Determination of the tolerable upper intake level of leucine in adult men

Analytical Chemistry of Impurities in Amino Acids Used as Nutrients: Recommendations for Regulatory Risk Management

Proteinogenic amino acids are natural nutrients ingested daily from standard foods. Commercially manufactured amino acids are added to a wide range of nutritional products, including dietary supplements and regular foods. Currently, the regulatory risk management of amino acids is conducted by means of setting daily maximum limits of intake. However, there have been no reported adverse effects of amino acid overdosing, while impurities in low-quality amino acids have been identified as causative agents in several health hazard events. This paper reviews the analytical chemistry of impurities in amino acids and highlights major variations in the purity of commercial products. Furthermore, it examines the international standards and global regulatory risk assessment of amino acids utilized in dietary supplements and foods, recommending (1) further research on analytical methods that can comprehensively separate impurities in amino acids, and (2) re-focusing on the regulatory risk management of amino acids to the analytical chemistry of impurities.

Whey Protein, L-Leucine and Vitamin D Supplementation for Preserving Lean Mass during a Low-Calorie Diet in Sarcopenic Obese Women

In sarcopenic obese subjects it is essential to reduce body weight and preserve lean mass, in order to avoid a worsening of muscle function. Several studies have shown that leucine supplementation can be useful to improve skeletal muscle mass in sarcopenic patients. The aim of our study was to evaluate the effectiveness of a short-term low-calorie diet (LCD) combined with supplementation with whey protein and leucine on weight loss, lean mass and muscle strength in sarcopenic, obese, hyperinsulinemic and post-menopausal women. Sixteen females with a mean age of 60 years (range: 50–70 years), BMI 37.6 kg/m² (range: 31.7–44.1 Kg/m²), HOMA-index ≥ 2.5 (range: 2.9–12) were assigned to an LCD regimen (1000 kcal/day) with supplementation of 18 g whey proteins which 4.1 g of leucine for 45 days. Anthropometric indexes, blood and urine chemistry, body composition by DEXA, muscle strength by handgrip test and Short Physical Performance Battery (SPPB) were assessed at baseline and at the end of the treatment. A significant reduction in BMI (37.6 vs. 35.7 Kg/m²), waist circumference (107 vs. 102.4 cm), HOMA index (4.8 vs. 2.3) and fasting insulin (17.4 vs. 10.4 μIU/mL) was observed in all patients. Women preserved total lean body mass (55 vs. 5%) and significantly improved their muscle strength, as measured by handgrip (15.3 vs. 20.1 Kg), and their muscle function, as measured by SPPB (7.5 vs. 8.9). A significant increase in BUN was also observed (36.1 vs. 46.3). We conclude that LCD with adequate protein intake and supplementation with whey protein and leucine should be promoted to maintain muscle mass and improve muscle strength in post-menopausal women with sarcopenic obesity.

Pyruvate Might Bridge Gut Microbiota and Muscle Health in Aging Mice After Chronic High Dose of Leucine Supplementation

Background: The previous studies demonstrated that there might be complex and close relationships among leucine supplementation, gut microbiota, and muscle health, which still needs further investigation.

Aims: This study aimed to explore the associations of gut microbiota with muscle health after leucine intake.

Methods: In this study, 19-month-old male C57BL/6j mice (n = 12/group) were supplemented with ultrapure water, low dose of leucine (500 mg/kg·d), and high dose of leucine (1,250 mg/kg·d) for 12 weeks by oral gavage. The mice fecal samples in each group before and after supplementation were collected for baseline and endpoint gut microbiota analysis by using 16S rDNA amplicon sequencing. Meanwhile, ultrasound measurement, H&E staining, myofiber cross-sectional area (CSA) measurement, and western blotting were performed in the quadriceps subsequently. The pyruvate levels were detected in feces.

Results: Improvement in muscle of histology and ultrasonography were observed after both low and high dose of leucine supplementation. High dose of leucine supplementation could promote skeletal muscle health in aging mice via regulating AMPKα/SIRT1/PGC-1α. The richness and diversities of microbiota as well as enriched metabolic pathways were altered after leucine supplementation. Firmicutes-Bacteroidetes ratio was significantly decreased in high-leucine group. Moreover, pyruvate fermentation to propanoate I were negatively associated with differential species and the pyruvate levels were significantly increased in feces after high dose of leucine supplementation.

Conclusions: Chronic high dose of leucine supplementation changed gut microbiota composition and increased pyruvate levels in the feces, which possibly provides a novel direction for promoting muscle health in aging mice.

Tolerable amounts of amino acids for human supplementation: summary and lessons from published peer-reviewed studies

Amino acid supplementation may be indicated to correct for insufficient amino acid intake in healthy individuals, and in specific physiological or pathophysiological situations. However, there is a concern to not supplement beyond the tolerable upper intake level (UL) by determining parameters of no-observed-adverse-effect level (NOAEL) or lowest-observed-adverse-effect level (LOAEL) for each amino acid. Since the NOAEL and LOAEL values are at least one order of magnitude different when comparing the values obtained in rats and humans, the aim of this review is to evaluate to what extent the amino acid UL measured in the rat model, when referenced to the dietary usual consumption (UC) and dietary requirement (RQ) for indispensable amino acids, may be used as an approximation of the UL in humans. This review then compares the ratios of the NOAEL or LOAEL over UC and RQ in the rat model with the same ratios calculated in humans for the nine amino acids (arginine, serine, glycine, histidine, leucine, lysine, methionine, phenylalanine, and tryptophan) for which this comparison can be done. From the calculations made, it appears that for these 9 amino acids, the calculated ratios for rats and humans, although rather different for several amino acids, remains for all of them in the same order of magnitude. For tryptophan, tyrosine, and valine, the ratios calculated in rats are markedly different according to the sex of animals, raising the view that it may be also the case in humans.

Vcp Overexpression and Leucine Supplementation Increase Protein Synthesis and Improve Fear Memory and Social Interaction of Nf1 Mutant Mice

Neurofibromatosis type 1 (NF1) is a dominant genetic disorder manifesting, in part, as cognitive defects. Previous study indicated that neurofibromin (NF1 protein) interacts with valosin-containing protein (VCP)/P97 to control dendritic spine formation, but the mechanism is unknown. Here, using Nf1^+/- mice and transgenic mice overexpressing wild-type Vcp/p97, we demonstrate that neurofibromin acts with VCP to control endoplasmic reticulum (ER) formation and consequent protein synthesis and regulates dendritic spine formation, thereby modulating contextual fear memory and social interaction. To validate the role of protein synthesis, we perform leucine supplementation in vitro and in vivo. Our results suggest that leucine can effectively enter the brain and increase protein synthesis and dendritic spine density of Nf1^+/- neurons. Contextual memory and social behavior of Nf1^+/- mice are also restored by leucine supplementation. Our study suggests that the "ER-protein synthesis" pathway downstream of neurofibromin and VCP is a critical regulator of dendritic spinogenesis and brain function.

Low-fat, lactose-free and leucine-enriched chocolate cow milk prototype: A preliminary study on sensorial acceptability and gastrointestinal complaints following exhaustive exercise

BACKGROUND

Chocolate milk has gained recent scientific support as a recovery drink. However, it is known that high exercise-demand triggers gastrointestinal discomfort which continues post-exercise, thereby hindering this nutritional strategy. In addition, those who are lactose intolerant cannot benefit from a milk-based beverage. Thus, the aim of this preliminary study was to develop a low-fat, lactose-free, and leucine-enriched chocolate cow milk prototype (CML) representing nutrition-related recommendations for football players, as well as assess athletes' individual subjective outcomes for gastrointestinal complaints and sensorial acceptability in a field-based setting following strenuous team-sport physical demands.

METHODS

This study followed a single group and repeated-measured design with 10 football players (23 ± 2 yrs., 74 ± 14 kg, 174 ± 5 cm) who consumed CML following a 90-min football match simulation protocol (FMP). The total CML intake to achieve 0.150 g leucine·kg [BW]·h^- 1 occurred in aliquots of 50, 30 and 20% at 0-, 45- and 75-min post-FMP, respectively. Athletes were evaluated by the prevalence, the type and severity (bloating, nausea, flatulence, and gastric reflux) of gastrointestinal complaints and sensorial acceptability (overall perception, appearance, consistency, and flavour) after drinking each aliquot in a 4-h recovery period.

RESULTS

The CML showed higher scores for "Product Acceptability Index" (88%) and sensorial acceptability (~ 8 in 9-point hedonic scale). Kendall's W with bootstrapped resample (95%CI) revealed agreement among respondents as "moderate" (overall perception, flavour) to "strong" (appearance, consistency) and with no significant agreement differences between rater response in the timeline analysis (0.57 up to 0.87; p > 0.05). Agresti-Caffo add-4 analysis (95% confidence interval, [95%CI]) revealed no differences in each time-point analysis versus baseline for athletes classified as having severe gastrointestinal symptoms, but confirmed concern with bloating (three athletes showed a transient response at 2-h and only one continued until 3-h; p = 0.051).

CONCLUSIONS

These preliminary findings suggest that CML presents good taste and high acceptability by the sampled athletes. Thus, CML may be an alternative sport drink for immediate post-workout supplementation to overcome the energy deficit, offer co-ingested leucine, maintain palatability and adherence including lactose intolerance following a team sport-specific fatigue.

TRIAL REGISTRATION

RBR-2vmpz9 , 10/12/2019, retrospectively registered.

Effects of oral administration of β-hydroxy β-methylbutyrate on lean body mass in older adults: a systematic review and meta-analysis

OBJECTIVE

Beta-hydroxy beta-methylbutyrate (HMB), a metabolite of leucine, is currently widely used in athletes to increase muscle mass and strength and has also been used as a nutritional supplement in recent years to maintain muscle mass in muscular atrophic diseases of older people. However, the effects of HMB supplementation on muscle mass, muscle strength, and physical function in older people remain controversial. The purpose of this review was to explore the effects of HMB on body composition in older adults.

METHODS

The PubMed, EMBASE and Cochrane Central Register of Controlled Trials databases were searched to obtain the randomized controlled trials needed as a basis for systematic review and meta-analysis.

RESULT

A total of 9 studies (448 participants) were eventually found eligible. The pooled results showed that HMB supplementation significantly increased fat-free mass in older people compared with the control group (effect size: 0.37; 95% Cl 0.16, 0.58; Z value = 3.47, P = 0.001; Fixed-effect model). But it had no effect on fat mass (effect size: - 0.04 95% CI - 0.26, 0.18; Z value = 0.36, P = 0.716, fixed-effect model). Subgroup analysis of HMB supplementation alone showed a significant improvement in fat-free mass in older people (effect size: 0.59; 95% CI 0.32, 0.87; Z = 4.24, P < 0.001; fixed-effect model), whereas HMB supplementation combined with exercise intervention showed no additional fat-free mass change (effect size: 0.06; 95% CI - 0.26, 0.38; Z = 0.38, P = 0.705; Fixed-effect model).

CONCLUSION

HMB supplementation is beneficial for improving body composition in older people. However, the effect of HMB supplementation combined with exercise therapy to improve muscle mass is not obvious. Exercise programs need to be designed according to the different physical health of older people.

International Regulations on Amino Acid Use in Foods and Supplements and Recommendations to Control Their Safety Based on Purity and Quality

We examined international regulatory developments related to the use of proteinogenic amino acids in human nutrition and concluded that the current risk-assessment practices tend to focus exclusively on setting maximum daily limits. In this brief review we argue that controlling the standards of purity and ingredient quality are the key safety issues that should be considered during risk assessment. Moreover, if maximum intake limits on amino acids are implemented, they should be defined using a well-established rationale for the health risks associated with high intakes. This would avoid setting limits that are so low that they render the dietary supplements ineffective and which, therefore, could mislead the consumer. We further suggest that there should be greater regional concordance in how the use of amino acids as ingredients is regulated and use the capacity of industry to oversee pre-competitive issues, such as standards of purity and scientific research on the safety of generic ingredients. Our arguments are based on clinical safety scientific research and oversights of amino acid purity standards conducted in the last decade by the not-for-profit international association, the International Council on Amino Acid Science.

Proposals for Upper Limits of Safe Intake for Methionine, Histidine, and Lysine in Healthy Humans

Based on research presented during the 10th Amino Acid Assessment Workshop, no observed adverse effect levels (NOAELs) for supplemental methionine at 46 mg/(kg·d) (∼3.2 g/d), for supplemental histidine at 8.0 g/d, and for supplemental lysine at 6.0 g/d have been proposed. These NOAELs are relevant to healthy adults and are applicable only to high-purity amino acids administered in fortified foods or dietary supplements. Because individuals are exposed to the above supplemental amino acids in the context of complex combinations of essential amino acids or individually in dietary supplements for various physiologic benefits, such as body fat reduction, skin conditioning, mental energy increase, or herpes simplex treatments, the above safety recommendations will make an important contribution to regulatory and nutritional practices.

Safety and tolerability of 6-month supplementation with a vitamin D, calcium and leucine-enriched whey protein medical nutrition drink in sarcopenic older adults

Aims

Safety and tolerability of prolonged supplementation with a vitamin D, calcium and leucine-enriched whey protein medical nutrition drink (WP-MND) was evaluated in sarcopenic older adults.

Methods

A 13-week double-blinded, randomized, isocaloric placebo-controlled trial (PROVIDE study; n = 380) was extended with a voluntary 13-week open-label extension (OLE). OLE participants were randomized to receive daily 1 or 2 servings of WP-MND (21 g protein, 3 g leucine, 10 µg vitD and 500 mg calcium per serving). Gastro-intestinal tolerability, kidney function and serum levels of calcidiol, parathyroid hormone (PTH) and calcium were evaluated at week 0, 13 and 26.

Results and discussion

In response to the high daily protein intake (median1.5; IQR: 1.3, 1.7 g/kg BW/day), the estimated glomerular filtration rate (eGFR) increased in the test group during the RCT (p = 0.013). The same trend was observed for those participants with moderate chronic kidney disease. During OLE no eGFR change was observed in any of the groups. Serum calcidiol and calcium reached a plateau after 13-week WP-MND supplementation. As expected, PTH significantly changed in the opposite direction, decreasing during RCT in the test group (T vs C: p < 0.001) and during OLE in former control groups. During RCT, 20/366 participants with normal baseline calcidiol reached levels ≥ 100 nmol/L (T: n = 18; C: n = 2) and 6 developed albumin-corrected calcium levels > 2.55 mmol/L (T: n = 3; C: n = 3), without associated adverse events.

Conclusion

A 6 months intervention with up to 2 servings of WP-MND did neither result in kidney function deterioration nor symptoms of vitamin D or calcium toxicity. The product was overall well tolerated.

The Effect of Beta-Alanine versus Alkaline Agent Supplementation Combined with Branched-Chain Amino Acids and Creatine Malate in Highly-Trained Sprinters and Endurance Athletes: A Randomized Double-Blind Crossover Study

The study aimed to verify the effect of intra- (beta-alanine—BA) versus extra- (alkaline agents—ALK) cellular buffering agent supplementation, combined with customarily used branched-chain amino acids (BCAAs) and creatine malate (TCM) treatment in natural training conditions. Thirty-one elite athletes (11 sprinters and 20 endurance athletes) participated in the study. Eight-week randomized double-blind, crossover, combined supplementation with BA-ALKpla_BCAA&TCM and ALK-BApla_BCAA&TCM was implemented. In the course of the experiment, body composition, aerobic capacity, and selected blood markers were assayed. After BA-ALKpla_BCAA&TCM supplementation, total fat-free mass increased in sprinters (p = 0.009). No other differences were found in body composition, respiratory parameters, aerobic capacity, blood lactate concentration, and hematological indices after BA-ALKpla_BCAA&TCM/ALK-BApla_BCAA&TCM supplementation. The maximum post-exercise blood ammonia (NH₃) concentration decreased in both groups after BA-ALKpla_BCAA&TCM supplementation (endurance, p = 0.002; sprint, p < 0.0001). Also, lower NH₃ concentrations were observed in endurance athletes in the post-exercise recovery period. The results of our study indicate that combined BCAA, TCM, and BA supplementation is more effective than combined BCAA, TCM and ALK supplementation for an increase in fat-free mass and exercise adaptation, but not for aerobic capacity improvement. Besides, it seems that specific exercise stimuli and the training status are key factors affecting exercise performance, even in athletes using efficient supplementation.

Leucine Promotes Milk Synthesis in Bovine Mammary Epithelial Cells via the PI3K-DDX59 Signaling

Leucine is an essential amino acid in the milk production of bovine mammary glands, but the regulatory roles and molecular mechanisms of leucine are still not known well. This study investigated the roles of leucine on milk synthesis and explored the corresponding mechanism in bovine mammary epithelial cells (BMECs). Leucine (0, 0.25, 0.5, 0.75, 1.0, and 1.25 mM) was added to BMECs that were cultured in FBS-free OPTI-MEM medium. Leucine significantly promoted milk protein and milk fat synthesis and also increased phosphorylation of mTOR signaling protein and the protein expression levels of SREBP-1c, with the most significant effects at 0.75 mM concentration. Leucine increased the expression and nuclear localization of DDX59, and loss and gain of gene function experiments further reveal that DDX59 mediates the stimulation of leucine on the mRNA expression variation of mTOR and SREBP-1c genes. PI3K inhibition experiment further detected that leucine upregulated expression of DDX59 and its downstream signaling via PI3K activation. ChIP-qPCR analysis further proved the binding of DDX59 to the promoter regions of mTOR and SREBP-1c. In summary, these data prove that DDX59 positively regulates the mTOR and SREBP-1c signaling pathways leading to synthesis of milk, and leucine regulates these two signaling pathways through the PI3K-DDX59 signaling.

Supplements with purported effects on muscle mass and strength

PURPOSE

Several supplements are purported to promote muscle hypertrophy and strength gains in healthy subjects, or to prevent muscle wasting in atrophying situations (e.g., ageing or disuse periods). However, their effectiveness remains unclear.

METHODS

This review summarizes the available evidence on the beneficial impacts of several popular supplements on muscle mass or strength.

RESULTS

Among the supplements tested, nitrate and caffeine returned sufficient evidence supporting their acute beneficial effects on muscle strength, whereas the long-term consumption of creatine, protein and polyunsaturated fatty acids seems to consistently increase or preserve muscle mass and strength (evidence level A). On the other hand, mixed or unclear evidence was found for several popular supplements including branched-chain amino acids, adenosine triphosphate, citrulline, β-Hydroxy-β-methylbutyrate, minerals, most vitamins, phosphatidic acid or arginine (evidence level B), weak or scarce evidence was found for conjugated linoleic acid, glutamine, resveratrol, tribulus terrestris or ursolic acid (evidence level C), and no evidence was found for other supplements such as ornithine or α-ketoglutarate (evidence D). Of note, although most supplements appear to be safe when consumed at typical doses, some adverse events have been reported for some of them (e.g., caffeine, vitamins, α-ketoglutarate, tribulus terrestris, arginine) after large intakes, and there is insufficient evidence to determine the safety of many frequently used supplements (e.g., ornithine, conjugated linoleic acid, ursolic acid).

CONCLUSION

 In summary, despite their popularity, there is little evidence supporting the use of most supplements, and some of them have been even proven ineffective or potentially associated with adverse effects.

Branched-chain amino acids promote endothelial dysfunction through increased reactive oxygen species generation and inflammation

Branched‐chain amino acids (BCAA: leucine, isoleucine and valine) are essential amino acids implicated in glucose metabolism and maintenance of correct brain function. Elevated BCAA levels can promote an inflammatory response in peripheral blood mononuclear cells. However, there are no studies analysing the direct effects of BCAA on endothelial cells (ECs) and its possible modulation of vascular function. In vitro and ex vivo studies were performed in human ECs and aorta from male C57BL/6J mice, respectively. In ECs, BCAA (6 mmol/L) increased eNOS expression, reactive oxygen species production by mitochondria and NADPH oxidases, peroxynitrite formation and nitrotyrosine expression. Moreover, BCAA induced pro‐inflammatory responses through the transcription factor NF‐κB that resulted in the release of intracellular adhesion molecule‐1 and E‐selectin conferring endothelial activation and adhesion capacity to inflammatory cells. Pharmacological inhibition of mTORC1 intracellular signalling pathway decreased BCAA‐induced pro‐oxidant and pro‐inflammatory effects in ECs. In isolated murine aorta, BCAA elicited vasoconstrictor responses, particularly in pre‐contracted vessels and after NO synthase blockade, and triggered endothelial dysfunction, effects that were inhibited by different antioxidants, further demonstrating the potential of BCAA to induce oxidative stress with functional impact. In summary, we demonstrate that elevated BCAA levels generate inflammation and oxidative stress in ECs, thereby facilitating inflammatory cells adhesion and endothelial dysfunction. This might contribute to the increased cardiovascular risk observed in patients with elevated BCAA blood levels.

High concentration of branched-chain amino acids promotes oxidative stress, inflammation and migration of human peripheral blood mononuclear cells via mTORC1 activation

Leucine, isoleucine and valine are essential aminoacids termed branched-chain amino acids (BCAA) due to its aliphatic side-chain. In several pathological and physiological conditions increased BCAA plasma concentrations have been described. Elevated BCAA levels predict insulin resistance development. Moreover, BCAA levels higher than 2mmol/L are neurotoxic by inducing microglial activation in maple syrup urine disease. However, there are no studies about the direct effects of BCAA in circulating cells. We have explored whether BCAA could promote oxidative stress and pro-inflammatory status in peripheral blood mononuclear cells (PBMCs) obtained from healthy donors. In cultured PBMCs, 10mmol/L BCAA increased the production of reactive oxygen species (ROS) via both NADPH oxidase and the mitochondria, and activated Akt-mTOR signalling. By using several inhibitors and activators of these molecular pathways we have described that mTOR activation by BCAA is linked to ROS production and mitochondrial dysfunction. BCAA stimulated the activation of the redox-sensitive transcription factor NF-κB, which resulted in the release of pro-inflammatory molecules, such as interleukin-6, tumor necrosis factor-α, intracellular adhesion molecule-1 or CD40L, and the migration of PBMCs. In conclusion, elevated BCAA blood levels can promote the activation of circulating PBMCs, by a mechanism that involving ROS production and NF-κB pathway activation. These data suggest that high concentrations of BCAA could exert deleterious effects on circulating blood cells and therefore contribute to the pro-inflammatory and oxidative status observed in several pathophysiological conditions.

Leucine supplementation enhances integrative myofibrillar protein synthesis in free-living older men consuming lower- and higher-protein diets: a parallel-group crossover study

BACKGROUND

Leucine co-ingestion with lower-protein (LP)-containing meals may overcome the blunted muscle protein synthetic response to food intake in the elderly but may be effective only in individuals who consume LP diets.

OBJECTIVE

We examined the impact of leucine co-ingestion with mixed macronutrient meals on integrated 3-d rates of myofibrillar protein synthesis (MyoPS) in free-living older men who consumed higher protein (HP) (1.2 g · kg^-1 · d^-1) or LP (0.8 g · kg^-1 · d^-1) in rested and resistance exercise (REX) conditions.

DESIGN

In a crossover design, 20 healthy older men [aged 65-85 y] were randomly assigned to receive LP or HP diets while ingesting a placebo (days 0-2) and Leu supplement (5 g leucine/meal; days 3-5) with their 3 main daily meals. A bout of unilateral REX was performed during the placebo and Leu treatments. Ingested ²H₂O and skeletal muscle biopsies were used to measure the 3-d integrated rate of MyoPS during the placebo and Leu treatments in the rested and REX legs.

RESULTS

Leucinemia was higher with Leu treatment than with placebo treatment (P < 0.001). MyoPS was similar in LP and HP during both treatments (P = 0.39) but was higher with Leu treatment than with placebo treatment in the rested (pooled mean ± SD: Leu, 1.57% ± 0.11%/d; placebo, 1.48% ± 0.08%/d; main effect of treatment: P < 0.001) and REX (pooled mean: Leu, 1.87% ± 0.09%/d; placebo, 1.71 ± 0.10%/d; main effect of treatment: P < 0.001) legs.

CONCLUSIONS

Leu co-ingestion with daily meals enhances integrated MyoPS in free-living older men in rested and REX conditions and is equally effective in older men who consume daily protein intakes greater than or equal to the RDA. This trial was registered at clinicaltrials.gov as NCT02371278.

Proposals for Upper Limits of Safe Intake for Arginine and Tryptophan in Young Adults and an Upper Limit of Safe Intake for Leucine in the Elderly

On the basis of research presented during the 9th Amino Acid Assessment Workshop, a No Observed Adverse Effect Level (NOAEL) for diet-added arginine (added mostly in the form of dietary supplements) of 30 g/d and an upper limit of safe intake (ULSI) for diet-added tryptophan (added mostly in the form of dietary supplements) of 4.5 g/d have been proposed. Both recommendations apply to healthy young adults. The total dietary leucine ULSI proposed for elderly individuals is 500 mg · kg^-1 · d^-1 All 3 recommendations are relevant only to high-quality amino acid-containing products with specifications corresponding to those listed in the US Pharmacopeia Because the above amino acids are extensively utilized as dietary supplements for various real or perceived benefits, such as vasodilation, spermatogenesis, sleep, mood regulation, or muscle recovery, the above safety recommendations will have an important impact on regulatory and nutritional practices.

Efficacy and Safety of Leucine Supplementation in the Elderly

Leucine supplementation has grown in popularity due to the discovery of its anabolic effects on cell signaling and protein synthesis in muscle. The current recommendation is a minimum intake of 55 mg ⋅ kg^-1. d^-1 Leucine acutely stimulates skeletal muscle anabolism and can overcome the anabolic resistance of aging. The value of chronic leucine ingestion for muscle growth is still unclear. Most of the research into leucine consumption has focused on efficacy. To our knowledge, very few studies have sought to determine the maximum safe level of intake. Limited evidence suggests that intakes of ≤1250 mg ⋅ kg^-1. d^-1 do not appear to have any health consequences other than short-term elevated plasma ammonia concentrations. Similarly, no adverse events have been reported for the leucine metabolite β-hydroxy-β-methylbutyrate (HMB), although no studies have tested HMB toxicity in humans. Therefore, future research is needed to evaluate leucine and HMB toxicity in the elderly and in specific health conditions.

The Measurement of Ammonia in Human Breath and its Potential in Clinical Diagnostics

Ammonia is an important component of metabolism and is involved in many physiological processes. During normal physiology, levels of blood ammonia are between 11 and 50 µM. Elevated blood ammonia levels are associated with a variety of pathological conditions such as liver and kidney dysfunction, Reye's syndrome and a variety of inborn errors of metabolism including urea cycle disorders (UCD), organic acidaemias and hyperinsulinism/hyperammonaemia syndrome in which ammonia may reach levels in excess of 1 mM. It is highly neurotoxic and so effective measurement is critical for assessing and monitoring disease severity and treatment. Ammonia is also a potential biomarker in exercise physiology and studies of drug metabolism. Current ammonia testing is based on blood sampling, which is inconvenient and can be subject to significant analytical errors due to the quality of the sample draw, its handling and preparation for analysis. Blood ammonia is in gaseous equilibrium with the lungs. Recent research has demonstrated the potential use of breath ammonia as a non-invasive means of measuring systemic ammonia. This requires measurement of ammonia in real breath samples with associated temperature, humidity and gas characteristics at concentrations between 50 and several thousand parts per billion. This review explores the diagnostic applications of ammonia measurement and the impact that the move from blood to breath analysis could have on how these processes and diseases are studied and managed.

Alterations in protein and amino acid metabolism in rats fed a branched-chain amino acid- or leucine-enriched diet during postprandial and postabsorptive states

Background

Many people believe in favourable effects of branched-chain amino acids (BCAAs; valine, leucine, and isoleucine), especially leucine, on muscle protein balance and consume BCAAs for many years. We determined the effects of the chronic intake of a BCAA- or leucine-enriched diet on protein and amino acid metabolism in fed and postabsorptive states.

Methods

Rats were fed a standard diet, a diet with a high content of valine, leucine, and isoleucine (HVLID), or a high content of leucine (HLD) for 2 months. Half of the animals in each group were sacrificed in the fed state on the last day, and the other half were sacrificed after overnight fast. Protein synthesis was assessed using the flooding dose method (L-[3,4,5-³H]phenylalanine), proteolysis on the basis of chymotrypsin-like activity (CHTLA) of proteasome and cathepsin B and L activities.

Results

Chronic intake of HVLID or HLD enhanced plasma levels of urea, alanine and glutamine. HVLID also increased levels of all three BCAA and branched-chain keto acids (BCKA), HLD increased leucine, ketoisocaproate and alanine aminotransferase and decreased valine, ketovaline, isoleucine, ketoisoleucine, and LDL cholesterol. Tissue weight and protein content were lower in extensor digitorum longus muscles in the HLD group and higher in kidneys in the HVLID and HLD groups. Muscle protein synthesis in postprandial state was higher in the HVLID group, and CHTLA was lower in muscles of the HVLID and HLD groups compared to controls. Overnight starvation enhanced alanine aminotransferase activity in muscles, and decreased protein synthesis in gastrocnemius (in HVLID group) and extensor digitorum longus (in HLD group) muscles more than in controls. Effect of HVLID and HLD on CHTLA in muscles in postabsorptive state was insignificant.

Conclusions

The results failed to demonstrate positive effects of the chronic consumption of a BCAA-enriched diet on protein balance in skeletal muscle and indicate rather negative effects from a leucine-enriched diet. The primary effects of both diets are an activated catabolism of BCAAs, which leads to an enhanced production of BCKA, alanine and glutamine and their utilization in visceral tissues and an impaired protein synthesis in postabsorptive state, particularly in fast-twitch (white) muscles.

Branched-chain amino acids as biomarkers in diabetes

PURPOSE OF REVIEW

Numerous human studies have consistently demonstrated that concentrations of branched-chain amino acids (BCAAs) in plasma and urine are associated with insulin resistance and have the quality to predict diabetes development. However, it is not known how altered BCAA levels link to insulin action and diabetes. This review addresses some recent findings in BCAA metabolism and discusses their role as reporter molecules of insulin sensitivity and diabetes and their possible contribution to disease progression.

RECENT FINDINGS

Changes in plasma and urine levels result mainly from altered metabolism in tissues and recent studies have thus focused on organ-specific changes in BCAA handling using animal models and human tissue samples. A decreased mitochondrial oxidation has been demonstrated in peripheral tissues and that was shown to be associated with an increased inflammatory tone and changes in adipokine levels (adiponectin and leptin). These changes appear already before insulin resistance is established. Key findings demonstrating the discordance between changes in BCAA and insulin resistance are derived from studies using insulin sensitizers and from data collected in patients undergoing Roux-en-Y bypass bariatric surgery. Intermediates derived from BCAA breakdown rather than BCAA itself were recently proposed to contribute to the development of insulin resistance and studies now explore the biomarker qualities of these metabolites.

SUMMARY

Understanding the mechanisms and putative causalities in the alterations in BCAA levels as found in obesity, metabolic syndrome and diabetes is crucial for any intervention options but also for the use of BCAA and derivatives as biomarkers in clinical routine.

Defining meal requirements for protein to optimize metabolic roles of amino acids

Dietary protein provides essential amino acids (EAAs) for the synthesis of new proteins plus an array of other metabolic functions; many of these functions are sensitive to postprandial plasma and intracellular amino acid concentrations. Recent research has focused on amino acids as metabolic signals that influence the rate of protein synthesis, inflammation responses, mitochondrial activity, and satiety, exerting their influence through signaling systems including mammalian/mechanistic target of rapamycin complex 1 (mTORC1), general control nonrepressed 2 (GCN2), glucagon-like peptide 1 (GLP-1), peptide YY (PYY), serotonin, and insulin. These signals represent meal-based responses to dietary protein. The best characterized of these signals is the leucine-induced activation of mTORC1, which leads to the stimulation of skeletal muscle protein synthesis after ingestion of a meal that contains protein. The response of this metabolic pathway to dietary protein (i.e., meal threshold) declines with advancing age or reduced physical activity. Current dietary recommendations for protein are focused on total daily intake of 0.8 g/kg body weight, but new research suggests daily needs for older adults of ≥1.0 g/kg and identifies anabolic and metabolic benefits to consuming at least 20-30 g protein at a given meal. Resistance exercise appears to increase the efficiency of EAA use for muscle anabolism and to lower the meal threshold for stimulation of protein synthesis. Applying this information to a typical 3-meal-a-day dietary plan results in protein intakes that are well within the guidelines of the Dietary Reference Intakes for acceptable macronutrient intakes. The meal threshold concept for dietary protein emphasizes a need for redistribution of dietary protein for optimum metabolic health.

Effects of leucine-rich protein supplements on anthropometric parameter and muscle strength in the elderly: a systematic review and meta-analysis

OBJECTIVE

The primary objective of the present systematic review and meta-analysis was to synthesize the available literature relating to leucine supplementation in the elderly with respect to its effects on anthropometrical parameters and muscle strength. The secondary aim was to perform a selective subgroup analysis when possible differentiating between healthy and sarcopenic subjects.

METHODS

Literature search was performed using the electronic databases MEDLINE, EMBASE, SportDiscus, and the Cochrane Central Register of trials with restrictions to randomized controlled trials or studies following a cross-over design. Parameters taken into account were body weight, body mass index, lean body mass, fat mass, percentage of body fat, hand grip strength, and knee extension strength. Moreover, biomarkers of glucose metabolism (fasting glucose, fasting insulin, albumin, and HOMA index) were extracted when possible. For each outcome measure of interest, a meta-analysis was performed in order to determine the pooled effect of the intervention in terms of weighted mean differences between the post-intervention (or differences in means) values of the leucine and the respective control groups. Data analysis was performed using the Review Manager 5.2.4. software.

RESULTS

A total of 16 studies enrolling 999 subjects met the inclusion criteria. Compared with control groups, leucine supplementation significantly increased gain in body weight [mean differences 1.02 kg, 95%-CI (0.19, 1.85), p=0.02], lean body mass [mean differences 0.99 kg, 95%-CI (0.43, 1.55), p=0.0005], and body mass index [mean differences 0.33 kg/m2, 95%-CI (0.13, 0.53), p=0.001], when compared to the respective control groups. With respect to body weight and lean body mass, leucine supplementation turned out to be more effective in the subgroup of study participants with manifested sarcopenia. All other parameters under investigation were not affected by leucine supplementation in a fashion significantly different from controls.

CONCLUSIONS

It is concluded that leucine supplementation was found to exert beneficial effects on body weight, body mass index, and lean body mass in older persons in those subjects already prone to sarcopenia, but not muscle strength. However, due to the heterogeneity between the trials included in this systematic review, further studies adopting a homogenous design with respect to participant characteristics duration as well as the kind and amount of daily supplement in use are required.

Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation

The popularity of natural bodybuilding is increasing; however, evidence-based recommendations for it are lacking. This paper reviewed the scientific literature relevant to competition preparation on nutrition and supplementation, resulting in the following recommendations. Caloric intake should be set at a level that results in bodyweight losses of approximately 0.5 to 1%/wk to maximize muscle retention. Within this caloric intake, most but not all bodybuilders will respond best to consuming 2.3-3.1 g/kg of lean body mass per day of protein, 15-30% of calories from fat, and the reminder of calories from carbohydrate. Eating three to six meals per day with a meal containing 0.4-0.5 g/kg bodyweight of protein prior and subsequent to resistance training likely maximizes any theoretical benefits of nutrient timing and frequency. However, alterations in nutrient timing and frequency appear to have little effect on fat loss or lean mass retention. Among popular supplements, creatine monohydrate, caffeine and beta-alanine appear to have beneficial effects relevant to contest preparation, however others do not or warrant further study. The practice of dehydration and electrolyte manipulation in the final days and hours prior to competition can be dangerous, and may not improve appearance. Increasing carbohydrate intake at the end of preparation has a theoretical rationale to improve appearance, however it is understudied. Thus, if carbohydrate loading is pursued it should be practiced prior to competition and its benefit assessed individually. Finally, competitors should be aware of the increased risk of developing eating and body image disorders in aesthetic sport and therefore should have access to the appropriate mental health professionals.

Effect of leucine supplementation on fat free mass with prolonged hypoxic exposure during a 13-day trek to Everest Base Camp: a double-blind randomized study

Loss of body weight and fat-free mass (FFM) are commonly noted with prolonged exposure to hypobaric hypoxia. Recent evidence suggests protein supplementation, specifically leucine, may potentially attenuate loss of FFM in subcaloric conditions during normoxia. The purpose of this study was to determine if leucine supplementation would prevent the loss of FFM in subcaloric conditions during prolonged hypoxia. Eighteen physically active male (n = 10) and female (n = 8) trekkers completed a 13-day trek in Nepal to Everest Base Camp with a mean altitude of 4140 m (range 2810–5364 m). In this double-blind study, participants were randomized to ingest either leucine (LEU) (7 g leucine, 93 kcal, 14.5 g whey-based protein) or an isocaloric isonitrogenous control (CON) (0.3 g LEU, 93 kcal, 11.3 g collagen protein) twice daily prior to meals. Body weight, body composition, and circumferences of bicep, thigh, and calf were measured pre- and post-trek. There was a significant time effect for body weight (−2.2% ± 1.7%), FFM (−1.7% ± 1.5%), fat mass (−4.0% ± 6.9%), and circumferences (p < 0.05). However, there was no treatment effect on body weight (CON −2.3 ± 2.0%; LEU −2.2 ± 1.5%), FFM (CON −2.1 ± 1.5%; LEU −1.2 ± 1.6%), fat mass (CON −2.9% ± 5.9%; LEU −5.4% ± 8.1%), or circumferences. Although a significant loss of body weight, FFM, and fat mass was noted in 13 days of high altitude exposure, FFM loss was not attenuated by leucine. Future studies are needed to determine if leucine attenuates loss of FFM with longer duration high altitude exposure.

Application of Key Events Dose Response Framework to defining the upper intake level of leucine in young men

Leucine is sold in large doses in health food stores and is ingested by weight-training athletes. The safety of ingestion of large doses of leucine is unknown. Before designing chronic high-dose leucine supplementation experiments, we decided to determine the effect of graded doses of leucine in healthy participants. The Key Events Dose Response Framework is an organizational and analytical framework that dissects the various biologic steps (key events) that occur between exposure to a substance and an eventual adverse effect. Each biologic event is looked at for its unique dose-response characteristics. For nutrients, there are a number of biologic homeostatic mechanisms that work to keep circulating/tissue levels in a safe, nontoxic range. If a response mechanism at a particular key event is especially vulnerable and easily overwhelmed, this is known as a determining event, because this event drives the overall slope or shape of the dose-response relationship. In this paper, the Key Events Dose Framework has been applied to the problem of leucine toxicity and leucine's tolerable upper level. After analyzing the experimental data vis a vis key events for leucine leading to toxicity, it became evident that the rate of leucine oxidation was the determining event. A dose-response study has been conducted to graded intakes of leucine in healthy human adult male volunteers. All participants were started at the mean requirement level of leucine [50 mg/(kg · d)] and the highest leucine intake was 1250 mg/( kg · d), which is 25 times the mean requirement. No gut intolerance was seen. Blood glucose fell progressively but remained within normal values without any changes in plasma insulin. Maximal leucine oxidation levels occurred at an intake of 550 mg leucine/( kg · d), after which plasma leucine progressively increased and plasma ammonia also increased in response to leucine intakes >500 mg/( kg · d). Thus, the "key determining event" appears to be when the participants reach their maximal leucine oxidation level, after which the risk of metabolic adverse effects progressively increased.

Summary of workshop discussions on establishing upper limits for amino acids with specific attention to available data for the essential amino acids leucine and tryptophan

The morning of the first day of the 8th Amino Acid Assessment Workshop was organized and co-sponsored by the International Council on Amino Acid Science (ICAAS) and the International Life Sciences Institute Research Foundation and was focused on the International Life Sciences Institute Research Foundation's approach to establishing upper limits of nutrients. The remainder of d 1 and all of d 2 were focused on the safety of leucine and tryptophan, with special emphasis on determining the upper level of the safe range of intake. It was recognized that some toxicological frameworks, mainly the key-events dose response framework, might be applicable to amino acids and provide appropriate assistance to regulators in establishing upper limits for amino acids as a group of nutrients used in dietary supplements. ICAAS-funded projects for determining the upper intake limits for the essential amino acid leucine provided the main pool of leucine data discussed at the workshop. The acute clinical study suggests 500 mg/(kg · d) as a possible upper limit for leucine in healthy humans, but the safety margin needed to widen this limit to the general population has not been determined. For tryptophan, the workshop participants found less ground for consensus. Older efficacy studies suggested that tryptophan at 8-15 g/d was well tolerated, but human research was abruptly terminated in the late 1980s and no new data are available. Animal results obtained in pigs and rodents were discussed and 2 possible strategies for applying those outcomes to humans were described.

Lessons learned regarding symptoms of tryptophan deficiency and excess from animal requirement studies

Tryptophan is the precursor for several neurotransmitters and metabolic regulators, which, although quantitatively of little importance in determining the dietary requirement, have major importance for interpreting symptoms of dietary tryptophan deficiency and excess. The quantitative dietary tryptophan requirement appears to vary widely across species, so intakes relative to requirements are more appropriate expressions for comparison of adverse effects across species than daily intake or diet concentration. Symptoms of tryptophan deficiency may occur at intakes as little as 25% below the requirement. Symptoms include reduced feed intake and reduced growth rate but also impaired skeletal development and aberrant behavior. Older animals appear less susceptible than younger animals to tryptophan deficiency and females less than males. Symptoms of excess tryptophan intake include reduced food intake and growth rate. In growing animals, it appears that tryptophan intakes of >10 times the requirement are necessary before there are detrimental effects on growth performance. At still greater intakes, fatty liver and fibrotic changes in muscles, lung, and pancreas and the serotonin syndrome may develop. In pigs, tryptophan intake of 60 times the daily requirement did not cause mortality. The maximal tryptophan oxidation rate, measured in vivo using (13)C universally labeled tryptophan, may be a possible marker of the intake above which increasing intake increases the risk of adverse effects. The advantage of the oxidation technique is that it does not necessarily rely on but still allows the identification and measurement of amino acid metabolites and is therefore simpler and more universally applicable.

Knowledge gained from studies of leucine consumption in animals and humans

Leucine's wide-ranging metabolic influences have made it subject to special interest. It is abundant in the diet, especially in some milk and cereal proteins, in part due to its allocation of 6 codons in the genetic code, and individual dietary intakes range up to >250 mg · kg(-1) · d(-1). It influences many cell functions by various mechanisms, which include allosteric activation of enzymes, enabling ATP generation and insulin secretion from the pancreatic islet cell, and activation of signaling pathways. It is a mediator of the anabolic drive of dietary amino acids, stimulating anabolic hormone secretion and directly signaling protein deposition and growth through the stimulation of protein synthesis and restraint of proteolysis. Its signaling may involve the mammalian target of rapamycin complex and rapamycin-insensitive pathways responding to a leucine "transceptor," which combines leucine cellular transport, fueled by the intracellular-extracellular glutamine gradient, and a signaling response to changes in ionic and water balance and cell volume. In animal studies, dietary leucine supplementation has reversed many of the adverse influences of a high-fat diet, consistent with a benefit for healthy weight maintenance in humans for which evidence is accumulating. The implications for safety of leucine-supplemented diets are discussed in terms of adversely lowering valine and isoleucine concentrations and inducing hyperammonemia through overloading peripheral glutamine synthetic pathways. Finally, the apparently high human leucine requirement is explained in terms of both an adaptive metabolic demand model of requirements and the design and analysis of human studies, which may overestimate values.

A proposal for an upper limit of leucine safe intake in healthy adults

Based on recent research, an upper limit of safe intake (ULSI) for leucine is proposed for healthy adults: 0.53 g/(kg·d). Because leucine has been used as a dietary supplement for many years in people practicing exercise and sport, further study with long-term exposure to leucine in this specific subpopulation should be performed to eventually adjust the ULSI.