-V differences of glutamine and V-A differences of amino acids released from the intestine were analyzed. Mean extraction rate of glutamine by the small intestine was 28.45%, approximately 3 times higher than 9.41% in the distal colon. Citrulline, proline, alanine, glycine, and arginine were released by the small intestine into the portal circulation. Positive correlations were found between glutamine uptake and the production of citrulline (r=0.814, P=0.0013) and glycine (r=0.734, P=0.0080). In conclusion, the synthesis of citrulline from glutamine by the small intestine is highly suspected, and the contribution of gut glutamine extraction to the release of glycine into the portal circulation is also supposed.

Glycine production in severe childhood undernutrition

BACKGROUND

Although nutritionally dispensable amino acids are not essential in the diet, from a biochemical standpoint, dispensable amino acids such as glycine are essential for life. This is especially true under unique circumstances, such as when the availability of labile nitrogen for dispensable amino acid synthesis is reduced, as in severe childhood undernutrition.

OBJECTIVE

We aimed to measure glycine production in children with edematous and nonedematous severe childhood undernutrition.

DESIGN

Glycine flux and splanchnic glycine extraction were measured in 2 groups of children with edematous (n = 8) and nonedematous (n = 9) severe childhood undernutrition when they were infected and malnourished (clinical phase 1), when they were still severely malnourished but no longer infected (clinical phase 2), and when they were recovered (clinical phase 3).

RESULTS

Total and endogenous glycine flux and splanchnic glycine uptake did not differ significantly between the edematous and nonedematous groups during any clinical phase. In both groups of subjects, none of the glycine kinetic parameters changed significantly from clinical phase 1 through phases 2 and 3. Compared with the value at clinical phase 3, plasma glycine concentrations were not significantly lower during clinical phase 1 or 2 in either group.

CONCLUSIONS

These findings suggest that children with severe childhood undernutrition can increase their de novo glycine synthesis to compensate for the reduced contribution from chronic food deprivation. The maintenance of the plasma glycine concentration suggests that the rate of glycine production was sufficient to satisfy metabolic demands in these children when they were acutely undernourished and infected.

Responses of whole body protein synthesis, nitrogen retention and glucose kinetics to supplemental starch in goats

An isotope dilution experiment was conducted to determine the effect of metabolizable energy intake (MEI) as starch on whole body protein synthesis (WBPS), nitrogen (N) retention and glucose irreversible loss rate (ILR) in four adult goats (Capra hircus). The goats were fed isonitrogenous diets containing three different metabolizable energy (1.0, 1.5 and 2.0 times maintenance) twice daily. Energy above maintenance was supplemented with cornstarch. The WBPS and glucose ILR during 5 to 7 h after feeding were measured by a primed-continuous infusion of [2H5]phenylalanine, [2H2]tyrosine, [2H4]tyrosine and [13C6]glucose for 4 h, with measurements of plasma concentrations of metabolites and insulin. Ruminal characteristics were also determined. Increasing MEI improved N retention, despite decreased digestible N. Increasing MEI decreased ruminal pH and ammonia nitrogen. In plasma, decreased urea N, increased total amino N and tyrosine, and trends for increases in phenylalanine and insulin resulted from increasing MEI. Increasing MEI increased ILR of glucose, phenylalanine and tyrosine, and hydroxylation rate of phenylalanine and WBPS. We conclude that in goats increasing MEI as starch enhances WBPS in the absorptive state and N retention, despite a decrease in digestible N. These changes are probably associated with both decreased ammonia absorption and increased amino acid absorption.

Protein kinetic differences between children with edematous and nonedematous severe childhood undernutrition in the fed and postabsorptive states

BACKGROUND

Pathogenic factors that cause a child to develop the edematous instead of the nonedematous form of severe childhood undernutrition (SCU) during food deprivation are not clear. It was hypothesized that, in edematous but not nonedematous SCU, impaired protein breakdown leading to inadequate amino acids for maintenance of important organ systems was a factor.

OBJECTIVE

We measured protein kinetics in children with edematous and nonedematous SCU.

DESIGN

Endogenous leucine flux, an index of whole-body protein breakdown rate, was determined in 4 groups of children with edematous or nonedematous SCU in the malnourished and recovered states. Two groups were studied in the postabsorptive state, and 2 groups were studied in the fed state.

RESULTS

In the postabsorptive state, leucine flux was slower (P < 0.01) in the edematous group than in the nonedematous group in the malnourished state, but in the recovered state, it was faster (P < 0.05) in the children who previously had edematous SCU. When compared with the malnourished state value, leucine flux at recovery doubled in the group that previously had edematous SCU, but it did not change in the other group. In the fed state, leucine flux was slower (P < 0.01) in the edematous group than in the nonedematous group in the malnourished state but not in the recovered state. In the recovered state, enteral leucine extraction by splanchnic tissues trended higher in the group that previously had edematous SCU than in the nonedematous group.

CONCLUSION

These findings indicate different protein breakdown responses to food deprivation between children with edematous and nonedematous SCU and inherent differences in protein metabolism when they have recovered.

Effect of cystine on the methionine requirement of healthy Indian men determined by using the 24-h indicator amino acid balance approach

BACKGROUND

The 1985 FAO/WHO/UNU requirement for methionine in healthy adults consuming a cystine-free diet is 13 mg.kg(-1).d(-1). It is unclear whether this daily requirement is influenced by dietary cystine.

OBJECTIVE

We assessed the effect of 2 intakes of cystine (5 and 12 mg.kg(-1).d(-1)) on methionine requirements in well-nourished Indian men by using 7 test methionine intakes (3, 6, 9, 13, 18, 21 and 24 mg.kg(-1).d(-1)) and the 24-h indicator amino acid oxidation (24-h IAAO) and balance (24-h IAAB) methods. We combined these data with those from an experiment with zero cystine intake and in which the exact same method was used.

DESIGN

Two studies were performed in which a diet containing either 5 or 12 mg cystine.kg(-1).d(-1) was fed to 21 well-nourished Indian men over three 7-d periods. The 24-h IAAO and 24-h IAAB values were measured on day 7 with the use of a 24-h intravenous [13C]leucine tracer infusion. The breakpoints in the relation between these values and methionine intake in each study were assessed by two-phase linear regression.

RESULTS

Breakpoints in the response curve were obtained at methionine intakes of 20 (95% Fiellers CI: 17, 26) and 10 (95% Fiellers CI: 8, 16) mg.kg(-1).d(-1) with cystine intakes of 5 and 12 mg.kg(-1).d(-1) intakes, respectively, which suggested a sparing effect of cystine. Although the 5- and 12-mg cystine breakpoints differed from one another, they did not differ significantly from that estimated previously with 0 mg cystine.

CONCLUSION

Cystine may spare the methionine requirement in healthy men, although the amount of sparing is difficult to quantify.

Supplementation with aromatic amino acids improves leucine kinetics but not aromatic amino acid kinetics in infants with infection, severe malnutrition, and edema

We investigated whether supplementation with an aromatic amino acid (AAA) cocktail consisting of 0.5 mmol each of phenylalanine, tryptophan, and tyrosine compared with isonitrogenous amounts of alanine (Ala) would improve measures of protein kinetics in 14 (8 with AAA, 6 Ala) children with edematous malnutrition (aged 6-24 mo) during the infected acute malnourished state. Supplementation started immediately after the baseline experiment, 2 d postadmission and continued to the end of the acute phase of treatment. The second (postsupplementation) experiment was done approximately 12 d postadmission. We measured leucine kinetics, phenylalanine and tyrosine fluxes, using an i.g. 8-h prime continuous infusion of (2)H(3)-leucine, and an i.v. 6-h prime continuous infusion of (13)C-leucine, (2)H(2)-tyrosine, and (2)H(5)-phenylalanine in the fed state. Leucine flux tended to be faster (P = 0.06) in the AAA group compared with Ala group after supplementation (mean difference +/- SEM): 22.6 +/- 10.9 micromol/(kg . h). The rate of leucine appearance from protein breakdown [28.1 +/- 9.4 micromol/(kg . h)] and the nonoxidative disposal of leucine [i.e., leucine to protein synthesis; 35.4 +/- 12.9 micromol/(kg . h)] were faster (P < 0.02) in the AAA group than in the Ala group. There was no significant effect of supplementation on leucine splanchnic metabolism, phenylalanine, and tyrosine fluxes. These findings are consistent with the hypothesis that the blunting of the protein catabolic response to infection in children with edematous malnutrition syndrome is due to limited availability of aromatic amino acids.

Response of hepatic proteins to the lowering of habitual dietary protein to the recommended safe level of intake

The plasma concentrations of albumin, HDL apolipoprotein A1 (apoA1), retinol-binding protein (RBP), transthyretin (TTR), haptoglobulin, and fibrinogen were measured, and a stable isotope infusion protocol was used to determine the fractional and absolute synthesis rates of RBP, TTR, and fibrinogen in 12 young adults on three occasions during a reduction of their habitual protein intake from 1.13 to 0.75 g x kg(-1) x day(-1) for 10 days. This study was performed to determine whether healthy adults could maintain the rates of synthesis of selected nutrient transport and positive acute-phase proteins when consuming a protein intake of 0.75 g x kg(-1) x day(-1). During the lower protein intake, the plasma concentration of all the proteins, other than HDL-apoA1, remained unchanged. HDL-apoA1 concentration was significantly reduced (P < 0.05) after 3 days of the lower protein intake, but not at 10 days. The rates of synthesis of RBP and TTR declined significantly (P < 0.05), whereas the rate of synthesis of fibrinogen remained unchanged. The results indicate that, when normal adults consume the recommended safe level of protein, 0.75 g x kg(-1) x day(-1), there is a slower rate of turnover of nutrient transport proteins than on their habitual diet. Hence, healthy individuals consuming this amount of protein may be less able to mount an adequate metabolic response to a stressful stimulus.

Synthesis of erythrocyte glutathione in healthy adults consuming the safe amount of dietary protein

BACKGROUND

The finding that plasma glutathione turnover decreases as dietary protein intake decreases suggests that the safe amount of dietary protein, although sufficient for maintenance of nitrogen balance, may be insufficient for maintenance of cellular glutathione.

OBJECTIVE

Our objective was to determine the effect of the safe protein intake on the erythrocyte glutathione synthesis rate and its relation with urinary 5-L-oxoproline excretion.

DESIGN

Erythrocyte glutathione synthesis and urinary 5-L-oxoproline excretion were measured in young adults (6 men and 6 women) by using an infusion of [(13)C(2)]glycine on 3 occasions: initially during the subjects' habitual protein intake (1.13 g.kg(-1).d(-1)) and on days 3 and 10 of consumption of a diet providing the safe protein intake (0.75 g.kg(-1).d(-1)).

RESULTS

Compared with baseline values, the fractional synthesis rate of erythrocyte glutathione was significantly lower (P < 0.05) on days 3 and 10 of the diet with the safe protein intake. Urinary 5-L-oxoproline excretion increased significantly (P < 0.05) above baseline by the third day of the diet with the safe protein intake and remained elevated. Erythrocyte glutathione concentrations and absolute synthesis rates decreased by day 3 but recovered to baseline values by day 10. Erythrocyte concentrations of cysteine, methionine, and serine remained unchanged, whereas erythrocyte concentrations of glycine, glutamic acid, and glutamine increased significantly by day 10.

CONCLUSION

During adaptation to the safe amount of dietary protein, there are changes in the concentration and kinetics of erythrocyte glutathione that suggest a reduced antioxidant capacity and possible increased susceptibility to oxidant stress.