Muscle protein synthesis and balance responsiveness to essential amino acids ingestion in the presence of elevated plasma free fatty acid concentrations

Association of HindIII Polymorphism of the Lipoprotein Lipase (LPL) Gene (rs320) and Plasma Metabolic Parameters in a Nigerian Population

Genetic variations in the lipoprotein lipase (LPL) gene including the HindIII polymorphism (rs320) have been reported to modify fat metabolism, adiposity, and body weight. However, little attention has been given to the African population. The present study aimed to investigate the relationship between the rs320 gene polymorphism and a number of metabolic and anthropometric parameters in a sample of the Nigerian population. We recruited 236 participants for the study. The participants were required to sign informed consent forms after which information related to their calorie intake and utilization as well as anthropometric measurements were recorded. Plasma metabolic parameters were subsequently determined using an autoanalyzer. Genotyping for HindIII polymorphism was performed using the PCR-RFLP method. The frequencies (n) of T and G alleles were 0.841 (397) and 0.158 (75), while the frequencies (n) of TT, TG, and GG were 0.691(163), 0.301(71), and 0.01(2), respectively. The population was not in Hardy-Weinberg equilibrium (χ2 = 3.717, df = 1, p = 0.841). The anthropometric parameters, the fasting blood glucose, and low-density lipoprotein cholesterol showed no association with the alleles, while plasma high-density lipoprotein cholesterol and total cholesterol were significantly higher among the G allele carriers. However, triglyceride and total protein were significantly higher among the non-G allele carriers. The LPL HindIII gene polymorphism is associated with changes in plasma lipid profile in a sample of the Nigerian population.

Consideration of the role of protein quality in determining dietary protein recommendations

The quality of a dietary protein refers to its ability to provide the EAAs necessary to meet dietary requirements. There are 9 dietary amino acids that cannot be metabolically produced in the body and therefore must be consumed as part of the diet to avoid adverse metabolic consequences. These essential amino acids (EAAs) serve a variety of roles in the body. The amount and profile of the dietary EAAs relative to the individual EAA requirements and the digestibility of the dietary protein are the key factors that determine its quality. Currently the Digestible Indispensable Amino Acid Score (DIAAS) is the best available approach to quantifying protein quality. The most prominent metabolic role of dietary EAAs is to stimulate protein synthesis by serving as signals to activate molecular mechanisms responsible for the initiation of protein synthesis and, most importantly, to provide the necessary precursors for the synthesis of complete proteins. Current dietary recommendations generally do not consider protein quality. Accounting for protein quality in dietary patterns can be accomplished while staying within established ranges for dietary protein consumption. Poor protein quality can be compensated for to some extent by eating more low-quality protein, but to be effective ("complementary") the limiting EAA must differ between the low-quality protein and the base diet to which it is being supplemented. Adding a high-quality protein to a dietary pattern based on low-quality protein is more effective in meeting EAA goals than increasing the amount of low-quality protein, even if the low-quality proteins are complementary. Further, reliance entirely on low-quality protein food sources, particularly in circumstances that may benefit from a level of dietary EAAs greater than minimal requirements, is likely to include excessive caloric consumption. While protein consumption in high-income nations is generally perceived to be adequate or even excessive, assessment of dietary patterns indicates that a significant percentage of individuals may fall short of meeting optimal levels of EAA consumption, especially in circumstances such as aging in which the optimal EAA consumption is greater than basal values for healthy young individuals. The case is made that protein quality is an important consideration in meeting EAA requirements.

The anabolic response to a ground beef patty and soy-based meat alternative: a randomized controlled trial

BACKGROUND

Soy-based meat alternatives (SBMA) are becoming increasingly popular, but it is unclear if they have the same anabolic effect on skeletal muscle as animal meat.

OBJECTIVES

We aimed to compare the stimulation of skeletal muscle protein synthesis by consumption of 1 or two 4 oz patties of SBMA with 4 oz (80% protein/20% fat) beef.

METHODS

The study design was a randomized controlled trial. Participants were aged 18-40 y of age and in good general health with a body mass index (kg/m2) between 20 and 32. Stable isotope tracer methods were used (L-[ring-2H5] phenylalanine, [U-13C9-15N]- tyrosine, and L-[ring-2H4] tyrosine) to quantify the response of muscle protein fractional synthetic rate (FSR) to consumption of a single beef (4 oz), single SBMA (4 oz), or two 4 oz SBMA patties (8 oz). Whole-body rates of protein synthesis, breakdown, and net balance, as well as plasma essential amino acid concentrations, were also measured.

RESULTS

The increase above basal in muscle protein FSR following consumption of the 4 oz beef patty (0.020 ± 0.016%/h) was significantly greater than the increase following consumption of 4 oz SBMA (P = 0.021; 0.003 ± 0.010%/h) but not 8 oz SBMA (P = 0.454; 0.013 ± 0.016%/h). The maximal essential amino acid concentration was significantly correlated (P = 0.046; r = 0.411) with the change in muscle FSR from the basal to the postprandial period. In addition, the change in muscle FSR from the basal to postprandial period was significantly correlated (P = 0.046; r = 0.412) with the corresponding change in whole-body protein synthesis.

CONCLUSIONS

Consumption of a 4 oz beef patty stimulates muscle and whole-body protein synthesis >4 oz SBMA patty and similarly to 8 oz of SBMA. This trial was registered at clinicaltrials.gov as NCT05197140.

The impact of forearm immobilization and acipimox administration on muscle amino acid metabolism and insulin sensitivity in healthy, young volunteers

Although the mechanisms underpinning short-term muscle disuse atrophy and associated insulin resistance remain to be elucidated, perturbed lipid metabolism might be involved. Our aim was to determine the impact of acipimox administration [i.e., pharmacologically lowering circulating nonesterified fatty acid (NEFA) availability] on muscle amino acid metabolism and insulin sensitivity during short-term disuse. Eighteen healthy individuals (age: 22 ± 1 years; body mass index: 24.0 ± 0.6 kg·m-2) underwent 2 days forearm immobilization with placebo (PLA; n = 9) or acipimox (ACI; 250 mg Olbetam; n = 9) ingestion four times daily. Before and after immobilization, whole body glucose disposal rate (GDR), forearm glucose uptake (FGU; i.e., muscle insulin sensitivity), and amino acid kinetics were measured under fasting and hyperinsulinemic-hyperaminoacidemic-euglycemic clamp conditions using forearm balance and l-[ring-2H5]-phenylalanine infusions. Immobilization did not affect GDR but decreased insulin-stimulated FGU in both groups, more so in ACI (from 53 ± 8 to 12 ± 5 µmol·min-1) than PLA (from 52 ± 8 to 38 ± 13 µmol·min-1; P < 0.05). In ACI only, and in contrast to our hypothesis, fasting arterialized NEFA concentrations were elevated to 1.3 ± 0.1 mmol·L-1 postimmobilization (P < 0.05), and fasting forearm NEFA balance increased approximately fourfold (P = 0.10). Forearm phenylalanine net balance decreased following immobilization (P < 0.10), driven by an increased rate of appearance [from 32 ± 5 (fasting) and 21 ± 4 (clamp) preimmobilization to 53 ± 8 and 31 ± 4 postimmobilization; P < 0.05] while the rate of disappearance was unaffected by disuse or acipimox. Disuse-induced insulin resistance is accompanied by early signs of negative net muscle amino acid balance, which is driven by accelerated muscle amino acid efflux. Acutely elevated NEFA availability worsened muscle insulin resistance without affecting amino acid kinetics, suggesting increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not cause anabolic resistance.NEW & NOTEWORTHY We demonstrate that 2 days of forearm cast immobilization in healthy young volunteers leads to the rapid development of insulin resistance, which is accompanied by accelerated muscle amino acid efflux in the absence of impaired muscle amino acid uptake. Acutely elevated fasting nonesterified fatty acid (NEFA) availability as a result of acipimox supplementation worsened muscle insulin resistance without affecting amino acid kinetics, suggesting increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not cause anabolic resistance.

The impact of short-term forearm immobilization and acipimox administration on muscle amino acid metabolism and insulin sensitivity in healthy, young volunteers

The mechanisms underpinning short-term muscle disuse atrophy remain to be elucidated, but perturbations in lipid metabolism may be involved. Specifically, positive muscle non-esterified fatty acid (NEFA) balance has been implicated in the development of disuse-induced insulin and anabolic resistance. Our aim was to determine the impact of acipimox administration (i.e. pharmacologically lowering circulating NEFA availability) on muscle amino acid metabolism and insulin sensitivity during short-term disuse. Eighteen healthy individuals (age 22±1 years, BMI 24.0±0.6 kg·m-2) underwent 2 days of forearm cast immobilization with placebo (PLA; n=9, 5M/4F) or acipimox (ACI; 250 mg Olbetam; n=9, 4M/5F) ingestion four times daily. Before and after immobilization, whole-body glucose disposal rate (GDR), forearm glucose uptake (FGU, i.e. muscle insulin sensitivity), and amino acid kinetics were measured under fasting and hyperinsulinaemic-hyperaminoacidaemic-euglycaemic clamp conditions using arteriovenous forearm balance and intravenous L-[ring-2H5]phenylalanine infusions. Immobilization did not affect GDR but decreased insulin-stimulated FGU in both groups, but to a greater degree in ACI (from 53±8 to 12±5 μmol·min-1) than in PLA (from 52±8 to 38±13 μmol·min-1; P<0.05). In ACI only, fasting arterialised NEFA concentrations were elevated to 1.3±0.1 mmol·L-1 post-immobilization (P<0.05), and fasting forearm NEFA balance increased ~4-fold (P=0.10). Forearm phenylalanine net balance tended to decrease following immobilization (P<0.10), driven by increases in phenylalanine rates of appearance (from 32±5 (fasting) and 21±4 (clamp) pre-immobilization to 53±8 and 31±4 post-immobilization; P<0.05) while rates of disappearance were unaffected and no effects of acipimox observed. Altogether, we show disuse-induced insulin resistance is accompanied by early signs of negative net muscle amino acid balance, which is driven by accelerated muscle amino acid efflux. Acutely elevated NEFA availability worsened muscle insulin resistance without affecting muscle amino acid kinetics, suggesting that disuse-associated increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not represent an early mechanism causing anabolic resistance.

(Dys)regulation of Protein Metabolism in Skeletal Muscle of Humans With Obesity

Studies investigating the proteome of skeletal muscle present clear evidence that protein metabolism is altered in muscle of humans with obesity. Moreover, muscle quality (i.e., strength per unit of muscle mass) appears lower in humans with obesity. However, relevant evidence to date describing the protein turnover, a process that determines content and quality of protein, in muscle of humans with obesity is quite inconsistent. This is due, at least in part, to heterogeneity in protein turnover in skeletal muscle of humans with obesity. Although not always evident at the mixed-muscle protein level, the rate of synthesis is generally lower in myofibrillar and mitochondrial proteins in muscle of humans with obesity. Moreover, alterations in the synthesis of protein in muscle of humans with obesity are manifested more readily under conditions that stimulate protein synthesis in muscle, including the fed state, increased plasma amino acid availability to muscle, and exercise. Current evidence supports various biological mechanisms explaining impairments in protein synthesis in muscle of humans with obesity, but this evidence is rather limited and needs to be reproduced under more defined experimental conditions. Expanding our current knowledge with direct measurements of protein breakdown in muscle, and more importantly of protein turnover on a protein by protein basis, will enhance our understanding of how obesity modifies the proteome (content and quality) in muscle of humans with obesity.

Impaired skeletal muscle hypertrophy signaling and amino acid deprivation response in Apoe knockout mice with an unhealthy lipoprotein distribution

This study explores if unhealthy lipoprotein distribution (LPD) impairs the anabolic and amino acid sensing responses to whey-protein feeding. Thus, if impairment of such anabolic response to protein consumption is seen by the LPD this may negatively affect the skeletal muscle mass. Muscle protein synthesis (MPS) was measured by puromycin labeling in Apolipoprotein E knockout (Apoe KO), characterized by an unhealthy LPD, and wild type mice post-absorptive at 10 and 20 weeks, and post-prandial after whey-protein feeding at 20 weeks. Hypertrophy signaling and amino acid sensing mechanisms were studied and gut microbiome diversity explored. Surprisingly, whey-protein feeding did not affect MPS. p-mTOR and p-4E-BP1 was increased 2 h after whey-protein feeding in both genotypes, but with general lower levels in Apoe KO compared to wild type. At 20 weeks of age, Apoe KO had a greater mRNA-expression for SNAT2, CD98, ATF4 and GCN2 compared to wild type. These responses were not associated with gut microbiota compositional differences. Regardless of LPD status, MPS was similar in Apoe KO and wild type. Surprisingly, whey-protein did not stimulate MPS. However, Apoe KO had lower levels of hypertrophy signaling, was amino acid deprived, and had impaired amino acid sensing mechanisms.

Pre- and Post-Surgical Nutrition for Preservation of Muscle Mass, Strength, and Functionality Following Orthopedic Surgery

Nutritional status is a strong predictor of postoperative outcomes and is recognized as an important component of surgical recovery programs. Adequate nutritional consumption is essential for addressing the surgical stress response and mitigating the loss of muscle mass, strength, and functionality. Especially in older patients, inadequate protein can lead to significant muscle atrophy, leading to a loss of independence and increased mortality risk. Current nutritional recommendations for surgery primarily focus on screening and prevention of malnutrition, pre-surgical fasting protocols, and combating post-surgical insulin resistance, while recommendations regarding macronutrient composition and timing around surgery are less established. The goal of this review is to highlight oral nutrition strategies that can be implemented leading up to and following major surgery to minimize atrophy and the resultant loss of functionality. The role of carbohydrate and especially protein/essential amino acids in combating the surgical stress cascade and supporting recovery are discussed. Practical considerations for nutrient timing to maximize oral nutritional intake, especially during the immediate pre- and post- surgical periods, are also be discussed.

The anabolic role of the Warburg, Cori-cycle and Crabtree effects in health and disease

In evolution, genes survived that could code for metabolic pathways, promoting long term survival during famines or fasting when suffering from trauma, disease or during physiological growth. This requires utilization of substrates, already present in some form in the body. Carbohydrate stores are limited and to survive long, their utilization is restricted to survival pathways, by inhibiting glucose oxidation and glycogen synthesis. This leads to insulin resistance and spares muscle protein, because being the main supplier of carbon for new glucose production. In these survival pathways, part of the glucose is degraded in glycolysis in peripheral (muscle) tissues to pyruvate and lactate (Warburg effect), which are partly reutilized for glucose formation in liver and kidney, completing the Cori-cycle. Another part of the glucose taken up by muscle contributes, together with muscle derived amino acids, to the production of substrates consisting of a complete amino acid mix but extra non-essential amino acids like glutamine, alanine, glycine and proline. These support cell proliferation, matrix deposition and redox regulation in tissues, specifically active in host response and during growth. In these tissues, also glucose is taken up delivering glycolytic intermediates, that branch off and act as building blocks and produce reducing equivalents. Lactate is also produced and released in the circulation, adding to the lactate released by muscle in the Cori-cycle and completing secondary glucose cycles. Increased fluxes through these cycles lead to modest hyperglycemia and hyperlactatemia in states of healthy growth and disease and are often misinterpreted as induced by hypoxia.

Effect of acute and short-term dietary fat ingestion on postprandial skeletal muscle protein synthesis rates in middle-aged, overweight, and obese men

Muscle anabolic resistance to dietary protein is associated with obesity and insulin resistance. However, the contribution of excess consumption of fat to anabolic resistance is not well studied. The aim of these studies was to test the hypothesis that acute and short-term dietary fat overload will impair the skeletal muscle protein synthetic response to dietary protein ingestion. Eight overweight/obese men [46.4 ± 1.4 yr, body mass index (BMI) 32.3 ± 5.4 kg/m²] participated in the acute feeding study, which consisted of two randomized crossover trials. On each occasion, subjects ingested an oral meal (with and without fat emulsion), 4 h before the coingestion of milk protein, intrinsically labeled with [1-¹³C]phenylalanine, and dextrose. Nine overweight/obese men (44.0 ± 1.7 yr, BMI 30.1 ± 1.1 kg/m²) participated in the chronic study, which consisted of a baseline, 1-wk isocaloric diet, followed by a 2-wk high-fat diet (+25% energy excess). Acutely, incorporation of dietary amino acids into the skeletal muscle was twofold higher (P < 0.05) in the lipid trial compared with control. There was no effect of prior lipid ingestion on indices of insulin sensitivity (muscle glucose uptake, pyruvate dehydrogenase complex activity, and Akt phosphorylation) in response to the protein/dextrose drink. Fat overfeeding had no effect on muscle protein synthesis or glucose disposal in response to whey protein ingestion, despite increased muscle diacylglycerol C16:0 (P = 0.06) and ceramide C16:0 (P < 0.01) levels. Neither acute nor short-term dietary fat overload has a detrimental effect on the skeletal muscle protein synthetic response to dietary protein ingestion in overweight/obese men, suggesting that dietary-induced accumulation of intramuscular lipids per se is not associated with anabolic resistance.

Consumption of a Specially-Formulated Mixture of Essential Amino Acids Promotes Gain in Whole-Body Protein to a Greater Extent than a Complete Meal Replacement in Older Women with Heart Failure

Heart failure in older individuals is normally associated with a high body mass index and relatively low lean body mass due to, in part, a resistance to the normal anabolic effect of dietary protein. In this study we have investigated the hypothesis that consumption of a specially-formulated composition of essential amino acids (HiEAAs) can overcome anabolic resistance in individuals with heart failure and stimulate the net gain of body protein to a greater extent than a commercially popular protein-based meal replacement beverage with greater caloric but lower essential amino acid (EAA) content (LoEAA). A randomized cross-over design was used. Protein kinetics were determined using primed continuous infusions of L-(²H₅)phenylalanine and L-(²H₂)tyrosine in the basal state and for four hours following consumption of either beverage. Both beverages induced positive net protein balance (i.e., anabolic response). However, the anabolic response was more than two times greater with the HiEAA than the LoEAA (p < 0.001), largely through a greater suppression of protein breakdown (p < 0.001). Net protein accretion (g) was also greater in the HiEAA when data were normalized for either amino acid or caloric content (p < 0.001). We conclude that a properly formulated EAA mixture can elicit a greater anabolic response in individuals with heart failure than a protein-based meal replacement. Since heart failure is often associated with obesity, the minimal caloric value of the HiEAA formulation is advantageous.

Malnutrition and sarcopenia

Risk for or established malnutrition is frequent in older adults, accompanied by functional limitations, increased morbidity and mortality. Protein-energy malnutrition is often observed and leads besides other predisposing factors to sarcopenia, the increased loss of muscle mass with aging. Sarcopenia is an integral correlate of the physical component of the frailty syndrome. Even though sarcopenia often reaches levels where mobility, balance and functionality on overall are hampered, its diagnosis has not become part of the standard diagnostic and therapeutic repertoire of geriatric medicine. This will hopefully change with a recently published revised international definition of sarcopenia, as well an own ICD-number. From a pathophysiological point of view, both malnutrition and sarcopenia share many components, a low-inflammatory state (inflamm-aging) being an important one. Nutritional interventions with and without parallel physical activity programs can prevent and often also reverse sarcopenia. It is hoped that upcoming even more potent nutritional treatment options-including for sarcopenic obesity-will lower the burden of malnutrition and sarcopenia for many older adults.

A single bout of strenuous exercise overcomes lipid-induced anabolic resistance to protein ingestion in overweight, middle-aged men

High-circulating lipid availability attenuates protein feeding-induced muscle protein synthesis (MPS). Whether the combined effects of exercise and protein ingestion can rescue this inhibition is unknown. In a parallel-groups design, middle-aged sedentary males (n = 28) matched for fat-free mass and body mass index received a 5-h intravenous infusion of either saline/control (n = 9), 20% intralipid infusion (n = 9), or intralipid with concomitant exercise (n = 10). Two hours into each of these infusions, participants received a primed constant infusion of L-(ring-[¹³C]₆)-phenylalanine. Muscle biopsies were taken immediately after control and lipid infusions, at which time, a 30-g protein beverage was ingested. Further biopsies were taken 2 and 4 h after protein ingestion. Intralipid increased plasma free fatty acid concentrations from ∼0.4-2 mM, resulting in an attenuated MPS response to protein ingestion, which was prevented by exercise. Intralipid resulted in a lower peak aminoacidemia following protein ingestion that was exacerbated by prior exercise, suggesting efficiency of the working skeletal muscle to utilize amino acid substrate to drive the postprandial anabolic response. We conclude that in the face of high-fat availability, exercise preserves the sensitivity of skeletal muscle to the anabolic properties of amino acids.-Smiles, W. J., Churchward-Venne, T. A., van Loon, L. J. C., Hawley, J. A., Camera, D. M. A single bout of strenuous exercise overcomes lipid-induced anabolic resistance to protein ingestion in overweight, middle-aged men.

An Update on Protein, Leucine, Omega-3 Fatty Acids, and Vitamin D in the Prevention and Treatment of Sarcopenia and Functional Decline

Aging is associated with sarcopenia and functional decline, leading to frailty and disability. As a modifiable risk factor, nutrition may represent a target for preventing or postponing the onset of these geriatric conditions. Among nutrients, high-quality protein, leucine, vitamin D, and omega-3 polyunsaturated fatty acids (n-3 PUFA) are of particular interest for their demonstrated effects on skeletal muscle health. This narrative review aims to examine the recent observational and interventional evidence on the associations and the role of these nutrients in the muscle mass, strength, mobility, and physical function of free-living older adults, who are either healthy or at risk of frailty. Recent evidence supports a higher protein intake recommendation of 1.0⁻1.2 g/kg/day in healthy older adults; an evenly distributed mealtime protein intake or minimal protein per meal may be beneficial. In addition, vitamin D supplementation of 800⁻1000 IU, particularly when vitamin D status is low, and doses of ~3 g/day of n-3 PUFA may be favorable for physical function, muscle mass, and strength. Reviewed studies are highly heterogenous, yet the quantity, quality, and timing of intakes should be considered when designing intervention studies. Combined protein, leucine, vitamin D, and n-3 PUFA supplements may convey added benefits and may represent an intervention strategy in the prevention of sarcopenia and functional decline.

Protein-Rich Food Ingestion Stimulates Mitochondrial Protein Synthesis in Sedentary Young Adults of Different BMIs

CONTEXT

Excess fat mass may diminish the anabolic potency of protein-rich food ingestion to stimulate muscle protein subfractional synthetic responses. However, the impact of adiposity on mitochondrial protein synthesis (MPS) rates after protein-rich food ingestion has not been thoroughly examined in vivo in humans.

OBJECTIVE

We compared basal and postprandial MPS and markers of muscle inflammation [toll-like receptor 4 (TLR4) and myeloid differentiation primary response protein 88 (MyD88) protein content] in young adults with different body mass indices (BMIs).

METHODS

Ten normal-weight (NW; BMI = 22.7 ± 0.4 kg/m2), 10 overweight (OW; BMI = 27.1 ± 0.5 kg/m2), and 10 obese (OB; BMI = 35.9 ± 1.3 kg/m2) adults received primed continuous L-[ring-13C6]phenylalanine infusions, blood sampling, and skeletal muscle biopsies before and after the ingestion of 170 g of pork.

RESULTS

Pork ingestion increased muscle TLR4 and MyD88 protein content in the OB group (P < 0.05), but not in the NW or OW groups. Basal MPS was similar between groups (P > 0.05). Pork ingestion stimulated MPS (P < 0.001; 0 to 300 minutes) in the NW (2.5- ± 0.6-fold above baseline values), OW (1.7- ± 0.3-fold), and OB groups (2.4- ± 0.5-fold) with no group differences (P > 0.05).

CONCLUSIONS

Protein-dense food ingestion promotes muscle inflammatory signaling only in OB adults. However, the consumption of a dinner-sized amount of protein strongly stimulated a postprandial MPS response irrespective of BMI. Our data suggest that alterations in postprandial MPS are unlikely to contribute to compromised muscle macronutrient metabolism witnessed with obesity.

Anabolic resistance does not explain sarcopenia in patients with type 2 diabetes mellitus, compared with healthy controls, despite reduced mTOR pathway activity

BACKGROUND

Ageing and type 2 diabetes mellitus (T2DM) are risk factors for skeletal muscle loss. We investigated whether anabolic resistance to feeding might underlie accelerated muscle loss in older people with T2DM and whether dysregulated mTOR signalling was implicated.

SUBJECTS

8 obese men with T2DM, and 12 age-matched controls were studied (age 68 ± 3 vs. 68±6 y; BMI: 30 ± 2 vs. 27 ± 5 kg m^-2).

METHODS

Body composition was measured by dual-X-ray absorptiometry. Insulin and glucose were clamped at post-absorptive concentrations (13 ± 2 vs. 9 ± 3 mU l^-1; 7.4 ± 1.9 vs. 4.6 ± 0.4 mmol l^-1; T2DM vs. controls). Fractional synthetic rates (FSR) of myofibrillar and sarcoplasmic proteins were measured as the rate of incorporation of [¹³C] leucine during a primed, constant infusion of [1-¹³C] α-ketoisocaproic acid, 3 h after 10 or 20 g of essential amino acids (EAA) were orally administered. Protein expression of total and phosphorylated mTOR signalling proteins was determined by Western blot analysis.

RESULTS

Despite a significantly lower appendicular lean mass index and a greater fat mass index in T2DM vs. controls, basal myofibrillar and sarcoplasmic and post-prandial myofibrillar FSR were similar. After 20 g EAA, stimulation of sarcoplasmic FSR was slightly blunted in T2DM patients. Furthermore, feeding 20 g EAA increased phosphorylation of mTOR, p70^S6k and 4E-BP1 by 60-100% in controls with no response observed in T2DM.

CONCLUSIONS

There was clear dissociation between changes in mTOR signalling versus changes in protein synthesis rates. However, the intact anabolic response of myofibrillar FSR to feeding in both groups suggests anabolic resistance may not explain accelerated muscle loss in T2DM.

Effects of skeletal muscle energy availability on protein turnover responses to exercise

Skeletal muscle adaptation to exercise training is a consequence of repeated contraction-induced increases in gene expression that lead to the accumulation of functional proteins whose role is to blunt the homeostatic perturbations generated by escalations in energetic demand and substrate turnover. The development of a specific 'exercise phenotype' is the result of new, augmented steady-state mRNA and protein levels that stem from the training stimulus (i.e. endurance or resistance based). Maintaining appropriate skeletal muscle integrity to meet the demands of training (i.e. increases in myofibrillar and/or mitochondrial protein) is regulated by cyclic phases of synthesis and breakdown, the rate and turnover largely determined by the protein's half-life. Cross-talk among several intracellular systems regulating protein synthesis, breakdown and folding is required to ensure protein equilibrium is maintained. These pathways include both proteasomal and lysosomal degradation systems (ubiquitin-mediated and autophagy, respectively) and the protein translational and folding machinery. The activities of these cellular pathways are bioenergetically expensive and are modified by intracellular energy availability (i.e. macronutrient intake) and the 'training impulse' (i.e. summation of the volume, intensity and frequency). As such, exercise-nutrient interactions can modulate signal transduction cascades that converge on these protein regulatory systems, especially in the early post-exercise recovery period. This review focuses on the regulation of muscle protein synthetic response-adaptation processes to divergent exercise stimuli and how intracellular energy availability interacts with contractile activity to impact on muscle remodelling.

Insulin does not stimulate muscle protein synthesis during increased plasma branched-chain amino acids alone but still decreases whole body proteolysis in humans

Insulin stimulates muscle protein synthesis when the levels of total amino acids, or at least the essential amino acids, are at or above their postabsorptive concentrations. Among the essential amino acids, branched-chain amino acids (BCAA) have the primary role in stimulating muscle protein synthesis and are commonly sought alone to stimulate muscle protein synthesis in humans. Fourteen healthy young subjects were studied before and after insulin infusion to examine whether insulin stimulates muscle protein synthesis in relation to the availability of BCAA alone. One half of the subjects were studied in the presence of postabsorptive BCAA concentrations (control) and the other half in the presence of increased plasma BCAA (BCAA). Compared with that prior to the initiation of the insulin infusion, fractional synthesis rate of muscle protein (%/h) did not change (P > 0.05) during insulin in either the control (0.04 ± 0.01 vs 0.05 ± 0.01) or the BCAA (0.05 ± 0.02 vs. 0.05 ± 0.01) experiments. Insulin decreased (P < 0.01) whole body phenylalanine rate of appearance (μmol·kg^-1·min^-1), indicating suppression of muscle proteolysis, in both the control (1.02 ± 0.04 vs 0.76 ± 0.04) and the BCAA (0.89 ± 0.07 vs 0.61 ± 0.03) experiments, but the change was not different between the two experiments (P > 0.05). In conclusion, insulin does not stimulate muscle protein synthesis in the presence of increased circulating levels of plasma BCAA alone. Insulin's suppressive effect on proteolysis is observed independently of the levels of circulating plasma BCAA.

Prolonged Exposure of Primary Human Muscle Cells to Plasma Fatty Acids Associated with Obese Phenotype Induces Persistent Suppression of Muscle Mitochondrial ATP Synthase β Subunit

Our previous studies show reduced abundance of the β-subunit of mitochondrial H+-ATP synthase (β-F1-ATPase) in skeletal muscle of obese individuals. The β-F1-ATPase forms the catalytic core of the ATP synthase, and it is critical for ATP production in muscle. The mechanism(s) impairing β-F1-ATPase metabolism in obesity, however, are not completely understood. First, we studied total muscle protein synthesis and the translation efficiency of β-F1-ATPase in obese (BMI, 36±1 kg/m2) and lean (BMI, 22±1 kg/m2) subjects. Both total protein synthesis (0.044±0.006 vs 0.066±0.006%·h-1) and translation efficiency of β-F1-ATPase (0.0031±0.0007 vs 0.0073±0.0004) were lower in muscle from the obese subjects when compared to the lean controls (P<0.05). We then evaluated these same responses in a primary cell culture model, and tested the specific hypothesis that circulating non-esterified fatty acids (NEFA) in obesity play a role in the responses observed in humans. The findings on total protein synthesis and translation efficiency of β-F1-ATPase in primary myotubes cultured from a lean subject, and after exposure to NEFA extracted from serum of an obese subject, were similar to those obtained in humans. Among candidate microRNAs (i.e., non-coding RNAs regulating gene expression), we identified miR-127-5p in preventing the production of β-F1-ATPase. Muscle expression of miR-127-5p negatively correlated with β-F1-ATPase protein translation efficiency in humans (r = - 0.6744; P<0.01), and could be modeled in vitro by prolonged exposure of primary myotubes derived from the lean subject to NEFA extracted from the obese subject. On the other hand, locked nucleic acid inhibitor synthesized to target miR-127-5p significantly increased β-F1-ATPase translation efficiency in myotubes (0.6±0.1 vs 1.3±0.3, in control vs exposure to 50 nM inhibitor; P<0.05). Our experiments implicate circulating NEFA in obesity in suppressing muscle protein metabolism, and establish impaired β-F1-ATPase translation as an important consequence of obesity.

Protein Considerations for Optimising Skeletal Muscle Mass in Healthy Young and Older Adults

Skeletal muscle is critical for human health. Protein feeding, alongside resistance exercise, is a potent stimulus for muscle protein synthesis (MPS) and is a key factor that regulates skeletal muscle mass (SMM). The main purpose of this narrative review was to evaluate the latest evidence for optimising the amino acid or protein source, dose, timing, pattern and macronutrient coingestion for increasing or preserving SMM in healthy young and healthy older adults. We used a systematic search strategy of PubMed and Web of Science to retrieve all articles related to this review objective. In summary, our findings support the notion that protein guidelines for increasing or preserving SMM are more complex than simply recommending a total daily amount of protein. Instead, multifactorial interactions between protein source, dose, timing, pattern and macronutrient coingestion, alongside exercise, influence the stimulation of MPS, and thus should be considered in the context of protein recommendations for regulating SMM. To conclude, on the basis of currently available scientific literature, protein recommendations for optimising SMM should be tailored to the population or context of interest, with consideration given to age and resting/post resistance exercise conditions.

Metabolomic and Lipidomic Analysis of Serum Samples following Curcuma longa Extract Supplementation in High-Fructose and Saturated Fat Fed Rats

We explored, using nuclear magnetic resonance (NMR) metabolomics and fatty acids profiling, the effects of a common nutritional complement, Curcuma longa, at a nutritionally relevant dose with human use, administered in conjunction with an unbalanced diet. Indeed, traditional food supplements have been long used to counter metabolic impairments induced by unbalanced diets. Here, rats were fed either a standard diet, a high level of fructose and saturated fatty acid (HFS) diet, a diet common to western countries and that certainly contributes to the epidemic of insulin resistance (IR) syndrome, or a HFS diet with a Curcuma longa extract (1% of curcuminoids in the extract) for ten weeks. Orthogonal projections to latent structures discriminant analysis (OPLS-DA) on the serum NMR profiles and fatty acid composition (determined by GC/MS) showed a clear discrimination between HFS groups and controls. This discrimination involved metabolites such as glucose, amino acids, pyruvate, creatine, phosphocholine/glycerophosphocholine, ketone bodies and glycoproteins as well as an increase of monounsaturated fatty acids (MUFAs) and a decrease of n-6 and n-3 polyunsaturated fatty acids (PUFAs). Although the administration of Curcuma longa did not prevent the observed increase of glucose, triglycerides, cholesterol and insulin levels, discriminating metabolites were observed between groups fed HFS alone or with addition of a Curcuma longa extract, namely some MUFA and n-3 PUFA, glycoproteins, glutamine, and methanol, suggesting that curcuminoids may act respectively on the fatty acid metabolism, the hexosamine biosynthesis pathway and alcohol oxidation. Curcuma longa extract supplementation appears to be beneficial in these metabolic pathways in rats. This metabolomic approach highlights important serum metabolites that could help in understanding further the metabolic mechanisms leading to IR.

A new method to measure muscle protein synthesis in humans by endogenously introduced d9-leucine and using blood for precursor enrichment determination

Enrichment from the easily accessible blood amino acid pool is commonly used as precursor enrichment to calculate rates of muscle protein fractional synthesis in relevant human studies in lieu of the less accessible muscle fluid amino acid pool. However, the accuracy of this approach depends largely on the extent to which there is low discrepancy in free amino acid enrichment between blood and muscle. Steady-state gradient (i.e., ratio) of amino acid enrichment between blood and muscle fluid in the basal state and in response to amino acid infusion were determined in five healthy subjects, and in association with two separate tracers: d9-leucine, introduced endogenously by the metabolism of d10-leucine (i.e., l-[2,3,3,4,5,5,5,6,6,6-(2)H10]leucine) infused in blood, and (13)C6-phenylalanine introduced/infused in blood. The blood-to-muscle fluid amino acid enrichment ratio was lower (P < 0.05) for d9-leucine compared to (13)C6-phenylalanine both before (1.5 ± 0.1 vs. 2.5 ± 0.1) and during (1.1 ± 0.1 vs. 1.2 ± 0.1) amino acid infusion. Importantly, the decrease in this ratio in association with the amino acid infusion was considerably less for the d9-leucine than the (13)C6-phenylalanine (-0.38 ± 0.03 vs. -1.29 ± 0.07; P < 0.05). In conclusion, blood d9-leucine enrichment introduced endogenously by intravenous infusion of d10-leucine provides a closer estimate of the muscle fluid amino acid enrichment, and its associated changes, than blood phenylalanine enrichment to calculate rates of muscle protein synthesis in humans.

Lipid-induced insulin resistance is associated with an impaired skeletal muscle protein synthetic response to amino acid ingestion in healthy young men

The ability to maintain skeletal muscle mass appears to be impaired in insulin-resistant conditions, such as type 2 diabetes, that are characterized by muscle lipid accumulation. The current study investigated the effect of acutely increasing lipid availability on muscle protein synthesis. Seven healthy young male volunteers underwent a 7-h intravenous infusion of l-[ring-(2)H5]phenylalanine on two randomized occasions combined with 0.9% saline or 10% Intralipid at 100 mL/h. After a 4-h "basal" period, a 21-g bolus of amino acids was administered and a 3-h hyperinsulinemic-euglycemic clamp was commenced ("fed" period). Muscle biopsy specimens were obtained from the vastus lateralis at 1.5, 4, and 7 h. Lipid infusion reduced fed whole-body glucose disposal by 20%. Furthermore, whereas the mixed muscle fractional synthetic rate increased from the basal to the fed period during saline infusion by 2.2-fold, no change occurred during lipid infusion, despite similar circulating insulin and leucine concentrations. This "anabolic resistance" to insulin and amino acids with lipid infusion was associated with a complete suppression of muscle 4E-BP1 phosphorylation. We propose that increased muscle lipid availability may contribute to anabolic resistance in insulin-resistant conditions by impairing translation initiation.

Proteins and amino acids are fundamental to optimal nutrition support in critically ill patients

Proteins and amino acids are widely considered to be subcomponents in nutritional support. However, proteins and amino acids are fundamental to recovery and survival, not only for their ability to preserve active tissue (protein) mass but also for a variety of other functions. Understanding the optimal amount of protein intake during nutritional support is therefore fundamental to appropriate clinical care. Although the body adapts in some ways to starvation, metabolic stress in patients causes increased protein turnover and loss of lean body mass. In this review, we present the growing scientific evidence showing the importance of protein and amino acid provision in nutritional support and their impact on preservation of muscle mass and patient outcomes. Studies identifying optimal dosing for proteins and amino acids are not currently available. We discuss the challenges physicians face in administering the optimal amount of protein and amino acids. We present protein-related nutrition concepts, including adaptation to starvation and stress, anabolic resistance, and potential adverse effects of amino acid provision. We describe the methods for assessment of protein status, and outcomes related to protein nutritional support for critically ill patients. The identification of a protein target for individual critically ill patients is crucial for outcomes, particularly for specific subpopulations, such as obese and older patients. Additional research is urgently needed to address these issues.

Impaired protein metabolism: interlinks between obesity, insulin resistance and inflammation

Metabolic and structural changes in skeletal muscle that accompany obesity are often associated with the development of insulin resistance. The first events in the pathogenesis of this disorder are considered as an accumulation of lipids within skeletal muscle due to blunted muscle capacity to oxidize fatty acids. Fat infiltration is also associated with muscle fibre typology modification, decrease in muscle mass and impairments in muscle strength. Thus, as a result of obesity, mobility and quality of life are affected, and this is in part due to quantitative and qualitative impairments in skeletal muscle. In addition, the insulin resistance related to obesity results not only in defective insulin-stimulated glucose disposal but has also detrimental consequences on protein metabolism at the skeletal muscle level and whole-body level. This review highlights the involvement of fat accumulation and insulin resistance in metabolic disorders occurring in skeletal muscle during the development of obesity, and the impairments in the regulation of protein metabolism and protein turnover in the links between obesity, metabolic inflammation and insulin resistance.

Methionine and protein metabolism in non-alcoholic steatohepatitis: evidence for lower rate of transmethylation of methionine

Hepatic metabolism of methionine is the source of cysteine, the precursor of glutathione, the major intracellular antioxidant in the body. Methionine also is the immediate precursor of SAM (S-adenosylmethionine) the key methyl donor for phosphatidylcholine synthesis required for the export of VLDL (very-low-density lipoprotein) triacylglycerols (triglycerides) from the liver. We have examined the kinetics of methionine, its transmethylation and trans-sulfuration with estimates of whole body rate of protein turnover and urea synthesis in clinically stable biopsy-confirmed subjects with NASH (non-alcoholic steatohepatitis). Subjects with NASH were more insulin-resistant and had significantly higher plasma concentrations of usCRP (ultrasensitive C-reactive protein), TNFα (tumour necrosis factor α) and other inflammatory cytokines. There was no significant effect of insulin resistance and NASH on whole body rate of protein turnover [phenylalanine Ra (rate of appearance)] and on the rate of urea synthesis. The rates of methylation of homocysteine and transmethylation of methionine were significantly lower in NASH compared with controls. There was no difference in the rate of trans-sulfuration of methionine between the two groups. Enteric mixed nutrient load resulted in a significant increase in all the measured parameters of methionine kinetics. Heterozygosity for MTHFR (5,10-methylene-tetrahydrofolate reductase) (677C→T) did not have an impact on methionine metabolism. We speculate that, as a result of oxidant stress possibly due to high fatty acid oxidation, the activity of methionine adenosyltransferase is attenuated resulting in a lower rate of transmethylation of methionine and of SAM synthesis. These results are the first evidence for perturbed metabolism of methionine in NASH in humans and provide a rationale for the development of targeted intervention strategies.

β-Adrenergic receptor blockade blunts postexercise skeletal muscle mitochondrial protein synthesis rates in humans

β-Adrenergic receptor (AR) signaling is a regulator of skeletal muscle protein synthesis and mitochondrial biogenesis in mice. We hypothesized that β-AR blockade blunts postexercise skeletal muscle mitochondrial protein synthesis rates in adult humans. Six healthy men (mean ± SD: 26 ± 6 yr old, 39.9 ± 4.9 ml·kg(-1)·min(-1) peak O(2) uptake, 26.7 ± 2.0 kg/m(2) body mass index) performed 1 h of stationary cycle ergometer exercise (60% peak O(2) uptake) during 1) β-AR blockade (intravenous propranolol) and 2) administration of saline (control). Skeletal muscle mitochondrial, myofibrillar, and sarcoplasmic protein synthesis rates were assessed using [(2)H(5)]phenylalanine incorporation into skeletal muscle proteins after exercise. The mRNA content of signals for mitochondrial biogenesis was determined using real-time PCR. β-AR blockade decreased mitochondrial (from 0.217 ± 0.076 to 0.135 ± 0.031%/h, P < 0.05), but not myofibrillar or sarcoplasmic, protein synthesis rates. Peroxisome proliferator-activated receptor-γ coactivator-1α mRNA was increased ∼2.5-fold (P < 0.05) at 5 h compared with 1 h postexercise but was not influenced by β-AR blockade. We conclude that decreased β-AR signaling during cycling can blunt the postexercise increase in mitochondrial protein synthesis rates without affecting mRNA content.

Human muscle protein turnover--why is it so variable?

We undertook a comprehensive review of the literature to unravel the nature of the variability in the reported rate of human muscle protein synthesis. We analyzed the results from studies that report the protein fractional synthesis rate (FSR) in the vastus lateralis in healthy, nonobese, untrained adults ≤50 yr of age in the postabsorptive state at rest by using the primed, constant tracer amino acid infusion method according to experimental design characteristics. We hypothesized that if the variability is methodological (rather than physiological) in nature, systematic clustering of FSR values would be evident, and outliers would become apparent. Overall, as expected, the mixed muscle protein FSR values were significantly (P < 0.001) greater when the muscle vs. the plasma free amino acid enrichment is used as the surrogate precursor pool enrichment, and the average mixed muscle protein FSR values were significantly greater (P = 0.05) than the myofibrillar/myosin heavy chain FSR values. The within-study variability (i.e., population variance) was somewhat smaller in studies that used plasma amino acid/ketoacid enrichments vs. muscle free amino acid enrichment (∼24 vs. ∼31%), but this was not apparent in all circumstances. Furthermore, the between-study consistency of measured FSR values (i.e., interquartile range) was inversely correlated with the average duration between biopsies. Aside from that, the variation in reported FSR values could not be explained by differences in the experimental design and analytical methods, and none of the most commonly used approaches stood out as clearly superior in terms of consistency of results and/or within-study variability. We conclude that the variability in reported values is in part due to 1) differences in experimental design (e.g., choice of precursor pool) and 2) considerable within-subject variability. The summary of the results from our analysis can be used as guidelines for "normal" average basal FSR values at rest in healthy adults.

Timing of the initial muscle biopsy does not affect the measured muscle protein fractional synthesis rate during basal, postabsorptive conditions

The muscle protein fractional synthesis rate (FSR) is determined by monitoring the incorporation of an amino acid tracer into muscle protein during a constant-rate intravenous tracer infusion. Commonly two sequential muscle biopsies are obtained some time after starting the tracer infusion. However, other protocols, including those with an initial biopsy before starting the tracer infusion to measure the background enrichment and those with only a single biopsy after several hours of tracer infusion have been used. To assess the validity of these approaches, we compared the muscle protein FSR obtained by calculating the difference in [ring-(2)H(5)]phenylalanine and [5,5,5-(2)H(3)]leucine incorporation into muscle protein at approximately 3.5 h after starting the tracer infusion and 1) at 60 min; 2) before starting the tracer infusion (background enrichment); 3) a population average muscle protein background enrichment; and 4) by measuring the tracer incorporation into muscle protein at approximately 3.5 h assuming essentially no background enrichment. Irrespective of the tracer used, the muscle protein FSR calculated from the difference in the muscle protein labeling several hours after starting the tracer infusion and either the labeling at 60 min or the background enrichment were not different (e.g., 0.049 +/- 0.007%/h vs. 0.049 +/- 0.007%/h, respectively, with [(2)H(5)]phenylalanine; P = 0.99). However, omitting the initial biopsy and assuming no background enrichment yielded average FSR values that were approximately 15% (with [(2)H(5)]phenylalanine) to 80% (with [(2)H(3)]leucine) greater (P < or = 0.059); using a population average background enrichment reduced the difference to approximately 3% (P = 0.76) and 22% (P = 0.52) with [(2)H(5)]phenylalanine and [(2)H(3)]leucine, respectively. We conclude that during basal, postabsorptive conditions, valid muscle protein FSR values can be obtained irrespective of the timing of the initial biopsy so long as the protein labeling in two sequential biopsies is measured whereas the single biopsy approach should be avoided.