Differential effect of long-term leucine supplementation on skeletal muscle and adipose tissue in old rats: an insulin signaling pathway approach

Autophagy: Are Amino Acid Signals Dependent on the mTORC1 Pathway or Independent?

Autophagy is a kind of "self-eating" phenomenon that is ubiquitous in eukaryotic cells. It mainly manifests in the damaged proteins or organelles in the cell being wrapped and transported by the autophagosome to the lysosome for degradation. Many factors cause autophagy in cells, and the mechanism of nutrient-deficiency-induced autophagy has been a research focus. It has been reported that amino-acid-deficiency-induced cellular autophagy is mainly mediated through the mammalian rapamycin target protein complex 1 (mTORC1) signaling pathway. In addition, some researchers also found that non-mTORC1 signaling pathways also regulate autophagy, and the mechanism of autophagy occurrence induced by the deficiency of different amino acids is not precisely the same. Therefore, this review aims to summarize the process of various amino acids regulating cell autophagy and provide a narrative review on the molecular mechanism of amino acids regulating autophagy.

Branched-chain amino acids at supraphysiological but not physiological levels reduce myotube insulin sensitivity

AIMS

Branched-chain amino acids (BCAA) are often emphasized in the diets of avid exercisers, yet population data demonstrates a correlation between circulating BCAA and insulin resistance. However, it is unclear if BCAA independently promote insulin resistance in otherwise healthy cells. The purpose of this study is to examine the effect of a BCAA mixture on muscle insulin signaling in vitro in both insulin resistant and sensitive cells.

MATERIALS AND METHODS

C2C12 myotubes were treated with a BCAA mixture containing leucine:isoleucine:valine at a ratio of 2:1:1 at 0.2, 2, or 20 mM (based on leucine content) for either 30 min, 1 day, or 6 days. Western blot was used to assess insulin sensitivity of cells treated with BCAA both with and without concurrent insulin resistance, and, with and without insulin stimulation.

RESULTS

BCAA treatment for 1 day significantly reduced basal, but not insulin-stimulated pAkt expression. BCAA treatment for 6 days resulted in significantly reduced basal insulin signaling in healthy cells and insulin-stimulated insulin signaling in insulin resistant (but not insulin sensitive) cells.

CONCLUSION

Similar to previous observations demonstrating BCAA may correlate with insulin resistance during metabolically stressed conditions, we demonstrate excessively high BCAA exposure can negatively influence basal insulin signaling, as well as insulin sensitivity in insulin resistant myotubes. However, given the intentionally high concentrations of BCAA used in this study, the extent to which these observations translate to in vivo models is unclear and warrants further investigation.

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.

Current progress in metabolomics of gestational diabetes mellitus

Gestational diabetes mellitus (GDM) is one of the most common metabolic disorders of pregnancy and can cause short- and long-term adverse effects in both pregnant women and their offspring. However, the etiology and pathogenesis of GDM are still unclear. As a metabolic disease, GDM is well suited to metabolomics study, which can monitor the changes in small molecular metabolites induced by maternal stimuli or perturbations in real time. The application of metabolomics in GDM can be used to discover diagnostic biomarkers, evaluate the prognosis of the disease, guide the application of diet or drugs, evaluate the curative effect, and explore the mechanism. This review provides comprehensive documentation of metabolomics research methods and techniques as well as the current progress in GDM research. We anticipate that the review will contribute to identifying gaps in the current knowledge or metabolomics technology, provide evidence-based information, and inform future research directions in GDM.

Important determinants to take into account to optimize protein nutrition in the elderly: solutions to a complex equation

During ageing, skeletal muscle develops anabolic resistance towards the stimulation of protein synthesis induced by dietary amino acids. The stimulation of muscle protein synthesis after food intake remains insufficient, even with a protein intake recommended for healthy adults. This alteration is one of the mechanisms known to be responsible for the decrease of muscle mass and function during ageing, namely sarcopenia. Increasing dietary protein intake above the current RDA(0⋅83 g/kg/d) has been strongly suggested to overcome the anabolic resistance observed. It is also specified that the dietary protein ingested should be of good quality. A protein of good quality is a protein whose amino acid (AA) composition covers the requirement of each AA when ingested at the RDA. However, the biological value of proteins may vary among dietary sources in which AA composition could be unbalanced. In the present review, we suggest that the quality of a dietary protein is also related to several other determinants. These determinants include the speed of digestion of dietary proteins, the presence of specific AA, the food matrix in which the dietary proteins are included, the processes involved in the production of food products (milk gelation and cooking temperature), the energy supply and its nature, and the interaction between nutrients before ingestion. Particular attention is given to plant proteins for nutrition of the elderly. Finally, the timing of protein intake and its association with the desynchronized intake of energetic nutrients are discussed.

Influence of Glutamine and Branched-Chain Amino Acids Supplementation during Refeeding in Activity-Based Anorectic Mice

Background: Optimizing the refeeding of patients with anorexia nervosa remains important to limit somatic complications of malnutrition, as well as to avoid disease relapses by targeting persistent mood and intestinal disorders. We aimed to evaluate the effects of glutamine (Gln) and branched-chain amino acids (BCAA) supplementation during refeeding in activity-based anorectic (ABA) mice. Method: Male C57Bl/6 mice were randomized in control and ABA groups. Once ABA-induced malnutrition was established, mice were progressively refed or not. Refed mice had free access to drinking water supplemented or not with 1% Gln or 2.5% BCAA for 10 days. Results: A progressive refeeding was associated with a partial restoration of body weight and lean mass, while a fat mass rebound was observed. In addition, refeeding restored glucose and leptin. Gln did not affect these parameters, while BCAA tended to increase body weight, fat mass, and glycaemia. In the colon, refeeding improved total protein synthesis and restored the LC3II/LC3I ratio, a marker of autophagy. Gln supplementation enhanced colonic protein synthesis, which was associated with an increased p-p70S6kinase/p70S6kinase ratio, whereas these effects were blunted by BCCA supplementation. Conclusions: In ABA mice, Gln and BCAA supplementations during a progressive refeeding fail to restore body weight and lean mass. However, Gln supplementation improves total colonic protein synthesis conversely to BCAA. Further studies are needed to decipher the underlying mechanisms involved in these opposite results.

Amino Acids in Health and Endocrine Function

Dietary amino acids play an important role in maintaining health. Branched chain amino acids can adversely increase blood pressure whereas arginine and citrulline can reduce it. D-amino acids play important roles in several cell types including testis, the nervous system and adrenal glands. Several amino acids also can have dramatic effects on diabetes; branched chain amino acids, phenylalanine and tyrosine have been implicated while others, namely arginine and citrulline can improve outcomes. Leucine has been shown to play important roles in muscle primarily through the mTOR pathway though this effect does not translate across every population. Glutamine, arginine and D-aspartate also exert their muscle effects through mTOR. Relationships between amino acids and endocrine function include that of glucocorticoids, thyroid function, glucagon-like peptide 1 (GLP-1), ghrelin, insulin-like growth factor-1 (IGF-1) and leptin. Leucine, for example, can alleviate the effect of dexamethasone on muscle protein accretion. Interestingly, amino acid transporters play an important role in thyroid function. Several amino acids have been shown to increase GLP-1 levels in non-diabetics when administered orally. Similarly, several amino acids increase ghrelin levels in different species while cysteine can decrease it in mice. There is evidence to suggest that the arginine/NO pathway may be involved in modulating some of the effects of ghrelin on cells. In regard to IGF-1, branched chain amino acids can increase levels in adults while tryptophan and phenylalanine have been shown to increase levels in infants. Finally, leptin levels can be elevated by branched chain amino acids while restricting leucine in high fat diets can increase leptin sensitivity.

Hypoglycaemic effect of resveratrol in streptozotocin-induced diabetic rats is impaired when supplemented in association with leucine

Studies have shown synergistic and independent effects of leucine and resveratrol (RSV) as possible therapeutic agents to ameliorate metabolic disorders. Thus, the objective of this study was to investigate the effects of supplementation with leucine and RSV, alone and in combination, on metabolic changes in white adipose tissue of neonatally STZ-induced diabetic rats. After weaning, the rats were treated with trans-resveratrol (0.6 mg/kg/dose) and/or leucine (1.35 mg/kg/dose) administered orally. The animals were euthanized at age 16 weeks for blood analyses. Subcutaneous (SC), periepididymal (PE) and retroperitoneal (RP) fat pads were weighed. Adipocytes from PE and RP pads were isolated for morphometric analysis. Long-term supplementation with RSV promoted adiposity recovery, prevented hypoinsulinemia and improved the metabolic profile of the diabetic rats. However, some of these effects were impaired when RSV was associated with leucine. The diabetic rats supplemented with leucine alone showed no significant improvement in metabolic disorders.

Metabolites as regulators of insulin sensitivity and metabolism

The cause of insulin resistance in obesity and type 2 diabetes mellitus (T2DM) is not limited to impaired insulin signalling but also involves the complex interplay of multiple metabolic pathways. The analysis of large data sets generated by metabolomics and lipidomics has shed new light on the roles of metabolites such as lipids, amino acids and bile acids in modulating insulin sensitivity. Metabolites can regulate insulin sensitivity directly by modulating components of the insulin signalling pathway, such as insulin receptor substrates (IRSs) and AKT, and indirectly by altering the flux of substrates through multiple metabolic pathways, including lipogenesis, lipid oxidation, protein synthesis and degradation and hepatic gluconeogenesis. Moreover, the post-translational modification of proteins by metabolites and lipids, including acetylation and palmitoylation, can alter protein function. Furthermore, the role of the microbiota in regulating substrate metabolism and insulin sensitivity is unfolding. In this Review, we discuss the emerging roles of metabolites in the pathogenesis of insulin resistance and T2DM. A comprehensive understanding of the metabolic adaptations involved in insulin resistance may enable the identification of novel targets for improving insulin sensitivity and preventing, and treating, T2DM.

Amino acids in post-stroke rehabilitation

Objectives: Patients with stroke are prone to disability due to muscle hypercatabolism. We aim to review this concept based on available data on benefits of amino acid supplementation in post-stroke rehabilitation.Method: The search was performed on Medline and Embase in January 2019. Randomized controlled studies, observational studies and case reports conducted in the last 15 years on the supplementation of amino acids in post-stroke rehabilitation patients were included.Result: Amino acids prevent muscle hypercatabolism in post stroke patients by suppressing myofibrillar protein and skeletal muscle degradation. Stroke patients supplemented with amino acids led to an improvement of functional and physical performance.Discussion: Muscle protein hypercatabolism and sequestration of amino acids from skeletal muscles occur cyclically in post-stroke patients to counter each other. There is a resultant deficit of amino acids which is unmet. Amino acids have antiproteolytic effect. Its supplementation prevents muscle wasting and improves rehabilitation by promoting physical performance, muscle strength, mass, and function.

Leucine modulates the IPEC-J2 cell proteome associated with cell proliferation, metabolism and phagocytosis

Leucine can affect intestinal protein expressions, and improve mucosal immune function. However, little study has been conducted to determine the change of protein component by leucine treatment in intestine epithelial cells. The present study was to cover the key proteins and cell pathways that could be regulated by leucine treatment in porcine intestinal epithelial cell line (IPEC-J2) cells with the approach of proteome analysis. A total number of 3,211 proteins were identified in our approach by searching the database of Uniprot sus scrofa. Among identified proteins, there were 101 proteins expressed differently between control group and leucine group. Compared with the control group, there were 50 up-regulated proteins and 51 down-regulated proteins in leucine group. In these proteins, leucine treatment decreased the expression of some proteins including pyruvate kinase, glyceraldehyde-3-phosphate dehydrogenase, E3 ubiquitin ligase, cathepsin D, caspase 3 and caspase 6, and increased the levels of some proteins, such as some eukaryotic translation initiation factors, ubiquitin carboxyl-terminal hydrolase, DNA-related RNA polymerase II, urokinase plasminogen activator, cyclin-dependent kinase inhibitor 2b, MutL homolog 1, 5-methylcytosine binding domain 4, polymerase δ, α-tubulin, syntaxin 18, Ras homolog D, actin related protein 2/3 complex and cofilin. Via the analysis of Gene Ontology and pathways, these proteins in IPEC-J2 cells were related with some physiological functions, such as protein metabolism, glycolysis, cell proliferation, apoptosis and phagocytosis. Thus, these results suggest that leucine affects gut barrier function possibly via regulating cell proliferation and apoptosis, metabolism and phagocytosis.

At same leucine intake, a whey/plant protein blend is not as effective as whey to initiate a transient post prandial muscle anabolic response during a catabolic state in mini pigs

BACKGROUND

Muscle atrophy has been explained by an anabolic resistance following food intake and an increase of dietary protein intake is recommended. To be optimal, a dietary protein has to be effective not only to initiate but also to prolong a muscle anabolic response in a catabolic state. To our knowledge, whether or not a dairy or a dairy/plant protein blend fulfills these criterions is unknown in a muscle wasting situation.

OBJECTIVE

Our aim was, in a control and a catabolic state, to measure continuously muscle anabolism in term of intensity and duration in response to a meal containing casein (CAS), whey (WHEY) or a whey/ plant protein blend (BLEND) and to evaluate the best protein source to elicit the best post prandial anabolism according to the physio-pathological state.

METHODS

Adult male Yucatan mini pigs were infused with U-13C-Phenylalanine and fed either CAS, WHEY or BLEND. A catabolic state was induced by a glucocorticoid treatment for 8 days (DEX). Muscle protein synthesis, proteolysis and balance were measured with the hind limb arterio-venous differences technique. Repeated time variance analysis were used to assess significant differences.

RESULTS

In a catabolic situation, whey proteins were able to initiate muscle anabolism which remained transient in contrast to the stimulated muscle protein accretion with WHEY, CAS or BLEND in healthy conditions. Despite the same leucine intake compared to WHEY, BLEND did not restore a positive protein balance in DEX animals.

CONCLUSIONS

Even with WHEY, the duration of the anabolic response was not optimal and has to be improved in a catabolic state. The use of BLEND remained of lower efficiency even at same leucine intake than whey.

Effects of long-term exposures to low iron and branched-chain amino acid containing diets on aging skeletal muscle of Fisher 344 × Brown Norway rats

Aging skeletal muscle displays an altered iron status that may promote oxidative stress and sarcopenia. A diet containing low iron (LI) could reduce muscle iron status and attenuate age-related muscle atrophy. Supplemental branched-chain amino acids (BCAA) may also alleviate sarcopenia by promoting muscle protein synthesis and iron status improvement. This study examined individual and combined effects of LI and BCAA diets on anabolic signaling and iron status in skeletal muscle of aging rats. Twenty-nine-month-old male Fisher 344 × Brown Norway rats consumed the following control-base diets: control + regular iron (35 mg iron/kg) (CR; n = 11); control + LI (∼6 mg iron/kg) (CL; n = 11); 2×BCAA + regular iron (BR; n = 10); and 2×BCAA + LI (BL; n = 12) for 12 weeks. Although LI and/or 2×BCAA did not affect plantaris muscle mass, 2×BCAA groups showed lower muscle iron content than did CR and CL groups (P < 0.05). p70 ribosomal protein S6 kinase phosphorylation was greater in 2×BCAA and LI animals compared with CR animals (P < 0.05). Interactions between IRON and BCAA were observed for proteins indicative of mitochondrial biogenesis (peroxisome proliferator-activated receptor gamma coactivator 1 alpha) and oxidative capacity (cytochrome c oxidase subunit 2 and citrate synthase) (P < 0.05) wherein the combined diet (BL) negated potential benefits of individual diets. Antioxidant capacity, superoxide dismutase activity, and oxidative injury (3-nitrotyrosine, protein carbonyls, and 4-hydroxynonenal) were similar between groups. In conclusion, 12 weeks of LI and 2×BCAA diets showed significant impacts on increasing anabolic signaling as well as ameliorating iron status; however, these interventions did not affect muscle mass.

Leucine and resistance training improve hyperglycemia, white adipose tissue loss, and inflammatory parameters in an experimental model of type 1 diabetes

BACKGROUND

Loss of white adipose tissue (WAT), associated with type 1 diabetes (DM1), contributes to increased chronic systemic inflammation.

AIM

The aim of this study was to investigate the effects of leucine supplementation and resistance training (RT) in attenuating WAT loss and improving inflammatory parameters and glucose metabolism in DM1 rats.

METHODS

Thirty-two male Wistar rats were distributed into four groups: DA (sedentary and supplemented with non-essential amino acids (NEAA)), DL (sedentary and supplemented with leucine), DTA (submitted to RT and supplemented with NEAA) and DTL (submitted to RT and supplemented with leucine). DM1 was induced by streptozotocin (STZ). An 8-week period of RT consisted of climbing a ladder with a progressively increased load, and supplementation was offered in the feed.

RESULTS

Glycemia, polyphagia and polydipsia were lower in DL, DTA and DTL groups compared with the DA group by approximately 20% ( p<.0001), 28% ( p=.004) and 64% ( p<.0001), respectively. Weight of total WAT and retroperitoneal adipose tissue (RPAT) were higher by approximately 21% ( p=.01) and 54% ( p=.0004), respectively, in DL, DTA and DTL groups compared with DA. However, gene expression of adiponectin and leptin in RPAT was only increased by RT (DTA and DTL) compared with DA and DL by approximately 93% ( p<.0001) and 78% ( p=.0002), respectively. Similarly, the levels of adiponectin in the serum, tissue IL-10 (RPAT) and serum IL-10 were only increased in DTA and DTL compared with DA and DL by approximately 31% ( p=.03), 45% ( p=.0009) and 35% ( p=.003), respectively.

CONCLUSIONS

Both interventions, isolated or together, reduced hyperglycemia and excessive loss of WAT, but RT was the main factor responsible for attenuating inflammation.

Endocrine responses and acute mTOR pathway phosphorylation to resistance exercise with leucine and whey

Leucine ingestion reportedly activates the mTOR pathway in skeletal muscle, contributing to a hypertrophy response. The purpose of the study was to compare the post-resistance exercise effects of leucine and whey protein supplementation on endocrine responses and muscle mTOR pathway phosphorylation. On visit 1, subjects (X±SD; n=20; age=27.8±2.8yrs) provided baseline blood samples for analysis of cortisol, glucose and insulin; a muscle biopsy of the vastus lateralis muscle to assess mTOR signaling pathway phosphorylation; and were tested for maximum strength on the leg press and leg extension exercises. For visits 2 and 3, subjects were randomized in a double-blind crossover design to ingest either leucine and whey protein (10g+10g; supplement) or a non-caloric placebo. During these visits, 5 sets of 10 repetitions were performed on both exercises, immediately followed by ingestion of the supplement or placebo. Blood was sampled 30 min post-, and a muscle biopsy 45 min post-exercise. Western blots quantified total and phosphorylated proteins. Insulin increased (α<.05) with supplementation with no change in glucose compared to placebo. Relative phosphorylation of AKT and rpS6 were greater with leucine and whey supplementation compared to placebo. Supplementation of leucine and whey protein immediately after heavy resistance exercise increases anabolic signaling in human skeletal muscle.

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.

Time Course of Molecular and Metabolic Events in the Development of Insulin Resistance in Fructose-Fed Rats

We aimed to determine the time-course of metabolic changes related to the early onset of insulin resistance (IR), trying to evidence breaking points preceding the appearance of the clinical IR phenotype. The model chosen was the fructose (FRU)-fed rat compared to controls fed with starch. We focused on the hepatic metabolism after 0, 5, 12, 30, or 45 days of FRU intake. The hepatic molecular metabolic changes followed indeed a multistep trajectory rather than a continuous progression. After 5 d of FRU feeding, we observed deep modifications in the hepatic metabolism, driven by the induction of lipogenic genes and important glycogen depletion. Thereafter, a steady-state period between days 12 and 30 was observed, characterized by a switch from carbohydrate to lipid utilization at the hepatic level and increased insulin levels aiming at alleviating lipid accumulation and hyperglycemia, respectively. The FRU-fed animals were only clinically IR at day 45 (altered homeostasis model assessment-estimated insulin resistance and muscle glucose transport). Furthermore, the urine metabolome revealed even earlier metabolic trajectory changes that precede the hepatic alterations. We identified several candidate metabolites linked to the tryptophan-nicotinamide metabolism and the installation of fasting hyperglycemia that suggest a role of this metabolic pathway on the development of the IR phenotype in the FRU-fed rats.

Hypertrophy-Promoting Effects of Leucine Supplementation and Moderate Intensity Aerobic Exercise in Pre-Senescent Mice

Several studies have indicated a positive influence of leucine supplementation and aerobic training on the aging skeletal muscle signaling pathways that control muscle protein balance and muscle remodeling. However, the effect of a combined intervention requires further clarification. Thirteen month old CD-1^® mice were subjected to moderate aerobic exercise (45 min swimming per day with 3% body weight workload) and fed a chow diet with 5% leucine or 3.4% alanine for 8 weeks. Serum and plasma were prepared for glucose, urea nitrogen, insulin and amino acid profile analysis. The white gastrocnemius muscles were used for determination of muscle size and signaling proteins involved in protein synthesis and degradation. The results show that both 8 weeks of leucine supplementation and aerobic training elevated the activity of mTOR (mammalian target of rapamycin) and its downstream target p70S6K and 4E-BP1, inhibited the ubiquitin-proteasome system, and increased fiber cross-sectional area (CSA) in white gastrocnemius muscle. Moreover, leucine supplementation in combination with exercise demonstrated more significant effects, such as greater CSA, protein content and altered phosphorylation (suggestive of increased activity) of protein synthesis signaling proteins, in addition to lower expression of proteins involved in protein degradation compared to leucine or exercise alone. The current study shows moderate aerobic training combined with 5% leucine supplementation has the potential to increase muscle size in fast-twitch skeletal muscle during aging, potentially through increased protein synthesis and decreased protein breakdown.

Metabolomic and Gene Expression Profiles Exhibit Modular Genetic and Dietary Structure Linking Metabolic Syndrome Phenotypes in Drosophila

Genetic and environmental factors influence complex disease in humans, such as metabolic syndrome, and Drosophila melanogaster serves as an excellent model in which to test these factors experimentally. Here we explore the modularity of endophenotypes with an in-depth reanalysis of a previous study by Reed et al. (2014), where we raised 20 wild-type genetic lines of Drosophila larvae on four diets and measured gross phenotypes of body weight, total sugar, and total triglycerides, as well as the endophenotypes of metabolomic and whole-genome expression profiles. We then perform new gene expression experiments to test for conservation of phenotype-expression correlations across different diets and populations. We find that transcript levels correlated with gross phenotypes were enriched for puparial adhesion, metamorphosis, and central energy metabolism functions. The specific metabolites L-DOPA and N-arachidonoyl dopamine make physiological links between the gross phenotypes across diets, whereas leucine and isoleucine thus exhibit genotype-by-diet interactions. Between diets, we find low conservation of the endophenotypes that correlate with the gross phenotypes. Through the follow-up expression study, we found that transcript-trait correlations are well conserved across populations raised on a familiar diet, but on a novel diet, the transcript-trait correlations are no longer conserved. Thus, physiological canalization of metabolic phenotypes breaks down in a novel environment exposing cryptic variation. We cannot predict the physiological basis of disease in a perturbing environment from profiles observed in the ancestral environment. This study demonstrates that variation for disease traits within a population is acquired through a multitude of physiological mechanisms, some of which transcend genetic and environmental influences, and others that are specific to an individual’s genetic and environmental context.

Dietary L-Lysine Suppresses Autophagic Proteolysis and Stimulates Akt/mTOR Signaling in the Skeletal Muscle of Rats Fed a Low-Protein Diet

Amino acids, especially L-leucine, regulate protein turnover in skeletal muscle and have attracted attention as a means of increasing muscle mass in people suffering from malnutrition, aging (sarcopenia), or a bedridden state. We previously showed that oral administration of L-lysine (Lys) by gavage suppressed proteolysis in skeletal muscles of fasted rats. However, the intake of Lys in the absence of other dietary components is unlikely in a non-experimental setting, and other dietary components may interfere with the suppressive effect of Lys on proteolysis. We supplemented Lys to a 10% casein diet and investigated the effect of Lys on proteolysis and autophagy, a major proteolytic system, in the skeletal muscle of rats. The rate of proteolysis was evaluated from 3-methylhisitidine (MeHis) released from isolated muscles, in plasma, and excreted in urine. Supplementing lysine with the 10% casein diet decreased the rate of proteolysis induced by intake of a low-protein diet. The upregulated autophagy activity [light chain 3 (LC3)-II/total LC3] caused by a low-protein diet was reduced, and the Akt/mTOR signaling pathway was activated by Lys. Importantly, continuous feeding of a Lys-rich 10% casein diet for 15 days increased the masses of the soleus and gastrocnemius muscles. Taken together, supplementation of Lys to a low-protein diet suppresses autophagic proteolysis through the Akt/mTOR signaling pathway, and continuous feeding of a Lys-rich diet may increase skeletal muscle mass.

Reviewing the Effects of L-Leucine Supplementation in the Regulation of Food Intake, Energy Balance, and Glucose Homeostasis

Leucine is a well-known activator of the mammalian target of rapamycin (mTOR). Because mTOR signaling regulates several aspects of metabolism, the potential of leucine as a dietary supplement for treating obesity and diabetes mellitus has been investigated. The objective of the present review was to summarize and discuss the available evidence regarding the mechanisms and the effects of leucine supplementation on the regulation of food intake, energy balance, and glucose homeostasis. Based on the available evidence, we conclude that although central leucine injection decreases food intake, this effect is not well reproduced when leucine is provided as a dietary supplement. Consequently, no robust evidence indicates that oral leucine supplementation significantly affects food intake, although several studies have shown that leucine supplementation may help to decrease body adiposity in specific conditions. However, more studies are necessary to assess the effects of leucine supplementation in already-obese subjects. Finally, although several studies have found that leucine supplementation improves glucose homeostasis, the underlying mechanisms involved in these potential beneficial effects remain unknown and may be partially dependent on weight loss.

Fast-digestive protein supplement for ten days overcomes muscle anabolic resistance in healthy elderly men

BACKGROUND/OBJECTIVE

Adequate protein intake is crucial to maintain muscle protein content in elderly subjects, but quality of dietary proteins should be considered. The aim was to determine whether soluble milk protein offers an original strategy to increase muscle anabolism in elderly subjects via a synergistic effect of fast-digesting proteins together with a unique essential AA content.

DESIGN

We investigated the effect of a 10-day adequate-protein (AP) or high-protein (HP) diet together with the protein source as caseins (CAS) or soluble milk proteins (PRO) on specific muscle protein fractional synthesis rates (FSRs) in healthy elderly men (71.8 ± 2.4 yr, n = 31). The isotopic study consisted of two periods of 4 h each: a post-absorptive and a postprandial period. The fed state was defined by consumption of either 15 g or 30 g of PRO or CAS, given fractionally every 20 min for 4 h. Soluble milk proteins are produced using a membrane process directly from pasteurized milk.

MEASUREMENTS

Specific muscle protein FSRs were measured during both postabsorptive and postprandial period using a continuous infusion of l-[1-(13)C]leucine.

RESULTS

FSR of sarcoplasmic muscle proteins and actin did not increase significantly in the postprandial state compared to postabsorptive state, whereas myosin FSR rate was increased by feeding whatever the protein source in HP groups (0.024 ± 0.005 vs 0.053 ± 0.011% h(-1), P < 0.05 and 0.026 ± 0.004 vs 0.050 ± 0.005% h(-1), P < 0.004 for PRO HP and CAS HP) but only with the PRO meal in the AP groups (0.031 ± 0.003 vs 0.062 ± 0.009% h(-1), P < 0.03 for PRO AP). Mitochondrial muscle protein FSR was also increased by feeding, irrespective of the protein quantity, but only in PRO meal groups (P < 0.02).

CONCLUSION

Fast-digesting soluble milk proteins improved postprandial muscle protein synthesis, especially mitochondrial muscle proteins and myosin fractional synthesis rates, in elderly subjects.

Lysine suppresses myofibrillar protein degradation by regulating the autophagic-lysosomal system through phosphorylation of Akt in C2C12 cells

The prevention of muscle wasting is important for maintaining quality of life, since loss of muscle mass can lead to a bedridden state and decreased resistance to diseases. The prevention of muscle wasting requires an increase in protein synthesis and a decrease in protein degradation in skeletal muscle. We previously showed that lysine (Lys) markedly suppressed myofibrillar protein degradation by inhibiting the autophagic-lysosomal system via the mammalian target of rapamycin (mTOR) and other signal molecules in C2C12 cells. In this study, we investigated the involvement of Akt and adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK), two regulators of autophagy, on the suppressive effects of Lys on myofibrillar protein degradation in C2C12 cells. Lys induced the phosphorylation of Akt, but the suppressive effects of Lys on myofibrillar protein degradation and autophagy were completely abolished in the presence of Akt1/2 kinase inhibitor (Akti). Lys suppressed the phosphorylation of AMPK, but this effect was also abolished by Akti. On the other hand, AMPK activation by 5-aminoimidazole-4-carboxamide-1-β-D-ribonucleoside (AICAR) did not affect either Akt activity or the autophagic-lysosomal system in C2C12 cells treated with Lys. These results indicate that regulation of AMPK activity is not essential for the regulation of autophagy by Lys. Taken together, our results show that Lys suppresses myofibrillar protein degradation by the autophagic-lysosomal system through the phosphorylation of Akt in C2C12 cells.

High whey protein intake delayed the loss of lean body mass in healthy old rats, whereas protein type and polyphenol/antioxidant supplementation had no effects

Our aim was to compare and combine 3 nutritional strategies to slow down the age-related loss of muscle mass in healthy old rats: 1) increase protein intake, which is likely to stimulate muscle protein anabolism; 2) use leucine rich, rapidly digested whey proteins as protein source (whey proteins are recognized as the most effective proteins to stimulate muscle protein anabolism). 3) Supplement animals with a mixture of chamomile extract, vitamin E, vitamin D (reducing inflammation and oxidative stress is also effective to improve muscle anabolism). Such comparisons and combinations were never tested before. Nutritional groups were: casein 12% protein, whey 12% protein, whey 18% protein and each of these groups were supplemented or not with polyphenols/antioxidants. During 6 months, we followed changes of weight, food intake, inflammation (plasma fibrinogen and alpha-2-macroglobulin) and body composition (DXA). After 6 months, we measured muscle mass, in vivo and ex-vivo fed and post-absorptive muscle protein synthesis, ex-vivo muscle proteolysis, and oxidative stress parameters (liver and muscle glutathione, SOD and total antioxidant activities, muscle carbonyls and TBARS). We showed that although micronutrient supplementation reduced inflammation and oxidative stress, the only factor that significantly reduced the loss of lean body mass was the increase in whey protein intake, with no detectable effect on muscle protein synthesis, and a tendency to reduce muscle proteolysis. We conclude that in healthy rats, increasing protein intake is an effective way to delay sarcopenia.

The effectiveness of leucine on muscle protein synthesis, lean body mass and leg lean mass accretion in older people: a systematic review and meta-analysis

In the present study, we performed a meta-analysis to assess the ability of leucine supplementation to increase the muscle protein fraction synthetic rate and to augment lean body mass or leg lean mass in elderly patients. A literature search was conducted on Medline, Cochrane, EMBASE and Google Scholar databases up to 31 December 2013 for clinical trials that investigated the administration of leucine as a nutrient that affects muscle protein metabolism and muscle mass in elderly subjects. The included studies were randomised controlled trials. The primary outcome for the meta-analysis was the protein fractional synthetic rate. Secondary outcomes included lean body mass and leg lean mass. A total of nine studies were included in the meta-analysis. The results showed that the muscle protein fractional synthetic rate after intervention significantly increased in the leucine group compared with the control group (pooled standardised difference in mean changes 1·08, 95% CI 0·50, 1·67; P< 0·001). No difference was found between the groups in relation to lean body mass (pooled standardised difference in mean changes 0·18, 95% CI - 0·18, 0·54; P= 0·318) or leg lean mass (pooled standardised difference in mean changes 0·006, 95% CI - 0·32, 0·44; P= 0·756). These findings suggest that leucine supplementation is useful to address the age-related decline in muscle mass in elderly individuals, as it increases the muscle protein fractional synthetic rate.

L-leucine supplementation worsens the adiposity of already obese rats by promoting a hypothalamic pattern of gene expression that favors fat accumulation

Several studies showed that l-leucine supplementation reduces adiposity when provided before the onset of obesity. We studied rats that were exposed to a high-fat diet (HFD) for 10 weeks before they started to receive l-leucine supplementation. Fat mass was increased in l-leucine-supplemented rats consuming the HFD. Accordingly, l-leucine produced a hypothalamic pattern of gene expression that favors fat accumulation. In conclusion, l-leucine supplementation worsened the adiposity of rats previously exposed to HFD possibly by central mechanisms.

Leucine modulates dynamic phosphorylation events in insulin signaling pathway and enhances insulin-dependent glycogen synthesis in human skeletal muscle cells

BACKGROUND

Branched-chain amino acids, especially leucine, are known to interact with insulin signaling pathway and glucose metabolism. However, the mechanism by which this is exerted, remain to be clearly defined. In order to examine the effect of leucine on muscle insulin signaling, a set of experiments was carried out to quantitate phosphorylation events along the insulin signaling pathway in human skeletal muscle cell cultures. Cells were exposed to insulin, leucine or both, and phosphorylation events of key insulin signaling molecules were tracked over time so as to monitor time-related responses that characterize the signaling events and could be missed by a single sampling strategy limited to pre/post stimulus events.

RESULTS

Leucine is shown to increase the magnitude of insulin-dependent phosphorylation of protein kinase B (AKT) at Ser473 and glycogen synthase kinase (GSK3β) at Ser21-9. Glycogen synthesis follows the same pattern of GSK3β, with a significant increase at 100 μM leucine plus insulin stimulus. Moreover, data do not show any statistically significant increase of pGSK3β and glycogen synthesis at higher leucine concentrations. Leucine is also shown to increase the magnitude of insulin-mediated extracellularly regulated kinase (ERK) phosphorylation; however, differently from AKT and GSK3β, ERK shows a transient behavior, with an early peak response, followed by a return to the baseline condition.

CONCLUSIONS

These experiments demonstrate a complementary effect of leucine on insulin signaling in a human skeletal muscle cell culture, promoting insulin-activated GSK3β phosphorylation and glycogen synthesis.

Beyond the role of dietary protein and amino acids in the prevention of diet-induced obesity

High-protein diets have been shown to prevent the development of diet-induced obesity and can improve associated metabolic disorders in mice. Dietary leucine supplementation can partially mimic this effect. However, the molecular mechanisms triggering these preventive effects remain to be satisfactorily explained. Here we review studies showing a connection between high protein or total amino nitrogen intake and obligatory water intake. High amino nitrogen intake may possibly lower lipid storage, and prevent insulin resistance. Suggestions are made for further systematical studies to explore the relationship between water consumption, satiety, and energy expenditure. Moreover, these examinations should better distinguish between leucine-specific and unspecific effects. Research in this field can provide important information to justify dietary recommendations and strategies in promoting long-term weight loss and may help to reduce health problems associated with the comorbidities of obesity.

Oral leucine supplementation is sensed by the brain but neither reduces food intake nor induces an anorectic pattern of gene expression in the hypothalamus

Leucine activates the intracellular mammalian target of the rapamycin (mTOR) pathway, and hypothalamic mTOR signaling regulates food intake. Although central infusion of leucine reduces food intake, it is still uncertain whether oral leucine supplementation is able to affect the hypothalamic circuits that control energy balance. We observed increased phosphorylation of p70s6k in the mouse hypothalamus after an acute oral gavage of leucine. We then assessed whether acute oral gavage of leucine induces the activation of neurons in several hypothalamic nuclei and in the brainstem. Leucine did not induce the expression of Fos in hypothalamic nuclei, but it increased the number of Fos-immunoreactive neurons in the area postrema. In addition, oral gavage of leucine acutely increased the 24 h food intake of mice. Nonetheless, chronic leucine supplementation in the drinking water did not change the food intake and the weight gain of ob/ob mice and of wild-type mice consuming a low- or a high-fat diet. We assessed the hypothalamic gene expression and observed that leucine supplementation increased the expression of enzymes (BCAT1, BCAT2 and BCKDK) that metabolize branched-chain amino acids. Despite these effects, leucine supplementation did not induce an anorectic pattern of gene expression in the hypothalamus. In conclusion, our data show that the brain is able to sense oral leucine intake. However, the food intake is not modified by chronic oral leucine supplementation. These results question the possible efficacy of leucine supplementation as an appetite suppressant to treat obesity.

Adipose tissue, diet and aging

Age related increase in body fat mass, visceral adipose tissue (AT), and ectopic fat deposition are strongly related to worse health conditions in the elderly. Moreover, with aging higher inflammation in adipose tissue may be observed and may contribute to inflammaging. Aging may significantly affect AT function by modifying the profile of adipokines produced by adipose cells, reducing preadipocytes number and their function and increasing AT macrophages infiltration. The initiating events of the inflammatory cascade promoting a greater AT inflammatory profile are not completely understood. Nutrients may determine changes in the amount of body fat, in its distribution as well as in AT function with some nutrients showing a pro-inflammatory effect on AT. Evidences are sparse and quite controversial with only a few studies performed in older subjects. Different dietary patterns are the result of the complex interaction of foods and nutrients, thus more studies are needed to evaluate the association between dietary patterns and changes in adipose tissue structure, distribution and function in the elderly.

Unilateral hindlimb casting induced a delayed generalized muscle atrophy during rehabilitation that is prevented by a whey or a high protein diet but not a free leucine-enriched diet

Sarcopenia is the general muscle mass and strength loss associated with ageing. Muscle atrophy could be made worse by exposure to acute periods of immobilization, because muscle disuse by itself is a stimulus for atrophy. Using a model of unilateral hindlimb casting in old adult rats, we have already demonstrated that the primary effect of immobilization was atrophy in the casted leg, but was also surprisingly associated with a retarded atrophy in the non-casted leg during rehabilitation. In search of mechanisms involved in this generalized atrophy, we demonstrated in the present study that contrary to pair-fed non-immobilized control animals, muscle protein synthesis in the non-immobilized limb was unable to adapt and to respond positively to food intake. Because pair-fed control rats did not lose muscle mass, this defect in muscle protein synthesis may represent one of the explanation for the muscle mass loss observed in the non-immobilized rats. Nevertheless, in order to stimulate protein turn over and generate a positive nitrogen balance required to maintain the whole muscle mass in immobilized rats, we tested a dietary free leucine supplementation (an amino acid known for its stimulatory effect on protein metabolism) during the rehabilitation period. Leucine supplementation was able to overcome the anabolic resistance in the non-immobilized limb. A greater muscle protein synthesis up-regulation associated with a stimulation of the mTOR signalling pathway was indeed recorded but it remained inefficient to prevent the loss of muscle in the non-immobilized limb. By contrast, we demonstrated here that whey protein or high protein diets were able to prevent the muscle mass loss of the non-immobilized limb by sustaining muscle protein synthesis during the entire rehabilitation period.

Chronic α-hydroxyisocaproic acid treatment improves muscle recovery after immobilization-induced atrophy

Muscle disuse atrophy is observed routinely in patients recovering from traumatic injury and can be either generalized resulting from extended bed rest or localized resulting from single-limb immobilization. The present study addressed the hypothesis that a diet containing 5% α-hydroxyisocaproic acid (α-HICA), a leucine (Leu) metabolite, will slow the loss and/or improve recovery of muscle mass in response to disuse. Adult 14-wk-old male Wistar rats were provided a control diet or an isonitrogenous isocaloric diet containing either 5% α-HICA or Leu. Disuse atrophy was produced by unilateral hindlimb immobilization ("casting") for 7 days and the contralateral muscle used as control. Rats were also casted for 7 days and permitted to recover for 7 or 14 days. Casting decreased gastrocnemius mass, which was associated with both a reduction in protein synthesis and S6K1 phosphorylation as well as enhanced proteasome activity and increased atrogin-1 and MuRF1 mRNA. Although neither α-HICA nor Leu prevented the casting-induced muscle atrophy, the decreased muscle protein synthesis was not observed in α-HICA-treated rats. Neither α-HICA nor Leu altered the increased proteasome activity and atrogene expression observed with immobilization. After 14 days of recovery, muscle mass had returned to control values only in the rats fed α-HICA, and this was associated with a sustained increase in protein synthesis and phosphorylation of S6K1 and 4E-BP1 of previously immobilized muscle. Proteasome activity and atrogene mRNA content were at control levels after 14 days and not affected by either treatment. These data suggest that whereas α-HICA does not slow the loss of muscle produced by disuse, it does speed recovery at least in part by maintaining an increased rate of protein synthesis.

Effect of gluteus medius muscle sample collection depth on postprandial mammalian target of rapamycin signaling in mature Thoroughbred mares

OBJECTIVE

To determine the effect of biopsy collection depth on the postprandial activation of mammalian target of rapamycin (mTOR) signaling factors, particularly protein kinase B, ribosomal protein S6 kinase, ribosomal protein S6, and eukaryotic initiation factor 4E binding protein 1 in middle-aged horses.

ANIMALS

6 healthy Thoroughbred mares (mean ± SD age, 13.4 ± 3.4 years).

PROCEDURES

Horses were fed a high-protein feed at 3 g/kg. Sixty minutes after horses were fed, the percutaneous needle biopsy technique was used to collect biopsy specimens from the gluteus medius muscle at 6, 8, and 10 cm below the surface of the skin. Muscle specimens were analyzed for the activation of upstream and downstream mTOR signaling factors, myosin heavy chain (MHC) isoform composition, and amino acid concentrations.

RESULTS

A 21% increase in MHC IIA isoform expression and a 21% decrease in MHC IIX isoform expression were identified as biopsy depth increased from 8 to 10 cm below the surface of the skin; however, no significant change was evident in the degree of MHC I expression with muscle depth. Biopsy depth had no significant effect on the phosphorylation of any of the mTOR signaling factors evaluated.

CONCLUSIONS AND CLINICAL RELEVANCE

Postprandial mTOR signaling could be compared between middle-aged horses when biopsy specimens were collected between 6 and 10 cm below the surface of the skin. Optimization of muscle biopsy techniques for evaluating mTOR signaling in horses will facilitate the design of future investigations into the factors that regulate muscle mass in horses.

Muscle wasting and resistance of muscle anabolism: the "anabolic threshold concept" for adapted nutritional strategies during sarcopenia

Skeletal muscle loss is observed in several physiopathological situations. Strategies to prevent, slow down, or increase recovery of muscle have already been tested. Besides exercise, nutrition, and more particularly protein nutrition based on increased amino acid, leucine or the quality of protein intake has generated positive acute postprandial effect on muscle protein anabolism. However, on the long term, these nutritional strategies have often failed in improving muscle mass even if given for long periods of time in both humans and rodent models. Muscle mass loss situations have been often correlated to a resistance of muscle protein anabolism to food intake which may be explained by an increase of the anabolic threshold toward the stimulatory effect of amino acids. In this paper, we will emphasize how this anabolic resistance may affect the intensity and the duration of the muscle anabolic response at the postprandial state and how it may explain the negative results obtained on the long term in the prevention of muscle mass. Sarcopenia, the muscle mass loss observed during aging, has been chosen to illustrate this concept but it may be kept in mind that it could be extended to any other catabolic states or recovery situations.

An enteral leucine supply modulates human duodenal mucosal proteome and decreases the expression of enzymes involved in fatty acid beta-oxidation

Leucine is well known to regulate protein metabolism in muscle. We recently reported that enteral leucine infusion decreased proteasome activity in human duodenal mucosa and enhanced intestinal cell proliferation, but its effects on gut proteome remain unknown. Therefore, we aimed to assess the effects of an enteral leucine infusion on the whole proteome of duodenal mucosa. In this work, 5 healthy volunteers received for 5h, on 2 occasions and in random order, an enteral supply of maltodextrins (0.25 g kg(-1) h(-1)) or maltodextrins supplemented with leucine (0.035 g kg(-1) h(-1)). At the end of infusion, endoscopic duodenal biopsy samples were collected and analyzed by 2D-PAGE. Eleven protein spots were differentially and significantly (P<0.05) expressed in response to the leucine-supplemented maltodextrins compared with maltodextrins alone. Forty percent of identified proteins by mass spectrometry were located in mitochondria. Four proteins were involved in lipid metabolism: HADHA, ACADVL and CPT2 expressions were reduced, whereas FABP1 expression was increased. In addition, the expression of DHA kinase involved in glycerol metabolism was also downregulated. Finally, leucine supplementation altered the duodenal mucosal proteome by regulating the expression of several enzymes mainly involved in lipid metabolism. These results suggest that leucine supplementation may slowdown fatty acid beta-oxidation in human duodenal mucosa.

Leucine improves glucose and lipid status in offspring from obese dams, dependent on diet type, but not caloric intake

Previously, we showed that offspring from obese rat dams were hyperphagic, with increased adiposity, hyperlipidaemia and glucose intolerance associated with increased orexigenic neuropeptide expression after fasting. Mammalian target of rapamycin (mTOR) can inhibit food intake through a hypothalamic action. As we previously showed that maternal obesity down-regulated hypothalamic mTOR, in the present study, we hypothesised that dietary leucine supplementation would activate hypothalamic mTOR to reduce food intake, thus limiting metabolic disorders in offspring from obese dams, regardless of postweaning diet. Obesity was induced in Sprague-Dawley females by high-fat diet (HFD) for 5 weeks before mating, throughout gestation and lactation. Male pups from HFD-fed mothers were weaned onto chow or HFD; within each dietary group, half were supplied with leucine via drinking water (1.5%) versus water control for 10 weeks. Those from chow-fed mothers were fed chow and water. Maternal obesity led to increased adiposity in chow-fed offspring. Postweaning HFD consumption exaggerated adiposity, hyperglycaemia, hyperinsulinaemia and hyperlipidaemia. Supplementation with leucine doubled leucine intake and increased hypothalamic mTOR activation; however, appetite regulation was not affected. A reduction in blood lipid levels was observed in offspring regardless of diet, as well as improved glucose tolerance in HFD-fed rats. In HFD-fed rats, up-regulated carnitine palmitoyl-transferase-1 and peroxisome-proliferator-activated receptor-γ coactivator-1α in muscle and glucose transporter 4 in fat suggested that leucine improved peripheral fat oxidation and glucose transport. Leucine is able to improve peripheral glucose and lipid metabolism independent of appetite and weight regulation, suggesting its potential application in the management of metabolic disorders.

Impact of leucine on energy balance

Body weight is determined by the balance between energy intake and energy expenditure. When energy intake exceeds energy expenditure, the surplus energy is stored as fat in the adipose tissue, which causes its expansion and may even lead to the development of obesity. Thus, there is a growing interest to develop dietary interventions that could reduce the current obesity epidemic. In this regard, data from a number of in vivo and in vitro studies suggest that the branched-chain amino acid leucine influences energy balance. However, this has not been consistently reported. Here, we review the literature related to the effects of leucine on energy intake, energy expenditure and lipid metabolism as well as its effects on the cellular activity in the brain (hypothalamus) and in peripheral tissues (gastro-intestinal tract, adipose tissue, liver and muscle) regulating the above physiological processes. Moreover, we discuss how obesity may influence the actions of this amino acid.

Excessive Leucine-mTORC1-Signalling of Cow Milk-Based Infant Formula: The Missing Link to Understand Early Childhood Obesity

Increased protein supply by feeding cow-milk-based infant formula in comparison to lower protein content of human milk is a well-recognized major risk factor of childhood obesity. However, there is yet no conclusive biochemical concept explaining the mechanisms of formula-induced childhood obesity. It is the intention of this article to provide the biochemical link between leucine-mediated signalling of mammalian milk proteins and adipogenesis as well as early adipogenic programming. Leucine has been identified as the predominant signal transducer of mammalian milk, which stimulates the nutrient-sensitive kinase mammalian target of rapamycin complex 1 (mTORC1). Leucine thus functions as a maternal-neonatal relay for mTORC1-dependent neonatal β-cell proliferation and insulin secretion. The mTORC1 target S6K1 plays a pivotal role in stimulation of mesenchymal stem cells to differentiate into adipocytes and to induce insulin resistance. It is of most critical concern that infant formulas provide higher amounts of leucine in comparison to human milk. Exaggerated leucine-mediated mTORC1-S6K1 signalling induced by infant formulas may thus explain increased adipogenesis and generation of lifelong elevated adipocyte numbers. Attenuation of mTORC1 signalling of infant formula by leucine restriction to physiologic lower levels of human milk offers a great chance for the prevention of childhood obesity and obesity-related metabolic diseases.

Leucine supplementation improves adiponectin and total cholesterol concentrations despite the lack of changes in adiposity or glucose homeostasis in rats previously exposed to a high-fat diet

BACKGROUND

Studies suggest that leucine supplementation (LS) has a therapeutic potential to prevent obesity and to promote glucose homeostasis. Furthermore, regular physical exercise is a widely accepted strategy for body weight maintenance and also for the prevention of obesity. The aim of this study was to determine the effect of chronic LS alone or combined with endurance training (ET) as potential approaches for reversing the insulin resistance and obesity induced by a high-fat diet (HFD) in rats.

METHODS

Forty-seven rats were randomly divided into two groups. Animals were fed a control diet-low fat (n = 10) or HFD (n = 37). After 15 weeks on HFD, all rats received the control diet-low fat and were randomly divided according to treatment: reference (REF), LS, ET, and LS+ET (n = 7-8 rats per group). After 6 weeks of treatment, the animals were sacrificed and body composition, fat cell volume, and serum concentrations of total cholesterol, HDL-cholesterol, triacylglycerol, glucose, adiponectin, leptin and tumor necrosis factor-alpha (TNF-α) were analyzed.

RESULTS

At the end of the sixth week of treatment, there was no significant difference in body weight between the REF, LS, ET and LS+ET groups. However, ET increased lean body mass in rats (P = 0.019). In addition, ET was more effective than LS in reducing adiposity (P = 0.019), serum insulin (P = 0.022) and TNF-α (P = 0.044). Conversely, LS increased serum adiponectin (P = 0.021) levels and reduced serum total cholesterol concentration (P = 0.042).

CONCLUSIONS

The results showed that LS had no beneficial effects on insulin sensitivity or adiposity in previously obese rats. On the other hand, LS was effective in increasing adiponectin levels and in reducing total cholesterol concentration.