Influence of leucine infusion on intracellular amino acids in humans

The effects of branched-chain amino acids on muscle protein synthesis, muscle protein breakdown and associated molecular signalling responses in humans: an update

Branched-chain amino acids (BCAA: leucine, isoleucine and valine) are three of the nine indispensable amino acids, and are frequently consumed as a dietary supplement by athletes and recreationally active individuals alike. The popularity of BCAA supplements is largely predicated on the notion that they can stimulate rates of muscle protein synthesis (MPS) and suppress rates of muscle protein breakdown (MPB), the combination of which promotes a net anabolic response in skeletal muscle. To date, several studies have shown that BCAA (particularly leucine) increase the phosphorylation status of key proteins within the mechanistic target of rapamycin (mTOR) signalling pathway involved in the regulation of translation initiation in human muscle. Early research in humans demonstrated that BCAA provision reduced indices of whole-body protein breakdown and MPB; however, there was no stimulatory effect of BCAA on MPS. In contrast, recent work has demonstrated that BCAA intake can stimulate postprandial MPS rates at rest and can further increase MPS rates during recovery after a bout of resistance exercise. The purpose of this evidence-based narrative review is to critically appraise the available research pertaining to studies examining the effects of BCAA on MPS, MPB and associated molecular signalling responses in humans. Overall, BCAA can activate molecular pathways that regulate translation initiation, reduce indices of whole-body and MPB, and transiently stimulate MPS rates. However, the stimulatory effect of BCAA on MPS rates is less than the response observed following ingestion of a complete protein source providing the full complement of indispensable amino acids.

Early lean mass sparing effect of high-protein diet with excess leucine during long-term bed rest in women

Muscle inactivity leads to muscle atrophy. Leucine is known to inhibit protein degradation and to promote protein synthesis in skeletal muscle. We tested the ability of a high-protein diet enriched with branched-chain amino acids (BCAAs) to prevent muscle atrophy during long-term bed rest (BR). We determined body composition (using dual energy x-ray absorptiometry) at baseline and every 2-weeks during 60 days of BR in 16 healthy young women. Nitrogen (N) balance was assessed daily as the difference between N intake and N urinary excretion. The subjects were randomized into two groups: one received a conventional diet (1.1 ± 0.03 g protein/kg, 4.9 ± 0.3 g leucine per day) and the other a high protein, BCAA-enriched regimen (1.6 ± 0.03 g protein-amino acid/kg, 11.4 ± 0.6 g leucine per day). There were significant BR and BR × diet interaction effects on changes in lean body mass (LBM) and N balance throughout the experimental period (repeated measures ANCOVA). During the first 15 days of BR, lean mass decreased by 4.1 ± 0.9 and 2.4 ± 2.1% (p < 0.05) in the conventional and high protein-BCAA diet groups, respectively, while at the end of the 60-day BR, LBM decreased similarly in the two groups by 7.4 ± 0.7 and 6.8 ± 2.4%. During the first 15 days of BR, mean N balance was 2.5 times greater (p < 0.05) in subjects on the high protein-BCAA diet than in those on the conventional diet, while we did not find significant differences during the following time intervals. In conclusion, during 60 days of BR in females, a high protein-BCAA diet was associated with an early protein-LBM sparing effect, which ceased in the medium and long term.

Leucine Supplementation in Cancer Cachexia: Mechanisms and a Review of the Pre-Clinical Literature

Cancer cachexia (CC) is a complex syndrome of bodily wasting and progressive functional decline. Unlike starvation, cachexia cannot be reversed by increased energy intake alone. Nonetheless, targeted nutritional support is a necessary component in multimodal syndrome management. Due to the highly catabolic nature of cancer cachexia, amino acid supplementation has been proposed. Interestingly, leucine has been found to increase protein synthesis and decrease protein degradation via mTORC1 pathway activation. Multiple pre-clinical studies have explored the impact of leucine supplementation in cachectic tumor-bearing hosts. Here, we provide an overview of leucine’s proposed modes of action to preserve lean mass in cachexia and review the current pre-clinical literature related to leucine supplementation during CC. Current research indicates that a leucine-rich diet may attenuate CC symptomology; however, these works are difficult to compare due to methodological differences. There is need for further pre-clinical work exploring leucine’s potential ability to modulate protein turnover and immune response during CC, as well as the impact of additive leucine on tumor growth.

The Effect of Leucine Supplementation on Sarcopenia-Related Measures in Older Adults: A Systematic Review and Meta-Analysis of 17 Randomized Controlled Trials

Background

The role of leucine in sarcopenia prevention remains unclear. We aimed to summarize the published data from randomized controlled trials (RCTs) to estimate the effect of leucine supplementation on sarcopenia-related measures in older adults.

Methods

A systematic literature search was performed using the electronic databases PubMed, Embase, and Web of Science with restriction to randomized controlled trials design from January 1, 2009 to March 19, 2022. Sarcopenia-related measures included handgrip strength, total lean mass, gait speed, leg press, 6-min walk test, short-physical performance battery, timed up-and-go test and 30-s chair-stand test. Fixed- and random-effects meta-analysis models were used to generate pooled weighted mean differences (WMDs) and 95% CIs. Heterogeneity was examined in subgroup and sensitivity analyses. Publication bias assessments were performed.

Results

A total of 17 RCTs enrolling 1418 subjects were identified. Leucine-isolated supplementation showed no effect on total lean mass (WMD = 0.03 kg, 95% CI: –0.51, 0.57, P = 0.917), handgrip strength (WMD = 1.23 kg, 95% CI: –0.58, 3.03, P = 0.183) and leg press (WMD = –1.35 kg, 95% CI: –7.46, 4.77, P = 0.666). However, leucine-combined supplementation including vitamin D showed a significant improvement in handgrip strength (WMD = 2.17 kg, 95% CI: 0.24, 4.10, P = 0.027) and gait speed (WMD = 0.03 m/s, 95% CI: 0.01, 0.05, P = 0.008).

Conclusion

Leucine-isolated supplementation did not improve muscle mass and strength in elderly. However, leucine-combined supplementation including vitamin D exhibited a significant benefit for muscle strength and performance including handgrip strength and gait speed in older adults. A combination of nutritional supplements would be a viable option for improving sarcopenia.

Changes in plasma concentration of kynurenine following intake of branched-chain amino acids are not caused by alterations in muscle kynurenine metabolism

Administration of branched-chain amino acids (BCAA) has been suggested to enhance mitochondrial biogenesis, including levels of PGC-1α, which may, in turn, alter kynurenine metabolism. Ten healthy subjects performed 60 min of dynamic one-leg exercise at ~70% of Wmax on two occasions. They were in random order supplied either a mixture of BCAA or flavored water (placebo) during the experiment. Blood samples were collected during exercise and recovery, and muscle biopsies were taken from both legs before, after and 90 and 180 min following exercise. Ingestion of BCAA doubled their concentration in both plasma and muscle while causing a 30-40% reduction (P<0.05 vs. placebo) in levels of aromatic amino acids in both resting and exercising muscle during 3-h recovery. The muscle concentration of kynurenine decreased by 25% (P<0.05) during recovery, similar in both resting and exercising leg and with both supplements, although plasma concentration of kynurenine during recovery was 10% lower (P<0.05) when BCAA were ingested. Ingestion of BCAA reduced the plasma concentration of kynurenic acid by 60% (P<0.01) during exercise and recovery, while the level remained unchanged with placebo. Exercise induced a 3-4-fold increase (P<0.05) in muscle content of PGC-1a1 mRNA after 90 min of recovery under both conditions, whereas levels of KAT4 mRNA and protein were unaffected by exercise or supplement. In conclusion, the reduction of plasma levels of kynurenine and kynurenic acid caused by BCAA were not associated with any changes in the level of muscle kynurenine, suggesting that kynurenine metabolism was altered in tissues other than muscle.

Influence of Dietary Metformin on the Growth Performance and Plasma Concentrations of Amino Acids and Advanced Glycation End Products in Two Types of Chickens

Glycation is a non-enzymatic reaction inducing the bonding of glucose to amino acids and proteins. Glycated amino acids are not useful for protein synthesis, suggesting that glycation reduces the utilization of amino acids. Metformin (MF) is well known as a therapeutic drug for type II diabetes that inhibits glycation. It is possible that treatment with MF raises the utilization of amino acids by the inhibition of glycation, thereby improving the growth performance of chickens. In the present study, therefore, we investigated the influence of dietary MF on the growth performance, and plasma concentrations of free amino acids and N^ε-(Carboxymethyl)lysine (CML), which is an advanced glycation end product, in layer (Experiment 1) and broiler (Experiment 2) chickens. From 7 d of age, chicks were allowed free access to one of the experimental diets containing MF at 3 supplementation levels (0, 150, and 300 mg/kg diet) for 14 days. Body weight and feed intake were measured every week. At the end of the experiments, blood and breast muscle (M. pectoralis major) were collected for further analysis. Dietary MF did not affect weight gain, feed intake, or feed efficiency in both layer and broiler chickens. Dietary MF at the level of 150 mg/kg diet increased breast muscle weight in both layer and broiler chickens. Dietary MF increased plasma concentrations of branched chain amino acids and decreased concentrations of CML in layer chickens, although it did not affect plasma concentrations of glucose. The present study suggested that dietary MF might have the potency to increase breast muscle weight of layer chickens with an increment in plasma concentrations of branched-chain amino acids.

The effects of branch-chain amino acids on fatigue in the athletes

Research has shown that injection of a mixture of amino acids into humans led to stimulation of protein synthesis about 30 min after injection, and the synthesis rate remained at a high level for up to 90 min. Various theories have been proposed to justify this effect, such as increasing the availability of amino acids in the body to increase their transmission to muscle, and this increase in transmission can stimulate the synthesis of protein in muscle cells. A total of 30 volunteers with a history of 2–3 years in the field of Athletics were participated in this study. The experimental group (15 subjects) used BCAAs supplement during 6-week athletics training. Control group (15 subjects) participated in athletics training for 6 weeks without any supplementation. Blood was taken before and after 6 weeks supplementary. The findings of the t-test showed that basal levels of lactate and ammonium had not changed significantly after 42 days of supplementation. The results of this study showed that BCAAs supplementation has no effect on the reduction of lactate and ammonium indices as indicators of fatigue.

The muscle anabolic effect of protein ingestion during a hyperinsulinaemic euglycaemic clamp in middle-aged women is not caused by leucine alone

KEY POINTS

It has been suggested that leucine is primarily responsible for the increase in muscle protein synthesis after protein ingestion because leucine uniquely activates the mTOR-p70S6K signalling cascade. We compared the effects of ingesting protein or an amount of leucine equal to that in the protein during a hyperinsulinaemic-euglycaemic clamp (to eliminate potential confounding as a result of differences in the insulinogenic effect of protein and leucine ingestion) on muscle anabolic signalling and protein turnover in 28 women. We found that protein, but not leucine, ingestion increased muscle p-mTOR^Ser2448 and p-p70S6K^Thr389 , although only protein, and not leucine, ingestion decreased muscle p-eIF2α^Ser51 and increased muscle protein synthesis.

ABSTRACT

It has been suggested that leucine is primarily responsible for the increase in muscle protein synthesis (MPS) after protein ingestion because leucine uniquely activates the mTOR-p70S6K signalling cascade. We tested this hypothesis by measuring muscle p-mTOR^Ser2448 , p-p70S6K^Thr389 and p-eIF2α^Ser51 , as well as protein turnover (by stable isotope labelled amino acid tracer infusion in conjunction with leg arteriovenous blood and muscle tissue sampling), in 28 women who consumed either 0.45 g protein kg^-1 fat-free mass (containing 0.0513 g leucine kg^-1 fat-free mass) or a control drink (n = 14) or 0.0513 g leucine kg^-1 fat-free mass or a control drink (n = 14) during a hyperinsulinaemic-euglycaemic clamp procedure (HECP). Compared to basal conditions, the HECP alone (without protein or leucine ingestion) suppressed muscle protein breakdown by ∼20% and increased p-mTOR^Ser2448 and p-p70S6K^Thr389 by >50% (all P < 0.05) but had no effect on p-eIF2α^Ser51 and MPS. Both protein and leucine ingestion further increased p-mTOR^Ser2448 and p-p70S6K^Thr389 , although only protein, and not leucine, ingestion decreased (by ∼35%) p-eIF2α^Ser51 and increased (by ∼100%) MPS (all P < 0.05). Accordingly, leg net protein balance changed from negative (loss) during basal conditions to equilibrium during the HECP alone and the HECP with concomitant leucine ingestion and to positive (gain) during the HECP with concomitant protein ingestion. These results provide new insights into the regulation of MPS by demonstrating that leucine and mTOR signalling alone are not responsible for the muscle anabolic effect of protein ingestion during physiological hyperinsulinaemia, most probably because they fail to signal to eIF2α to initiate translation and/or additional amino acids are needed to sustain translation.

Double-blind, placebo-controlled pilot trial of L-Leucine-enriched amino-acid mixtures on body composition and physical performance in men and women aged 65-75 years

Background/Objectives:

Adequate protein intake is essential to retaining muscle and maintaining physical function, especially in the elderly, and L-Leucine has received attention as an essential amino acid (EAA) that enhances protein retention. The study's aim was to compare the efficacy of EAA mixtures on lean tissue mass (LTM) and functional performance (FP) in a healthy elderly population.

Subjects/Methods:

Thirty-six subjects (65–75 years) volunteered to receive capsules with EAAs (Groups A and B containing 20% and 40% L-Leucine, respectively) or placebo (lactose containing 0% L-Leucine, Group C) for 12 weeks. The daily amount ranged from 11 to 21 g (0.21 g/ kg/day) and was taken in two equal dosages alongside food, morning and evening. Main outcomes measured before and after intervention were LTM and FP (30-s arm-curl test; 30-s chair-stand test (30-CST); 6-min walk test (6-WT); and handgrip strength). Secondary outcomes included dietary intakes and physical activity.

Results:

Twenty-five subjects (11 male and 14 female) completed the study (Group A, n=8; Group B, n=8; Group C, n=9). Gains associated with medium effect sizes were noted in LTM (Group B, 1.1 ±1.1%, P=0.003) and FP (Group A in 30-CST (11.0±11.5%, P=0.02) and 6-WT (8.8±10.0%, P=0.02); Group B in 6-WT (5.8±6.6%, P=0.03) and a trend in 30-CST (13.2±16.0, P=0.06)). Significant differences between groups were not observed in secondary outcomes.

Conclusions:

Twice-daily supplementation of EAAs containing 20% or 40% L-Leucine improved aspects of functional status and at the higher level improved LTM. Further work to establish change in a larger sample and palatable supplemental format is now required.

Autophagy is essential to support skeletal muscle plasticity in response to endurance exercise

Physical exercise is a stress that can substantially modulate cellular signaling mechanisms to promote morphological and metabolic adaptations. Skeletal muscle protein and organelle turnover is dependent on two major cellular pathways: Forkhead box class O proteins (FOXO) transcription factors that regulate two main proteolytic systems, the ubiquitin-proteasome, and the autophagy-lysosome systems, including mitochondrial autophagy, and the MTORC1 signaling associated with protein translation and autophagy inhibition. In recent years, it has been well documented that both acute and chronic endurance exercise can affect the autophagy pathway. Importantly, substantial efforts have been made to better understand discrepancies in the literature on its modulation during exercise. A single bout of endurance exercise increases autophagic flux when the duration is long enough, and this response is dependent on nutritional status, since autophagic flux markers and mRNA coding for actors involved in mitophagy are more abundant in the fasted state. In contrast, strength and resistance exercises preferentially raise ubiquitin-proteasome system activity and involve several protein synthesis factors, such as the recently characterized DAGK for mechanistic target of rapamycin activation. In this review, we discuss recent progress on the impact of acute and chronic exercise on cell component turnover systems, with particular focus on autophagy, which until now has been relatively overlooked in skeletal muscle. We especially highlight the most recent studies on the factors that can impact its modulation, including the mode of exercise and the nutritional status, and also discuss the current limitations in the literature to encourage further works on this topic.

Absence of leucine in an essential amino acid supplement reduces activation of mTORC1 signalling following resistance exercise in young females

The purpose of the study was to investigate the specific effect of leucine on mTORC1 signalling and amino acid metabolism in connection with resistance exercise. Comparisons were made between ingestion of supplements with and without leucine. Eight young women performed leg press exercise on 2 occasions. In randomized order they received either an aqueous solution of essential amino acids with leucine (EAA) or without leucine (EAA-Leu), given as small boluses throughout the experiment. Muscle biopsies were taken after an overnight fast before exercise and 1 and 3 h postexercise and samples of blood were taken repeatedly during the experiment. Plasma and muscle concentrations of leucine rose 60%-140% (p < 0.05) with EAA and fell 35%-45% (p < 0.05) with the EAA-Leu supplement. In the EAA-trial, plasma and muscle levels of tyrosine (not present in the supplement) and the sum of the EAA were 15%-25% (p < 0.05) lower during recovery. Phosphorylation of mTOR and p70S6k was elevated to a larger extent following 1 h of recovery with leucine in the supplement (120% vs. 49% (p < 0.05) and 59- vs. 8-fold (p < 0.05) for EAA and EAA-Leu, respectively). The levels of MAFbx and MuRF-1 mRNA and of the corresponding proteins were not significantly altered after 3 h recovery from exercise. In conclusion, the presence of leucine in the supplement enhances the stimulatory effect on mTORC1 signalling and reduces the level of tyrosine and the sum of the EAA in muscle and plasma, suggesting a stimulation of protein synthesis and (or) inhibition of breakdown, leading to improvement in net protein balance.

Insulinotropic and muscle protein synthetic effects of branched-chain amino acids: potential therapy for type 2 diabetes and sarcopenia

The loss of muscle mass and strength with aging (i.e., sarcopenia) has a negative effect on functional independence and overall quality of life. One main contributing factor to sarcopenia is the reduced ability to increase skeletal muscle protein synthesis in response to habitual feeding, possibly due to a reduction in postprandial insulin release and an increase in insulin resistance. Branched-chain amino acids (BCAA), primarily leucine, increases the activation of pathways involved in muscle protein synthesis through insulin-dependent and independent mechanisms, which may help counteract the "anabolic resistance" to feeding in older adults. Leucine exhibits strong insulinotropic characteristics, which may increase amino acid availability for muscle protein synthesis, reduce muscle protein breakdown, and enhance glucose disposal to help maintain blood glucose homeostasis.

Intake of branched-chain amino acids influences the levels of MAFbx mRNA and MuRF-1 total protein in resting and exercising human muscle

Resistance exercise and amino acids are two major factors that influence muscle protein turnover. Here, we examined the effects of resistance exercise and branched-chain amino acids (BCAA), individually and in combination, on the expression of anabolic and catabolic genes in human skeletal muscle. Seven subjects performed two sessions of unilateral leg press exercise with randomized supplementation with BCAA or flavored water. Biopsies were collected from the vastus lateralis muscle of both the resting and exercising legs before and repeatedly after exercise to determine levels of mRNA, protein phosphorylation, and amino acid concentrations. Intake of BCAA reduced (P < 0.05) MAFbx mRNA by 30 and 50% in the resting and exercising legs, respectively. The level of MuRF-1 mRNA was elevated (P < 0.05) in the exercising leg two- and threefold under the placebo and BCAA conditions, respectively, whereas MuRF-1 total protein increased by 20% (P < 0.05) only in the placebo condition. Phosphorylation of p70(S6k) increased to a larger extent (∼2-fold; P < 0.05) in the early recovery period with BCAA supplementation, whereas the expression of genes regulating mTOR activity was not influenced by BCAA. Muscle levels of phenylalanine and tyrosine were reduced (13-17%) throughout recovery (P < 0.05) in the placebo condition and to a greater extent (32-43%; P < 0.05) following BCAA supplementation in both resting and exercising muscle. In conclusion, BCAA ingestion reduced MAFbx mRNA and prevented the exercise-induced increase in MuRF-1 total protein in both resting and exercising leg. Further-more, resistance exercise differently influenced MAFbx and MuRF-1 mRNA expression, suggesting both common and divergent regulation of these two ubiquitin ligases.

Repeated post-exercise administration with a mixture of leucine and glucose alters the plasma amino acid profile in Standardbred trotters

BACKGROUND

The branched chain amino acid leucine is a potent stimulator of insulin secretion. Used in combination with glucose it can increase the insulin response and the post exercise re-synthesis of glycogen in man. Decreased plasma amino acid concentrations have been reported after intravenous or per oral administration of leucine in man as well as after a single per oral dose in horses. In man, a negative correlation between the insulin response and the concentrations of isoleucine, valine and methionine have been shown but results from horses are lacking. This study aims to determine the effect of repeated per oral administration with a mixture of glucose and leucine on the free amino acid profile and the insulin response in horses after glycogen-depleting exercise.

METHODS

In a crossover design, after a glycogen depleting exercise, twelve Standardbred trotters received either repeated oral boluses of glucose, 1 g/kg body weight (BW) at 0, 2 and 4 h with addition of leucine 0.1 g/kg BW at 0 and 4 h (GLU+LEU), or repeated boluses of water at 0, 2 and 4 h (CON). Blood samples for analysis of glucose, insulin and amino acid concentrations were collected prior to exercise and over a 6 h post-exercise period. A mixed model approach was used for the statistical analyses.

RESULTS

Plasma leucine, isoleucine, valine, tyrosine and phenylalanine concentrations increased after exercise. Post-exercise serum glucose and plasma insulin response were significantly higher in the GLU+LEU treatment compared to the CON treatment. Plasma leucine concentrations increased after supplementation. During the post-exercise period isoleucine, valine and methionine concentrations decreased in both treatments but were significantly lower in the GLU+LEU treatment. There was no correlation between the insulin response and the response in plasma leucine, isoleucine, valine and methionine.

CONCLUSIONS

Repeated post-exercise administration with a mixture of leucine and glucose caused a marked insulin response and altered the plasma amino acid profile in horses in a similar manner as described in man. However, the decreases seen in plasma amino acids in horses seem to be related more to an effect of leucine and not to the insulin response as seen in man.

Co-ingestion of leucine with protein does not further augment post-exercise muscle protein synthesis rates in elderly men

Leucine has been suggested to have the potential to modulate muscle protein metabolism by increasing muscle protein synthesis. The objective of this study was to investigate the surplus value of the co-ingestion of free leucine with protein hydrolysate and carbohydrate following physical activity in elderly men. Eight elderly men (mean age 73 +/- 1 years) were randomly assigned to two cross-over treatments consuming either carbohydrate and protein hydrolysate (CHO+PRO) or carbohydrate, protein hydrolysate with additional leucine (CHO+PRO+leu) after performing 30 min of standardized physical activity. Primed, continuous infusions with L-[ring-(13)C(6)]phenylalanine and L-[ring-(2)H(2)]tyrosine were applied, and blood and muscle samples were collected to assess whole-body protein turnover as well as protein fractional synthetic rate in the vastus lateralis muscle over a 6 h period. Whole-body protein breakdown and synthesis rates were not different between treatments. Phenylalanine oxidation rates were significantly lower in the CHO+PRO+leu v. CHO+PRO treatment. As a result, whole-body protein balance was significantly greater in the CHO+PRO+leu compared to the CHO+PRO treatment (23.8 (SEM 0.3) v. 23.2 (SEM 0.3) micromol/kg per h, respectively; P < 0.05). Mixed muscle fractional synthetic rate averaged 0.081 (SEM 0.003) and 0.082 (SEM 0.006) %/h in the CHO+PRO+leu and CHO+PRO treatment, respectively (NS). Co-ingestion of leucine with carbohydrate and protein following physical activity does not further elevate muscle protein fractional synthetic rate in elderly men when ample protein is ingested.

Supplementation of soy protein with branched-chain amino acids alters protein metabolism in healthy elderly and even more in patients with chronic obstructive pulmonary disease

BACKGROUND

It is often suggested that chronic wasting diseases [eg, chronic obstructive pulmonary disease (COPD)] may benefit from branched-chain amino acid (BCAA) administration via improved protein metabolism.

OBJECTIVE

The aim was to examine whether adding BCAAs to a soy protein meal would enhance protein anabolism in COPD patients and in healthy elderly persons.

DESIGN

Eight normal-weight COPD patients and 8 healthy control subjects were examined on 2 test days. Simultaneous continuous intravenous infusion of l-[ring-(2)H(5)]phenylalanine (Phe) and l-[ring-(2)H(2)]tyrosine tracers was done postabsorptively and at 2 h of ingestion of a maltodextrin soy or maltodextrin soy + BCAA protein meal (rate of ingestion: 0.02 g protein.kg body weight(-1).20 min(-1)) in a crossover design. Together with the meal, oral ingestion of 1-[(13)C]Phe was performed to measure first-pass Phe splanchnic extraction (SPE(Phe)). The endogenous rate of Phe appearance [reflecting whole-body protein breakdown (WbPB)], whole-body protein synthesis (WbPS), and net WbPS (WbPS - WbPB) were calculated. Arterialized venous blood was sampled for amino acid enrichment and concentration analyses.

RESULTS

Soy feeding induced a reduction in WbPB and an increase in WbPS. BCAA supplementation of soy protein resulted in a significantly higher (P < 0.05) increase in WbPS than did soy protein alone in COPD patients but not in the healthy elderly. BCAA supplementation did not significantly alter the change in WbPB or net WbPS. Furthermore, BCAA supplementation decreased (absolute) SPE(Phe) (P < 0.05) but did not change the percentage Phe hydroxylation in the splanchnic area, which indicates a BCAA-related reduction in splanchnic protein synthesis.

CONCLUSION

BCAA supplementation to soy protein enhances WbPS in patients with COPD and alters interorgan protein metabolism in favor of the peripheral (muscle) compartment in healthy elderly and even more in COPD patients.

Eat the meat or feed the meat: protein turnover in remodeling muscle

PURPOSE OF REVIEW

The present review outlines the role of muscle protein turnover in muscle remodeling, with emphasis on the effects of nutrition and exercise.

RECENT FINDINGS

Progress in our understanding of the pathways signaling and regulating protein synthesis and degradation, and thus protein turnover, in skeletal muscle has been substantial over the past decade. Protein synthesis and degradation jointly allow the active remodeling of skeletal muscle to adapt to changes in mechanical and metabolic demand. Nutrition and exercise are potent ways to stimulate protein turnover. This occurs in an amino acid and exercise-type (resistance versus endurance) and mode (lengthening and shortening)-specific manner.

SUMMARY

For optimal muscle remodeling, the timing and type of feeding and exercise appear to be crucial.

Branched-chain amino acids as fuels and anabolic signals in human muscle

During exercise, there is an increase in amino acid (AA) oxidation accompanied by a depression in whole-body protein synthesis and an increase in protein breakdown. Leucine oxidation increases in proportion to energy expenditure, but the total contribution of BCAA to fuel provision during exercise is minor and insufficient to increase dietary protein requirements. When investigating the effects of AA on the control of muscle protein synthesis (MPS), we showed that increased availability of mixed AAs caused a rise in human MPS to about the same extent as complete meals. Leucine alone (and to some extent other essential, but not nonessential, AAs) can stimulate MPS for a short period, suggesting that leucine acts as a signal as well as a substrate. MPS stimulation by infused AAs shows tachyphylaxis, returning to basal rates after 2 h, possibly explaining why chronically elevated leucine delivery does not elevate MPS clinically. Increased availability of essential amino acids (EAAs) results in dose-related responses of MPS, but, in elderly subjects, there is blunted sensitivity and responsiveness associated with decreased total RNA and mRNA for signaling proteins and signaling activity. Increases of MPS due to EAAs are associated with elevation of signaling activity in the mammalian target of rapamycin (mTOR)/p70 ribosomal subunit S6 kinase eukaryotic initiation factor 4 binding protein 1 pathway, without requiring rises of plasma insulin availability above 10 microU/mL. However, at insulin of <5 microU/mL, AAs appear to stimulate MPS without increasing mTOR signaling. Further increasing availability of insulin to postprandial values increases signaling activity, but has no further effect on MPS.

Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise

BCAAs (leucine, isoleucine, and valine), particularly leucine, have anabolic effects on protein metabolism by increasing the rate of protein synthesis and decreasing the rate of protein degradation in resting human muscle. Also, during recovery from endurance exercise, BCAAs were found to have anabolic effects in human muscle. These effects are likely to be mediated through changes in signaling pathways controlling protein synthesis. This involves phosphorylation of the mammalian target of rapamycin (mTOR) and sequential activation of 70-kD S6 protein kinase (p70 S6 kinase) and the eukaryotic initiation factor 4E-binding protein 1. Activation of p70 S6 kinase, and subsequent phopsphorylation of the ribosomal protein S6, is associated with enhanced translation of specific mRNAs. When BCAAs were supplied to subjects during and after one session of quadriceps muscle resistance exercise, an increase in mTOR, p70 S6 kinase, and S6 phosphorylation was found in the recovery period after the exercise with no effect of BCAAs on Akt or glycogen synthase kinase 3 (GSK-3) phosphorylation. Exercise without BCAA intake led to a partial phosphorylation of p70 S6 kinase without activating the enzyme, a decrease in Akt phosphorylation, and no change in GSK-3. It has previously been shown that leucine infusion increases p70 S6 kinase phosphorylation in an Akt-independent manner in resting subjects; however, a relation between mTOR and p70 S6 kinase has not been reported previously. The results suggest that BCAAs activate mTOR and p70 S6 kinase in human muscle in the recovery period after exercise and that GSK-3 is not involved in the anabolic action of BCAAs on human muscle.

Body maintenance and repair: how food and exercise keep the musculoskeletal system in good shape

This article provides a personal view of how feeding and exercise acutely modify protein metabolism of human skeletal muscle, with discussion of the anabolic signalling mechanisms involved and some new findings on the metabolism of the turnover of collagen, tendon and bone.

Observations of branched-chain amino acid administration in humans

Since the in vitro study of Buse and Reid in 1975 showing a stimulatory effect of leucine upon rat muscle protein synthesis and reduction in proteolysis, a similar effect has been sought in humans. In 1978, Sherwin demonstrated in humans an improvement in N balance with infusion of leucine in obese subjects fasting to lose weight. A variety of subsequent studies have been performed in humans where leucine alone or the BCAAs have been administered in varying amounts and durations, and the effect upon protein metabolism has been measured. Measurements of changes in muscle amino acid metabolism were made by arteriovenous difference measurements and by biopsies. An anabolic effect of leucine and the branched-chain amino acids (BCAAs) on reduction of muscle protein breakdown was found in these studies, with no measured effect upon muscle protein synthesis. Later studies using stable isotope tracers to define both whole-body protein turnover and leg or arm protein metabolism have similarly concluded that leucine administration specifically induces a reduction in protein breakdown without increasing protein synthesis. This anabolic effect, produced through a reduction of protein breakdown in vivo in humans by leucine is contrary to in vitro studies of rat muscle where stimulation of protein synthesis, has been demonstrated by leucine. Likewise an increase in protein synthesis has also been demonstrated by insulin in rat muscle that is not seen in humans. Of the various studies administering BCAAs or leucine to humans for varying periods of time and amount, the results have been consistent. In addition, no untoward effects have been reported in any of these studies from infusion of the BCAAs at upward 3 times basal flux or 6 times normal dietary intake during the fed portion of the day.

Control of the Size of the Human Muscle Mass

This review is divided into two parts, the first dealing with the cell and molecular biology of muscle in terms of growth and wasting and the second being an account of current knowledge of physiological mechanisms involved in the alteration of size of the human muscle mass. Wherever possible, attempts have been made to interrelate the information in each part and to provide the most likely explanation for phenomena that are currently only partially understood. The review should be of interest to cell and molecular biologists who know little of human muscle physiology and to physicians, physiotherapists, and kinesiologists who may be familiar with the gross behavior of human muscle but wish to understand more about the underlying mechanisms of change.

Human muscle protein synthesis is modulated by extracellular, not intramuscular amino acid availability: a dose-response study

To test the hypothesis that muscle protein synthesis (MPS) is regulated by the concentration of extracellular amino acids, we investigated the dose-response relationship between the rate of human MPS and the concentrations of blood and intramuscular amino acids. We increased blood mixed amino acid concentrations by up to 240 % above basal levels by infusion of mixed amino acids (Aminosyn 15, 44-261 mg kg-1 h-1) in 21 healthy subjects, (11 men 10 women, aged 29 +/- 2 years) and measured the rate of incorporation of D5-phenylalanine or D3-leucine into muscle protein and blood and intramuscular amino acid concentrations. The relationship between the fold increase in MPS and blood essential amino acid concentration ([EAA], mM) was hyperbolic and fitted the equation MPS = (2.68 x [EAA])/(1.51 + [EAA]) (P < 0.01). The pattern of stimulation of myofibrillar, sarcoplasmic and mitochondrial protein was similar. There was no clear relationship between the rate of MPS and the concentration of intramuscular EAAs; indeed, when MPS was increasing most rapidly, the concentration of intramuscular EAAs was below basal levels. We conclude that the rates of synthesis of all classes of muscle proteins are acutely regulated by the blood [EAA] over their normal diurnal range, but become saturated at high concentrations. We propose that the stimulation of protein synthesis depends on the sensing of the concentration of extracellular, rather than intramuscular EAAs.

BCAA intake affects protein metabolism in muscle after but not during exercise in humans

Branched-chain amino acids (BCAA) or a placebo was given to seven subjects during 1 h of ergometer cycle exercise and a 2-h recovery period. Intake of BCAA did not influence the rate of exchange of the aromatic amino acids, tyrosine and phenylalanine, in the legs during exercise or the increase in their concentration in muscle. The increase was approximately 30% in both conditions. On the other hand, in the recovery period after exercise, a faster decrease in the muscle concentration of aromatic amino acids was found in the BCAA experiment (46% compared with 25% in the placebo condition). There was also a tendency to a smaller release (an average of 32%) of these amino acids from the legs during the 2-h recovery. The results suggest that BCAA have a protein-sparing effect during the recovery after exercise, either that protein synthesis has been stimulated and/or protein degradation has decreased, but the data during exercise are too variable to make any conclusions about the effects during exercise. The effect in the recovery period does not seem to be mediated by insulin.

Branched-chain amino acids

The branched-chain amino acids (BCAA), isoleucine, leucine and valine, are unique in that they are principally metabolized extrahepatically in the skeletal muscle. This observation led to the investigation of these nutrients in a number of clinical scenarios. By far the most intensively studied applications for BCAA have been in patients with liver failure and/or patients in catabolic disease states. However, the resulting studies have not demonstrated a clear clinical benefit for BCAA nutritional supplements. In patients with liver failure, the BCAA did improve nitrogen retention and protein synthesis, but their effect on patient outcome was less clear. Similarly, in critically ill septic patients, BCAA did not improve either survival or morbidity. The BCAA are important nutrients, and it seems that any specific benefits associated with their use will be based upon a greater understanding of the underlying cellular biology. Potential areas of further research may include the combination of BCAA supplements with other anabolic factors (e.g. growth hormone) in managing patients with catabolic disease states.

Leucine supplementation and intensive training

Leucine, isoleucine and valine, the branched-chain amino acids (BCAA), make up about one-third of muscle protein. Of these, leucine has been the most thoroughly investigated because its oxidation rate is higher than that of isoleucine or valine. Leucine also stimulates protein synthesis in muscle and is closely associated with the release of gluconeogenic precursors, such as alanine, from muscle. Significant decreases in plasma or serum levels of leucine occur following aerobic (11 to 33%), anaerobic lactic (5 to 8%) and strength exercise (30%) sessions. In skeletal muscle, there is a decrease in leucine level and a reduction in glycogen stores during exhaustive aerobic exercise. Basal fasting serum leucine levels decrease by 20% during 5 weeks of speed and strength training in power-trained athletes on a daily protein intake of 1.26 g/kg bodyweight. The leucine content of protein is assumed to vary between 5 and 10%. There are suggestions that the current recommended dietary intake of leucine be increased from 14 mg/kg bodyweight/day to a minimum of 45 mg/kg bodyweight/day for sedentary individuals, and more for those participating in intensive training in order to optimise rates of whole body protein synthesis. Consumption of BCAA (30 to 35% leucine) before or during endurance exercise may prevent or decrease the net rate of protein degradation, may improve both mental and physical performance and may have a sparing effect on muscle glycogen degradation and depletion of muscle glycogen stores. However, leucine supplementation (200 mg/kg bodyweight) 50 minutes before anaerobic running exercise had no effect on performance. During 5 weeks of strength and speed training, leucine supplementation of 50 mg/kg bodyweight/day, supplementary to a daily protein intake of 1.26 g/kg bodyweight/day, appeared to prevent the decrease in the serum leucine levels in power-trained athletes. According to 1 study, dietary supplementation of the leucine metabolite beta-hydroxy-beta-methylbutyrate (HMB) 3 g/day to humans undertaking intensive resistance training exercise resulted in an increased deposition of fat-free mass and an accompanying increase in strength. Muscle proteolysis was also decreased with HMB, accompanied by lower plasma levels of enzymes indicating muscle damage and an average 50% decrease in plasma essential amino acid levels. Furthermore, BCAA supplementation (76% leucine) in combination with moderate energy restriction has been shown to induce significant and preferential losses of visceral adipose tissue and to allow maintenance of a high level of performance. Caution must be paid when interpreting the limited number of studies in this area since, in many studies, leucine has been supplemented as part of a mixture of BCAA. Consequently, further research into the effects of leucine supplementation alone is needed.

Influence of ingesting a solution of branched-chain amino acids on plasma and muscle concentrations of amino acids during prolonged submaximal exercise

On two occasions, seven male endurance-trained cyclists performed sustained exhaustive exercise with reduced muscle glycogen stores. During exercise, the subjects were supplied in random order with an aqueous solution of branched-chain amino acids (BCAA) or flavored water (placebo). Ingestion of BCAA caused the concentration of these amino acids to increase by 135% in the plasma and by 57% in muscle tissue during exercise, whereas in the placebo trial there was no change or a slight decrease in the concentration in plasma and a decrease of 18% in the muscle. The plasma concentration of alanine increased by 48% during exercise when BCAA were ingested, and the increase in the muscle concentration of alanine during exercise was larger (70% versus 31% in the placebo trial), suggesting an increased rate of alanine production. Also, the plasma concentration of arginine increased by 14% during exercise when BCAA were ingested, whereas there was no change during exercise in the placebo trial. There was a smaller decrease in the muscle glutamate concentration during exercise in the BCAA trial (32% versus 47% in the placebo trial; p < 0.05), but, for the remaining amino acids, there was no difference between the BCAA and placebo trials. There was a significant decrease in the muscle glycogen concentration during exercise in the placebo trial, whereas only a small decrease was found in the BCAA trial (28 and 9 mmol/kg wet wt [p < 0.05] in the placebo and BCAA trial, respectively). This might indicate that an increased supply of BCAA has a sparing effect on muscle glycogen degradation during exercise.

Effect of branched-chain amino acid and carbohydrate supplementation on the exercise-induced change in plasma and muscle concentration of amino acids in human subjects

Five male endurance-trained subjects performed exhaustive exercise on a cycle ergometer at a work rate corresponding to 75% of their VO2max after reduction of their muscle glycogen stores. During exercise the subjects were given in random order a 6% carbohydrate solution continuing 7 g L-1 of branched-chain amino acids (BCAA), a 6% CHO solution and flavoured water. The physical performance was lowered in four of the five subjects when they were given flavoured water during exercise as compared with the two conditions when CHO was supplied. No difference in performance was found when the subjects were given CHO + BCAA or only CHO during exercise. When CHO + BCAA was supplied the plasma and muscle (vastus lateralis) concentrations of BCAA increased during exercise by 120 and 35%, respectively. In the other conditions there was no change or a slight decrease in the plasma concentrations of BCAA, but the muscle concentrations of BCAA were decreased after exercise. The plasma concentration of glutamine over the whole exercise period and 5 min after exercise was higher when CHO + BCAA were supplied during exercise compared with a supply of CHO alone or water. However, exercise caused no change in the muscle concentration of glutamine, whereas that of glutamate decreased in all three conditions. A supply of CHO + BCAA or CHO alone did not affect the exercise-induced increase in the plasma and muscle concentration of aromatic amino acids, indicating that neither BCAA nor CHO influenced the net protein degradation during exercise.

The separate and combined effect of leucine and insulin on muscle free amino acids

The effect of insulin and leucine on amino acid and protein metabolism in muscle is not fully understood. To characterize their separate and combined effects on free amino acids in muscle and plasma, 11 volunteers received an infusion of either leucine (1 g h-1, Group 1) or glucose (20 g h-1, Group 2) for 2 h followed by a combination of the two infusions for an additional 2-h period. In muscle both the leucine infusion and the leucine plus glucose infusion increased the concentration of free leucine significantly, while the sum of the other branched chain amino acids (BCAA), of the aromatic amino acids and of the basic amino acids decreased. Glucose infusion alone decreased the sum of the essential amino acids, the BCAA and the aromatic amino acids. The combination of leucine and glucose augmented the decreases, while the concentrations of glutamate, glutamine and alanine were unaffected. In plasma the leucine infusion doubled the leucine concentration and decreased alanine, valine, methionine, tyrosine, phenylalanine and the sum of the aromatic amino acids. Glucose infusion decreased methionine, serine, isoleucine and the sum of the essential amino acids and of the BCAA. The combination of leucine infusion and hyperinsulinaemia augmented the decreases. The plasma concentrations of the keto acids of valine and isoleucine decreased by the leucine infusion while the concentrations of the keto acid of leucine and isoleucine decreased by glucose infusion. The combination of leucine and glucose had an additive effect. These effects are attributed to a specific effect of leucine on the other two BCAA and a depression of muscle proteolysis by both leucine and insulin, resulting from glucose infusion.

Effect of branched-chain amino acid supplementation on the exercise-induced change in aromatic amino acid concentration in human muscle

A mixture of the three branched-chain amino acids (BCAAs) was supplied to subjects during two types of sustained intense exercise, a 30 km cross-country race and a full marathon, and the effect on plasma and muscle concentrations of aromatic and BCAAs was studied. When BCAAs (7.5-12 g) were taken during exercise, the plasma and muscle (vastus lateralis) concentration of these amino acids increased, while in the placebo groups the concentration of BCAAs decreased in the plasma and remained unchanged in the muscle. In the placebo group, both types of exercise caused a 20-40% increase in the muscle concentration of the aromatic amino acids, tyrosine and phenylalanine, and the plasma concentration of these amino acids was increased after the marathon. Since tyrosine and phenylalanine are neither taken up nor metabolized by skeletal muscle, the increases in their concentrations in muscle might indicate net protein degradation during exercise. However, when the subjects were supplied with BCAAs during exercise, the increases in tyrosine and phenylalanine concentrations in both muscle and plasma were prevented. These results suggest that an intake of BCAAs during exercise can prevent or decrease the net rate of protein degradation caused by heavy exercise.