Leucine reduces the duration of insulin-induced PI 3-kinase activity in rat skeletal muscle

Leucine improves the growth performance, carcass traits, and lipid nutritional quality of pork in Shaziling pigs

Leucine is involved in promoting fatty acid oxidation and lipolysis, mediating lipid metabolism and energy homeostasis, thus it has been widely used in livestock production. However, the effects of leucine on fat deposition and nutrition in Shaziling pigs remain unclear. A total of 72 Shaziling pigs (150 days old, weight 35.00 ± 1.00 kg) were randomly divided into 2 groups and fed with basal diet (control group) or basal diet containing 1% leucine (leucine group) for 60 days. The results showed that leucine significantly increased the average daily feed intake but decreased the ratio of feed to gain (P < 0.05), increased the loin muscle area and serum glucose content (P < 0.05) of Shaziling pigs. Besides, leucine regulated the re-distribution of fatty acids from adipose tissue to muscle as it significantly increased the contents of C18:1n-9 and C22:6n-3 (DHA) in the longissimus thoracis while decreased the contents of C22:5n-3 (DPA), C20:5n-3 (EPA), and DHA in the adipose tissue of Shaziling pigs (P < 0.05). Lipidomic analysis showed that the contents of phosphatidylethanolamines (PEs), cardiolipins (CLs), and phosphatidylglycerols (PGs) in the longissimus thoracis and the contents of lysophosphatidylethanolamines (LPEs), ceramides (Cers), phosphatidylinositols (PIs) in adipose tissue of Shaziling pigs were decreased in leucine group (P < 0.05). Collectively, this study clarified that dietary addition of 1% leucine have a better effect on growth performance and the deposition of beneficial fatty acids in the muscle of Shaziling pigs, which is conductive to the production of high quality and healthy pork. In addition, leucine altered the lipid composition of muscle and fat in Shaziling pigs. The related results provide a theoretical basis and application guidance for regulating fat deposition in Shaziling pigs, which is important for the healthy breeding of Shaziling pigs.

Estradiol replacement improves high-fat diet-induced insulin resistance in ovariectomized rats

The role of 17β-estradiol (E2) in high-fat diet (HFD)-induced alteration of the protein kinase B (Akt) signaling pathway in ovariectomized (OVX) rats is unclear. Therefore, we examined whether chronic estrogen replacement restores HFD-induced impairment in insulin sensitivity by its effects concomitant with alterations in the Akt isoform 2 (Akt2) and Akt substrate of 160 kDa (AS160) phosphorylation in muscles of OVX rats. Nine-week-old female Wistar rats underwent ovariectomy under anesthesia; after 4 weeks, subcutaneous implantation of either E2 or placebo (PL) pellets was performed, and HFD feeding was initiated. Intravenous glucose tolerance tests were performed to assess insulin sensitivity. Following insulin injection into rats' portal vein, the liver and gastrocnemius muscle were dissected for insulin signaling analysis. We observed that HFD increased energy intake and body weight in the PL group; however, it was temporarily decreased in the E2 group. Adipose tissue accumulation was larger in HFD-fed rats than in normal chow diet (NCD)-fed rats in the PL group; however, this difference was not observed in the E2 group. HFD reduced insulin sensitivity in the PL group only. In vivo insulin stimulation increased Akt2 phosphorylation in the muscles of NCD-fed rats in both groups. In contrast, HFD affected insulin-stimulated phosphorylation of Akt2 and AS160 in the muscles of rats in the PL group but not in the E2 group. Our data suggest that E2 replacement improves HFD-induced insulin resistance, and this effect is accompanied by the alterations in the Akt2 and AS160 phosphorylation in insulin-stimulated muscles of OVX rats.

Effect of Citrulline and Leucine Intake with Exercises on Body Composition, Physical Activity, and Amino Acid Concentration in Older Women: A Randomized Double-Blind Placebo-Controlled Study

The combined intake of citrulline (CIT) and leucine (LEU) can stimulate protein synthesis. Therefore, this study aimed to investigate the effect of combined intake of CIT and LEU accompanied by exercise for 20 weeks on body composition, physical activity (PA), and amino acid concentrations in older Japanese women with low body mass index (BMI) (16 to 21 kg/m²) using a randomized, double-blind, placebo-controlled design. The supplement was administered twice a day for 20 weeks (Ex (exercise) + CIT·LEU group, n = 10: mainly 0.8 g CIT and 1.6 g LEU; Ex + Placebo group, n = 13: mainly 3.5 g carbohydrate). Additionally, both groups exercised (weight-bearing exercise, square stepping exercise) once a week for 75 min. Body composition, PA, and amino acid concentrations in the plasma were measured. Body weight, BMI, body mass, household PA, total PA, and phenylalanine significantly increased in the Ex + CIT·LEU group (p < 0.05) post intervention. This study suggests that the combined intake of CIT and LEU accompanied by exercise can improve body weight, BMI, body mass, and PA in older women with low BMI, which may prevent sarcopenia and frailty.

Effects of ketoisocaproic acid and inflammation on glucose transport in muscle cells

Branched-chain amino acids (BCAAs) are regulators of protein metabolism. However, elevated levels of BCAAs and their metabolites are linked to insulin resistance. We previously demonstrated that the leucine metabolite, α-ketoisocaproate (KIC), inhibited insulin-stimulated glucose transport in myotubes. Like KIC, inflammatory factors are implicated in the development of insulin resistance. Here, we analyzed the effect of KIC and inflammatory factors (homocysteine [50 μM], TNF-α [10 ng/ml], and interleukin 6 (IL-6) [10 ng/ml]) on myotubes. Although KIC suppressed insulin-stimulated glucose transport, addition of the inflammatory factors did not worsen this effect. Depletion of branched-chain aminotransferase 2, the enzyme that catalyzes the conversion of leucine into KIC, abrogated the effect of KIC and the inflammatory factors. The effect of insulin on AKTS473 and S6K1T389 phosphorylation was not modified by treatments. There were no treatment effects on glycogen synthase phosphorylation. Depletion of E1α subunit of branched-chain α-keto acid dehydrogenase, the enzyme that catalyzes the oxidative decarboxylation of KIC, suppressed insulin-stimulated glucose transport, especially in cells incubated in KIC. Thus, defects in BCAA catabolism are contributory to insulin resistance of glucose transport in myotubes, especially in the presence of KIC. Interventions that increase BCAA catabolism may promote muscle glucose utilization and improve insulin resistance and its sequelae.

Metabolomic Effects of Hormone Therapy and Associations With Coronary Heart Disease Among Postmenopausal Women

BACKGROUND

In the WHI-HT trials (Women's Health Initiative Hormone Therapy), treatment with oral conjugated equine estrogens and medroxyprogesterone acetate (CEE+MPA) resulted in increased risk of coronary heart disease (CHD), whereas oral conjugated equine estrogens (CEE) did not.

METHODS

Four hundred eighty-one metabolites were measured at baseline and at 1-year in 503 and 431 participants in the WHI CEE and CEE+MPA trials, respectively. The effects of randomized HT on the metabolite profiles at 1-year was evaluated in linear models adjusting for baseline metabolite levels, age, body mass index, race, incident CHD, prevalent hypertension, and diabetes. Metabolites with discordant effects by HT type were evaluated for association with incident CHD in 944 participants (472 CHD cases) in the WHI-OS (Women's Health Initiative Observational Study), with replication in an independent cohort of 980 men and women at high risk for cardiovascular disease.

RESULTS

HT effects on the metabolome were profound; 62% of metabolites significantly changed with randomized CEE and 52% with CEE+MPA (false discovery rate-adjusted P value<0.05) in multivariable models. Concerted increases in abundance were seen within various metabolite classes including triacylglycerols, phosphatidylethanolamines, and phosphatidylcholines; decreases in abundance was observed for acylcarnitines, lysophosphatidylcholines, quaternary amines, and cholesteryl/cholesteryl esters. Twelve metabolites had discordant effects by HT type and were associated with incident CHD in the WHI-OS; a metabolite score estimated in a Least Absolute Shrinkage and Selection Operator regression was associated with CHD risk with an odds ratio of 1.47 per SD increase (95% CI, 1.27-1.70, P<10^-6). All twelve metabolites were altered in the CHD protective direction by CEE treatment. One metabolite (lysine) was significantly altered in the direction of increased CHD risk by CEE+MPA; the remaining 11 metabolites were not significantly changed by CEE+MPA. The CHD associations of a subset of 4 metabolites including C58:11 triacylglycerol, C54:9 triacylglycerol, C36:1 phosphatidylcholine and sucrose replicated in an independent dataset of 980 participants in the PREDIMED trial (Prevención con Dieta Mediterránea).

CONCLUSIONS

Randomized treatment with oral HT resulted in large metabolome shifts that generally favored CEE alone over CEE+MPA in term of CHD risk. Discordant metabolite effects between HT regimens may partially mediate the differences in CHD risk between the 2 WHI-HT trials.

Optimizing Adult Protein Intake During Catabolic Health Conditions

The DRIs define a range of acceptable dietary intakes for each nutrient. The range is defined from the minimum intake to avoid risk of inadequacy (i.e., the RDA) up to an upper limit (UL) based on a detectable risk of adverse effects. For most nutrients, the minimum RDA is based on alleviating a clear deficiency condition, whereas higher intakes are often recommended to optimize specific health outcomes. Evidence is accumulating that similar logic should be applied to dietary recommendations for protein. Although the RDA for protein of 0.8 g/kg body weight is adequate to avoid obvious inadequacies, multiple studies provide evidence that many adults may benefit from protein quantity, quality, and distribution beyond guidelines currently defined by the RDA. Further, the dietary requirement for protein is a surrogate for the constituent amino acids and, in particular, the 9 considered to be indispensable. Leucine provides an important example of an essential amino acid where the RDA of 42 mg/kg body weight is significantly less than the 100-110 mg/kg required to optimize metabolic regulation and skeletal muscle protein synthesis. This review will highlight the benefits of higher protein diets to optimize health during aging, inactivity, bed rest, or metabolic dysfunction such as type 2 diabetes.

Leucine decreases intramyocellular lipid deposition in an mTORC1-independent manner in palmitate-treated C2C12 myotubes

Higher intramyocellular lipid (IMCL) deposition in skeletal muscle is commonly observed in patients with obesity, resulting in mitochondrial damage. Palmitic acid, a saturated fatty acid, has been reported to induce obesogenic conditions in C2C12 myotubes. Leucine has been shown to improve obesity-related metabolic signatures; however, evidence for the effect of leucine on IMCL and the underlying mechanisms are still lacking. The objective of this study was to determine the effect of leucine on IMCL deposition and identify the potential mechanisms. Palmitate-treated C2C12 myotubes were used as an in vitro model of obesity. Two doses of leucine were used: 0.5 mM (postprandial physiological plasma concentration) and 1.5 mM (supraphysiological plasma concentration). Rapamycin was used to determine the role of mammalian target of rapamycin complex 1 (mTORC1) in leucine's regulation of lipid deposition in C2C12 myotubes. One-way ANOVA followed by Tukey's post hoc test was used to calculate differences between treatment groups. Our results demonstrate that leucine reduces IMCL deposition in an mTORC1-independent fashion. Furthermore, leucine acts independently of mTORC1 to upregulate gene expression related to fatty acid metabolism and works through both mTORC1-dependent and mTORC1-independent pathways to regulate mitochondrial biogenesis in palmitate-treated C2C12 myotubes. In agreement with increased mitochondrial biogenesis, increased mitochondrial content, circularity, and decreased autophagy are observed in the presence of 1.5 mM leucine. Taken together, the results indicate leucine reduces IMCL potentially through an mTORC1-independent pathway in palmitate-treated C2C12 myotubes.

Dietary Leucine Supplementation Restores Serum Glucose Levels, and Modifying Hepatic Gene Expression Related to the Insulin Signal Pathway in IUGR Piglets

Simple Summary

Intrauterine malnutrition may compromise the size and structure of fetal organs and tissues, which leads to lower birth weight and a slower rate of growth after weaning. Intrauterine growth restriction/retardation (IUGR) impairs pancreas function, resulting in the decreased glucose levels in serum. Leucine, one of branched chain amino acids, is an essential amino acid and the substrate of protein synthesis. Leucine also acts as a major regulator of hormone signal transduction, like insulin. Dietary branched chain amino acids or leucine have beneficial effects on the glucose metabolism and glycogen synthesis of muscle. Leucine supplementation improves the insulin sensitivity in liver and muscle and then influences the systemic glucose homeostasis. However, it is still unclear whether leucine supplementation would alter insulin sensitivity in IUGR neonatal piglets. Our results showed that dietary leucine supplementation restored serum glucose concentrations, increased insulin and creatinine concentrations, and enhanced protein kinase adenosine monophosphate-activated γ 3-subunit and glucose transporter type 2 expression. These findings suggest that leucine might play a positive role in hepatic lipid metabolism and glucose metabolism in IUGR.

Abstract

This study aimed to investigate the effects of leucine with different levels on the insulin resistance in intrauterine growth restriction/retardation (IUGR) piglets. Thirty-two weaned piglets were arranged in a 2 × 2 factorial design and four treatments (n = 8) were as follow: (1) normal weaned piglets fed a basal diet (CONT), (2) IUGR weaned piglets fed a basal diet (IUGR), (3) normal weaned piglets fed a basal diet with the addition of 0.35% l-leucine (C-LEU), and (4) IUGR fed a basal diet with the addition of 0.35% l-leucine (I-LEU) for a 21-days trial. The results showed that compared to the IUGR group, the I-LEU group had higher final body weight and body weight gain, higher serum glucose concentrations, and higher serum insulin concentrations (p < 0.05). The gene expression of phosphatidylinositol 3-kinase p110 gamma, protein kinase adenosine monophosphate-activated γ 3-subunit, glycogen synthase kinase-3 alpha, and glucose transporter type 2 were increased in the I-LEU group as compared to the IUGR group (p < 0.05). It was concluded that dietary leucine supplementation restored serum glucose concentrations, increased insulin and creatinine concentrations, and enhanced protein kinase adenosine monophosphate-activated γ 3-subunit and glucose transporter type 2 expression, suggesting that leucine might play a positive role in hepatic lipid metabolism and glucose metabolism in IUGR.

Estrogen replacement enhances insulin-induced AS160 activation and improves insulin sensitivity in ovariectomized rats

Menopause predisposes women to impaired glucose metabolism, but the role of estrogen remains unclear. In this study, we examined the effects of chronic estrogen replacement on whole body insulin sensitivity and insulin signaling in ovariectomized rats. Female Wistar rats aged 9 wk were ovariectomized under anesthesia. After 4 wk, pellets containing either 17β-estradiol (E₂) or placebo (Pla) were subcutaneously implanted in the rats. After 4 wk of treatment, the intra-abdominal fat accumulation was greater in the Pla group than that in the E₂ group. Hyperinsulinemic-euglycemic clamp analysis and intravenous glucose tolerance test revealed that insulin sensitivity was significantly lower in the Pla group than in the E₂ group. In addition, Western blotting showed that in vivo insulin stimulation increased protein kinase B (Akt) phosphorylation to a similar degree in the gastrocnemius and liver of both groups, but phosphorylated Akt2 Ser⁴⁷⁴ was enhanced in the muscle of the E₂ group compared with the Pla group. Moreover, insulin-stimulated phosphorylation of Akt substrate of 160 kDa (AS160) Thr⁶⁴² was observed only in the E₂ group, resulting in the difference between the two groups. Additionally, AS160 protein and mRNA levels were higher in muscle of the E₂ group than the Pla group. In contrast, E₂ replacement had no effect on glucose transporter 4 protein levels in muscle and glycogen synthase kinase-3β in muscle and liver. These results suggest that estrogen replacement improves insulin sensitivity by activating the Akt2/AS160 pathway in the insulin-stimulated muscle of ovariectomized rats.

BCAA Metabolism and Insulin Sensitivity - Dysregulated by Metabolic Status?

Branched-chain amino acids (BCAAs) appear to influence several synthetic and catabolic cellular signaling cascades leading to altered phenotypes in mammals. BCAAs are most notably known to increase protein synthesis through modulating protein translation, explaining their appeal to resistance and endurance athletes for muscle hypertrophy, expedited recovery, and preservation of lean body mass. In addition to anabolic effects, BCAAs may increase mitochondrial content in skeletal muscle and adipocytes, possibly enhancing oxidative capacity. However, elevated circulating BCAA levels have been correlated with severity of insulin resistance. It is hypothesized that elevated circulating BCAAs observed in insulin resistance may result from dysregulated BCAA degradation. This review summarizes original reports that investigated the ability of BCAAs to alter glucose uptake in consequential cell types and experimental models. The review also discusses the interplay of BCAAs with other metabolic factors, and the role of excess lipid (and possibly energy excess) in the dysregulation of BCAA catabolism. Lastly, this article provides a working hypothesis of the mechanism(s) by which lipids may contribute to altered BCAA catabolism, which often accompanies metabolic disease.

A High-Protein Diet Reduces Weight Gain, Decreases Food Intake, Decreases Liver Fat Deposition, and Improves Markers of Muscle Metabolism in Obese Zucker Rats

A primary factor in controlling and preventing obesity is through dietary manipulation. Diets higher in protein have been shown to improve body composition and metabolic health during weight loss. The objective of this study was to examine the effects of a high-protein diet versus a moderate-protein diet on muscle, liver and fat metabolism and glucose regulation using the obese Zucker rat. Twelve-week old, male, Zucker (fa/fa) and lean control (Fa/fa) rats were randomly assigned to either a high-protein (40% energy) or moderate-protein (20% energy) diet for 12 weeks, with a total of four groups: lean 20% protein (L20; n = 8), lean 40% protein (L40; n = 10), obese 20% protein (O20; n = 8), and obese 40% protein (O40; n = 10). At the end of 12 weeks, animals were fasted and euthanized. There was no difference in food intake between L20 and L40. O40 rats gained less weight and had lower food intake (p < 0.05) compared to O20. O40 rats had lower liver weight (p < 0.05) compared to O20. However, O40 rats had higher orexin (p < 0.05) levels compared to L20, L40 and O20. Rats in the L40 and O40 groups had less liver and muscle lipid deposition compared to L20 and L40 diet rats, respectively. O40 had decreased skeletal muscle mechanistic target of rapamycin complex 1 (mTORC1) phosphorylation and peroxisome proliferator-activated receptor gamma (PPARγ) mRNA expression compared to O20 (p < 0.05), with no difference in 5' AMP-activated protein kinase (AMPK), eukaryotic translation initiation factor 4E binding protein 1 (4EBP1), protein kinase B (Akt) or p70 ribosomal S6 kinase (p70S6K) phosphorylation. The data suggest that high-protein diets have the potential to reduce weight gain and alter metabolism, possibly through regulation of an mTORC1-dependent pathway in skeletal muscle.

Daily supplementation of dietary protein improves the metabolic effects of GLP-1-based pharmacotherapy in lean and obese rats

Glucagon-like peptide-1 (GLP-1) is an incretin hormone released from intestinal L-cells in response to food entering into the gastrointestinal tract. GLP-1-based pharmaceuticals improve blood glucose regulation and reduce feeding. Specific macronutrients, when ingested, may trigger GLP-1 secretion and enhance the effects of systemic sitagliptin, a pharmacological inhibitor of DPP-IV (an enzyme that rapidly degrades GLP-1). In particular, macronutrient constituents found in dairy foods may act as potent secretagogues for GLP-1, and acute preclinical trials show that ingestion of dairy protein may represent a promising adjunct behavioral therapy in combination with sitagliptin. To test this hypothesis further, chow-maintained or high-fat diet (HFD)-induced obese rats received daily IP injections of sitagliptin (6mg/kg) or saline in combination with a twice-daily 8ml oral gavage of milk protein concentrate (MPC; 80/20% casein/whey; 0.5kcal/ml), soy protein (non-dairy control; 0.5kcal/ml) or 0.9% NaCl for two months. Food intake and body weight were recorded every 24-48h; blood glucose regulation was examined at baseline and at 3 and 6.5weeks via a 2h oral glucose tolerance test (OGTT; 25% glucose; 2g/kg). MPC and soy protein significantly suppressed cumulative caloric intake in HFD but not chow-maintained rats. AUC analyses for OGTT show suppression in glycemia by sitagliptin with MPC or soy in chow- and HFD-maintained rats, suggesting that chronic ingestion of dairy or soy proteins may augment endogenous GLP-1 signaling and the glycemic- and food intake-suppressive effects of DPP-IV inhibition.

Amino Acid Intakes Are Inversely Associated with Arterial Stiffness and Central Blood Pressure in Women

BACKGROUND

Although data suggest that intakes of total protein and specific amino acids (AAs) reduce blood pressure, data on other cardiovascular disease risk factors are limited.

OBJECTIVE

We examined associations between intakes of AAs with known mechanistic links to cardiovascular health and direct measures of arterial stiffness, central blood pressure, and atherosclerosis.

METHODS

In a cross-sectional study of 1898 female twins aged 18-75 y from the TwinsUK registry, intakes of 7 cardioprotective AAs (arginine, cysteine, glutamic acid, glycine, histidine, leucine, and tyrosine) were calculated from food-frequency questionnaires. Direct measures of arterial stiffness and atherosclerosis included central systolic blood pressure (cSBP), mean arterial pressure (MAP), augmentation index (AI), pulse wave velocity (PWV), and intima-media thickness (IMT). ANCOVA was used to assess the associations between endpoints of arterial stiffness and intake (per quintile), adjusting for potential confounders.

RESULTS

In multivariable analyses, higher intakes of total protein and 7 potentially cardioprotective AAs were associated with lower cSBP, MAP, and PWV. Higher intakes of glutamic acid, leucine, and tyrosine were most strongly associated with PWV, with respective differences of -0.4 ± 0.2 m/s (P-trend = 0.02), -0.4 ± 0.2 m/s (P-trend = 0.03), and -0.4 ± 0.2 m/s (P-trend = 0.03), comparing extreme quintiles. There was a significant interaction between AA intakes and protein source, and higher intakes of AAs from vegetable sources were associated with lower central blood pressure and AI. Higher intakes of glutamic acid, leucine, and tyrosine from animal sources were associated with lower PWV.

CONCLUSIONS

These data provide evidence to suggest that intakes of several AAs are associated with cardiovascular benefits beyond blood pressure reduction in healthy women. The magnitude of the observed associations was similar to those previously reported for other lifestyle factors. Increasing intakes of these AAs could be an important and readily achievable way to reduce cardiovascular disease risk.

Effects of leucine supplementation and serum withdrawal on branched-chain amino acid pathway gene and protein expression in mouse adipocytes

The essential branched-chain amino acids (BCAA), leucine, valine and isoleucine, are traditionally associated with skeletal muscle growth and maintenance, energy production, and generation of neurotransmitter and gluconeogenic precursors. Recent evidence from human and animal model studies has established an additional link between BCAA levels and obesity. However, details of the mechanism of regulation of BCAA metabolism during adipogenesis are largely unknown. We interrogated whether the expression of genes and proteins involved in BCAA metabolism are sensitive to the adipocyte differentiation process, and responsive to nutrient stress from starvation or BCAA excess. Murine 3T3-L1 preadipocytes were differentiated to adipocytes under control conditions and under conditions of L-leucine supplementation or serum withdrawal. RNA and proteins were isolated at days 0, 4 and 10 of differentiation to represent pre-differentiation, early differentiation and late differentiation stages. Expression of 16 BCAA metabolism genes was quantified by quantitative real-time PCR. Expression of the protein levels of branched-chain amino acid transaminase 2 (Bcat2) and branched-chain alpha keto acid dehydrogenase (Bckdha) was quantified by immunoblotting. Under control conditions, all genes displayed induction of gene expression during early adipogenesis (Day 4) compared to Day 0. Leucine supplementation resulted in an induction of Bcat2 and Bckdha genes during early and late differentiation. Western blot analysis demonstrated condition-specific concordance between gene and protein expression. Serum withdrawal resulted in undetectable Bcat2 and Bckdha protein levels at all timepoints. These results demonstrate that the expression of genes related to BCAA metabolism are regulated during adipocyte differentiation and influenced by nutrient levels. These results provide additional insights on how BCAA metabolism is associated with adipose tissue function and extends our understanding of the transcriptomic response of this pathway to variations in nutrient availability.

Intraduodenal milk protein concentrate augments the glycemic and food intake suppressive effects of DPP-IV inhibition

Glucagon-like peptide-1 (GLP-1) is an incretin hormone released from intestinal L-cells in response to food entering into the gastrointestinal tract. GLP-1-based pharmaceuticals improve blood glucose regulation and may hold promise for obesity treatment, as GLP-1 drugs reduce food intake and body weight in humans and animals. In an effort to improve GLP-1 pharmacotherapies, we focused our attention on macronutrients that, when present in the gastrointestinal tract, may enhance GLP-1 secretion and improve glycemic regulation and food intake suppression when combined with systemic administration of sitagliptin, a pharmacological inhibitor of DPP-IV (enzyme responsible for GLP-1 degradation). In particular, previous data suggest that specific macronutrient constituents found in dairy foods may act as potent secretagogues for GLP-1 and therefore may potentially serve as an adjunct dietary therapy in combination with sitagliptin. To directly test this hypothesis, rats received intraperitoneal injections of sitagliptin (6 mg/kg) or saline vehicle followed by intraduodenal infusions of either milk protein concentrate (MPC; 80/20% casein/whey; 4 kcal), soy protein (nondairy control infusate; 4 kcal), or 0.9% NaCl. Food intake was assessed 30 min postinfusion. In separate studies, regulation of blood glucose was examined via a 2-h oral glucose tolerance test (2 g/kg) following identical sitagliptin treatment and intraduodenal nutrient infusions. Collectively, results show that intraduodenal MPC, but not soy protein, significantly enhances both the food intake suppression and improved control of blood glucose produced by sitagliptin. These data support the hypothesis that dietary intake of dairy protein may be beneficial as an adjunct behavioral therapy to enhance the glycemic and food intake suppressive effects of GLP-1-based pharmacotherapies.

The effects of whey protein on cardiometabolic risk factors

Obesity has reached epidemic proportions worldwide. The health consequences of obesity are more dangerous when associated with the metabolic syndrome and its components. Studies show that whey protein and its bioactive components can promote greater benefits compared to other protein sources such as egg and casein. The aim of this paper is to review the effects of whey protein on cardiometabolic risk factors. Using PubMed as the database, a review was conducted to identify current scientific literature on whey protein and the components of the metabolic syndrome published between 1970 and 2012. Consumption of whey protein seems to play an anti-obesity and muscle-protective role during dieting by increasing thermogenesis and maintaining lean mass. In addition, whey protein has been shown to improve glucose levels and insulin response, promote a reduction in blood pressure and arterial stiffness, and improve lipid profile. The collective view of current scientific literature indicates that the consumption of whey protein may have beneficial effects on some symptoms of the metabolic syndrome as well as a reduction in cardiovascular risk factors.

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.

An amino acid mixture is essential to optimize insulin-stimulated glucose uptake and GLUT4 translocation in perfused rodent hindlimb muscle

The purpose of this study was to investigate whether an amino acid mixture increases glucose uptake across perfused rodent hindlimb muscle in the presence and absence of a submaximal insulin concentration, and if the increase in glucose uptake is related to an increase in GLUT4 plasma membrane density. Sprague-Dawley rats were separated into one of four treatment groups: basal, amino acid mixture, submaximal insulin, or amino acid mixture with submaximal insulin. Glucose uptake was greater for both insulin-stimulated treatments compared with the non-insulin-stimulated treatment groups but amino acids only increased glucose uptake in the presence of insulin. Phosphatidylinositol 3-kinase (PI 3-kinase) activity was greater for both insulin-stimulated treatments with amino acids having no additional impact. Akt substrate of 160 kDa (AS160) phosphorylation, however, was increased by the amino acids in the presence of insulin, but not in the absence of insulin. AMPK was unaffected by insulin or amino acids. Plasma membrane GLUT4 protein concentration was greater in the rats treated with insulin compared with no insulin in the perfusate. In the presence of insulin, amino acids increased GLUT4 density in the plasma membrane but had no effect in the absence of insulin. AS160 phosphorylation and plasma membrane GLUT4 density accounted for 76% of the variability in muscle glucose uptake. Collectively, these findings suggest that the beneficial effects of an amino acid mixture on skeletal muscle glucose uptake, in the presence of a submaximal insulin concentration, are due to an increase in AS160 phosphorylation and plasma membrane-associated GLUT4, but independent of PI 3-kinase and AMPK activation.

Toward a unifying hypothesis of metabolic syndrome

Despite a lack of consistent diagnostic criteria, the metabolic syndrome (MetS) is increasingly evident in children and adolescents, portending a tsunami of chronic disease and mortality as this generation ages. The diagnostic criteria for MetS apply absolute cutoffs to continuous variables and fail to take into account aging, pubertal changes, and race/ethnicity. We attempt to define MetS mechanistically to determine its specific etiologies and to identify targets for therapy. Whereas the majority of studies document a relationship of visceral fat to insulin resistance, ectopic liver fat correlates better with dysfunctional insulin dynamics from which the rest of MetS derives. In contrast to the systemic metabolism of glucose, the liver is the primary metabolic clearinghouse for 4 specific foodstuffs that have been associated with the development of MetS: trans-fats, branched-chain amino acids, ethanol, and fructose. These 4 substrates (1) are not insulin regulated and (2) deliver metabolic intermediates to hepatic mitochondria without an appropriate "pop-off" mechanism for excess substrate, enhancing lipogenesis and ectopic adipose storage. Excessive fatty acid derivatives interfere with hepatic insulin signal transduction. Reactive oxygen species accumulate, which cannot be quenched by adjacent peroxisomes; these reactive oxygen species reach the endoplasmic reticulum, leading to a compensatory process termed the "unfolded protein response," driving further insulin resistance and eventually insulin deficiency. No obvious drug target exists in this pathway; thus, the only rational therapeutic approaches remain (1) altering hepatic substrate availability (dietary modification), (2) reducing hepatic substrate flux (high fiber), or (3) increasing mitochondrial efficiency (exercise).

The effects of an amino acid supplement on glucose homeostasis, inflammatory markers, and incretins after laparoscopic gastric bypass

BACKGROUND

Protein supplements are routinely used after a laparoscopic gastric bypass (LGB). The aim of this study was to evaluate the impact of an amino acid supplement on glucose homeostasis and hormonal and inflammatory markers after LGB.

STUDY DESIGN

Thirty patients undergoing LGB were randomized to receive or not 24 g of an oral supplement containing a leucine metabolite, glutamine, and arginine twice daily. Changes in weight, glucose, insulin, C-peptide, insulin sensitivity, interleukin (IL) 6, C-reactive protein (CRP), leptin, insulin-like growth factor (IGF) 1, ghrelin, and incretins were assessed preoperatively and 2 weeks and 8 weeks postoperatively.

RESULTS

Thirty patients (96.7% female, age 46.9 ± 8.4 years, body mass index 43.3 ± 4.1 kg/m(2)) were randomized. The experimental (n = 14) and control (n = 16) groups were not significantly different at baseline. Weight loss was similar for the 2 groups. Fasting glucose decreased significantly at 2 and 8 weeks compared with base line (p < 0.0001) with no difference between the experimental and control groups (p = 0.8), but insulin and calculated insulin sensitivity, which were similar at baseline, became significantly worse in the experimental group 8 weeks after surgery (p = 0.02 for insulin; p = 0.04 for the homeostasis model assessment of insulin resistance). CRP and IL-6, which were similar at baseline, were found to be significantly higher at 8 weeks in the experimental group (p = 0.018 and p = 0.05, respectively). Leptin and IGF-1 levels decreased significantly from baseline at 2 and 8 weeks (p < 0.0001), but there was no difference between the 2 groups. No significant changes in GLP-1, ghrelin, or gastric inhibitory polypeptide were noticed after 8 weeks.

CONCLUSIONS

An amino acid supplement had no effect on the early postoperative incretins after LGB. It may have a negative influence on glucose kinetics and degree of inflammation. Future studies are needed to clarify these effects.

Excess leucine intake enhances muscle anabolic signaling but not net protein anabolism in young men and women

Essential amino acids (EAA) stimulate skeletal muscle protein synthesis (MPS) in humans. Leucine may have a greater stimulatory effect on MPS than other EAA and/or decrease muscle protein breakdown (MPB). To determine the effect of 2 different leucine concentrations on muscle protein turnover and associated signaling, young men (n = 6) and women (n = 8) ingested 10 g EAA in 1 of 2 groups: composition typical of high quality proteins (CTRL; 1.8 g leucine) or increased leucine concentration (LEU; 3.5 g leucine). Participants were studied for 180 min postingestion. Fractional synthetic rate and leg phenylalanine and leucine kinetics were assessed on muscle biopsies using stable isotopic techniques. Signaling was determined by immunoblotting. Arterial leucine concentration and delivery to the leg increased in both groups and was significantly higher in LEU than in CTRL; however, transport into the muscle and intracellular availability did not differ between groups. MPS increased similarly in both groups 60 min postingestion. MPB decreased at 60 min only in LEU, but net muscle protein balance improved similarly. Components of mammalian target of rapamycin (mTOR) signaling were improved in LEU, but no changes were observed in ubiquitin-proteasome system signaling. Changes in light chain 3 and mTOR association with Unc-51-like kinase 1 indicate autophagy decreased more in LEU. We conclude that in 10 g of EAA, the leucine content typical of high quality proteins (~1.8 g) is sufficient to induce a maximal skeletal muscle protein anabolic response in young adults, but leucine may play a role in autophagy regulation.

Dairy protein attenuates weight gain in obese rats better than whey or casein alone

Evidence suggests that dietary calcium (Ca) and particularly dairy foods may attenuate weight gain and improve symptoms of the metabolic syndrome. The purpose of this study was to determine the effect of different Ca-enriched dairy protein sources on the prevention of weight gain in Sprague-Dawley diet-induced obese (DIO) rats. Twelve week-old DIO rats were assigned to one of eight ad libitum diets that varied in protein source (casein, whey, or complete dairy), Ca content (0.67 or 2.4%) and energy level (high fat/high sucrose (HFHS); or normal calorie density (NC)). Body composition and response to a meal tolerance test (MTT) were measured. Average daily caloric intake did not differ within normal or high energy density groups. At the end of 8 weeks, the dairy/HFHS/0.67% and 2.4% groups had significantly lower body weight than all other HFHS groups. The dairy/HFHS/0.67% and 2.4% groups also had lower body fat and greater lean mass expressed as a percent (P < 0.05). Homeostatic model assessment of insulin resistance (HOMA(IR)) was lowest for dairy/HFHS/0.67% and significantly different from whey/HFHS/0.67% and 2.4%. Independent of protein source, high Ca decreased plasma insulin at 30 min in the MTT more so than low Ca (P < 0.05). Hepatic sterol regulatory element-binding protein (SREBP1c) and peroxisome proliferator-activated receptor-gamma (PPARgamma) mRNA was downregulated by dairy and whey compared to casein in the HFHS/0.67% diets. Overall, these data suggest that complete dairy improves body composition and insulin sensitivity to a greater extent than whey or casein alone.

Signal transducer and activator of transcription 3 (STAT3) mediates amino acid inhibition of insulin signaling through serine 727 phosphorylation

Nutrient overload is associated with the development of obesity, insulin resistance, and type II diabetes. High plasma concentrations of amino acids have been found to correlate with insulin resistance. At the cellular level, excess amino acids impair insulin signaling, the mechanisms of which are not fully understood. Here, we report that STAT3 plays a key role in amino acid dampening of insulin signaling in hepatic cells. Excess amino acids inhibited insulin-stimulated Akt phosphorylation and glycogen synthesis in mouse primary hepatocytes as well as in human hepatocarcinoma HepG2 cells. STAT3 knockdown protected insulin sensitivity from inhibition by amino acids. Amino acids stimulated the phosphorylation of STAT3 at Ser(727), but not Tyr(705). Replacement of the endogenous STAT3 with wild-type, but not S727A, recombinant STAT3 restored the ability of amino acids to inhibit insulin signaling, suggesting that Ser(727) phosphorylation was critical for STAT3-mediated amino acid effect. Furthermore, overexpression of STAT3-S727D was sufficient to inhibit insulin signaling in the absence of excess amino acids. Our results also indicated that mammalian target of rapamycin was likely responsible for the phosphorylation of STAT3 at Ser(727) in response to excess amino acids. Finally, we found that STAT3 activity and the expression of its target gene socs3, known to be involved in insulin resistance, were both stimulated by excess amino acids and inhibited by rapamycin. In conclusion, our study reveals STAT3 as a novel mediator of nutrient signals and identifies a Ser(727) phosphorylation-dependent and Tyr(705) phosphorylation-independent STAT3 activation mechanism in the modulation of insulin signaling.

Leucine modulates contraction- and insulin-stimulated glucose transport and upstream signaling events in rat skeletal muscle

Leucine has profound effects on glucose metabolism in muscle; however, the effects of leucine on glucose transport in muscle have not been well documented. We investigated the effects of leucine on contraction- and insulin-stimulated glucose transport in isolated rat epitrochlearis muscle in vitro. In the absence of insulin, tetanic contraction increased 3-O-methyl-D-glucose (3-MG) transport and Thr(172) phosphorylation of the catalytic alpha-subunit of 5'-AMP-activated protein kinase (AMPK), a signaling intermediary leading to insulin-independent glucose transport. Leucine (2 mM, 30 min) significantly enhanced contraction-stimulated 3-MG transport and AMPK phosphorylation, accompanied by increased phosphorylation of p70 S6 kinase (p70S6K) Thr(389). The stimulatory effects of leucine on 3-MG transport and AMPK phosphorylation were canceled by STO-609 blockade of Ca(2+)/calmodulin-dependent protein kinase kinase (CaMKK) or rapamycin blockade of p70S6K. On the other hand, leucine blunted insulin-stimulated 3-MG transport and reduced insulin-stimulated Akt Thr(473) phosphorylation. Leucine increased insulin-stimulated p70S6K Thr(389) phosphorylation and enhanced the inhibitory phosphorylation of the insulin receptor substrate 1 (IRS1) Ser(636/639). Furthermore, the effects of leucine on insulin-stimulated 3-MG transport and IRS phosphorylation were abolished by rapamycin. These results indicate that leucine activates contraction-stimulated glucose transport and inhibits insulin-stimulated glucose transport in skeletal muscle by activating mammalian target of rapamycin (mTOR)/p70S6K signaling. Enhanced increases in contraction-stimulated AMPK Thr(172) phosphorylation and insulin-stimulated IRS1 Ser(636/639) phosphorylation might be responsible for these opposing effects of leucine, respectively.

Forskolin attenuates the action of insulin on the Akt–mTOR pathway in human skeletal muscle

Forskolin (FSK) is capable of both stimulating and inhibiting the intracellular signaling pathways of protein synthesis tissues other than skeletal muscle. The purpose of this investigation was to determine if FSK administration affects various elements of the protein kinase B (Akt)–mammalian target of rapamycin (mTOR) pathway in human skeletal muscle. Ten (n = 10) healthy, young (21.6 ± 1.3 years), nonobese (body mass index = 25.5 ± 3.5 kg·m–2), recreationally active males were selected for participation. Following an 8 h fast, 2 muscle biopsies of the vastus lateralis were performed. The samples were sectioned and exposed to 4 in vitro treatment conditions: basal, FSK, insulin (INS), and FSK+INS. The samples were then analyzed for total and phosphorylated levels of Akt, mTOR, S6 kinase (S6K1), and 4E binding protein (4EBP1). Akt phosphorylation was significantly greater in the INS-treated samples compared with the basal and FSK conditions (p = 0.007). Furthermore, the ratio of phosphorylated Akt to total Akt (P/T) was higher in the INS samples compared with the basal and FSK samples (p = 0.001). There were no differences in mTOR phosphorylation among the 4 groups; however, total mTOR was significantly greater in the FSK+INS group (p = 0.006). There were also no differences in phosphorylated or total levels of S6K1 among the 4 groups. However, 4EBP1 phosphorylation was significantly greater in the INS-treated samples compared with the basal (p = 0.003) and FSK (p = 0.004) treatments. There were no differences in the ratio of phosphorylated 4EBP1 to total 4EBP1 (P/T) among the 4 groups. These results indicate that FSK does not activate the Akt–mTOR pathway in human skeletal muscle; however, these results suggest that FSK may inhibit the actions of INS on this pathway.

Role of Dietary Proteins and Amino Acids in the Pathogenesis of Insulin Resistance

Dietary proteins and amino acids are important modulators of glucose metabolism and insulin sensitivity. Although high intake of dietary proteins has positive effects on energy homeostasis by inducing satiety and possibly increasing energy expenditure, it has detrimental effects on glucose homeostasis by promoting insulin resistance and increasing gluconeogenesis. Varying the quality rather than the quantity of proteins has been shown to modulate insulin resistance induced by Western diets and has revealed that proteins derived from fish might have the most desirable effects on insulin sensitivity. In vitro and in vivo data also support an important role of amino acids in glucose homeostasis through modulation of insulin action on muscle glucose transport and hepatic glucose production, secretion of insulin and glucagon, as well as gene and protein expression in various tissues. Moreover, amino acid signaling is integrated by mammalian target of rapamycin, a nutrient sensor that operates a negative feedback loop toward insulin receptor substrate 1 signaling, promoting insulin resistance for glucose metabolism. This integration suggests that modulating dietary proteins and the flux of circulating amino acids generated by their consumption and digestion might underlie powerful new approaches to treat various metabolic diseases such as obesity and diabetes.

Effect of protein source and resistance training on body composition and sex hormones

Background

Evidence suggests an inverse relationship between soy protein intake and serum concentrations of male sex hormones. Anecdotal evidence indicates that these alterations in serum sex hormones may attenuate changes in lean body mass following resistance training. However, little empirical data exists regarding the effects of soy and milk-based proteins on circulating androgens and exercise induced body composition changes.

Methods

For 12 weeks 20 subjects were supplemented with 50 g per day of one of four different protein sources (Soy concentrate; Soy isolate; Soy isolate and whey blend, and Whey blend only) in combination with a resistance-training program. Body composition, testosterone, estradiol and sex hormone binding globulin (SHBG) were measured at baseline and week 12.

Results

Protein supplementation resulted in a significant increase in lean body mass independent of protein source (0.5 ± 1.1 and 0.9 ± 1.4 kg, p = 0.006, p = 0.007). No significant differences were observed between groups for total and free testosterone, SHBG, percentage body fat, BMI or body weight. The Testosterone/Estradiol ratio increased across all groups (+13.4, p = 0.005) and estradiol decreased (p = 0.002). Within group analysis showed significant increases in the Testosterone/Estradiol ratio in soy isolate + whey blend group (+16.3, p = 0.030). Estradiol was significantly lower in the whey blend group (-9.1 ± 8.7 pg/ml, p = 0.033).

Conclusion

This investigation shows that 12 week supplementation with soy protein does not decrease serum testosterone or inhibit lean body mass changes in subjects engaged in a resistance exercise program.

Hypoglycemic effect of isoleucine involves increased muscle glucose uptake and whole body glucose oxidation and decreased hepatic gluconeogenesis

Isoleucine, a branched chain amino acid, plays an important role in the improvement of glucose metabolism as evidenced by the increase of insulin-independent glucose uptake in vitro. This study evaluated the effect of isoleucine on glucose uptake and oxidation in fasted rats and on gluconeogenesis in vivo and in vitro. Oral administration of isoleucine decreased the plasma glucose level by 20% and significantly increased muscle glucose uptake by 71% without significant elevation of the plasma insulin level compared with controls at 60 min after administration. Furthermore, expiratory excretion of 14CO2 from [U-14C]glucose in isoleucine-administered rats was increased by 19% compared with controls. Meanwhile, isoleucine decreased AMP levels in the liver but did not affect hepatic glycogen synthesis. Under insulin-free conditions, isoleucine significantly inhibited glucose production when alanine was used as a glucogenic substrate in isolated hepatocytes. This inhibition by isoleucine was also associated with a decline in mRNA levels for phosphoenolpyruvate carboxykinase and glucose-6-phosphatase (G6Pase) and a decreased activity of G6Pase in isolated hepatocytes. These findings suggest that a reduction of gluconeogenesis in liver, along with an increase of glucose uptake in the muscle, is also involved in the hypoglycemic effect of isoleucine. In conclusion, isoleucine administration stimulates both glucose uptake in the muscle and whole body glucose oxidation, in addition to depressing gluconeogenesis in the liver, thereby leading to the hypoglycemic effect in rats.

Dietary protein and lactose increase translation initiation factor activation and tissue protein synthesis in neonatal pigs

Protein synthesis and eukaryotic initiation factor (eIF) activation are increased in muscle and liver of pigs parenterally infused with amino acids and insulin. To examine the effects of enteral protein and carbohydrate on protein synthesis, pigs (n = 42, 1.7 kg body wt) were fed isocaloric milk diets containing three levels of protein (5, 15, and 25 g x kg body wt(-1) x day(-1)) and two levels of lactose (low = 11 and high = 23 g x kg body wt(-1) x day(-1)) from 1 to 6 days of age. On day 7, pigs were gavage fed after 4-h food deprivation, and tissue protein synthesis rates and biomarkers of mRNA translation were assessed. Piglet growth and protein synthesis rates in muscle and liver increased with dietary protein and plateaued at 15 g x kg body wt(-1) x day(-1) (P < 0.001). Growth tended to be greater in high-lactose-fed pigs (P = 0.07). Plasma insulin was lowest in pigs fed 5 g x kg body wt(-1) x day(-1) protein (P < 0.0001). Plasma branched-chain amino acids increased as protein intake increased (P < 0.0001). Muscle (P < 0.001) and liver (P < or = 0.001) ribosomal protein S6 kinase-1 and eIF4E-binding protein phosphorylation increased with protein intake and plateaued at 15 g x kg body wt(-1) x day(-1). The results indicate that growth and protein synthesis rates in neonatal pigs are influenced by dietary protein and lactose intake and might be mediated by plasma amino acids and insulin levels. However, feeding protein well above the piglet's requirement does not further stimulate the activation of translation initiation or protein synthesis in skeletal muscle and liver.

Modulation of insulin action by dietary proteins and amino acids: role of the mammalian target of rapamycin nutrient sensing pathway

PURPOSE OF REVIEW

An increasing number of studies point towards an important role of dietary proteins and amino acids in the modulation of insulin action in peripheral tissues. The purpose of this review is to discuss how these nutrients affect insulin sensitivity and the potential mechanism by which they exert their action.

RECENT FINDINGS

Increased plasma amino acid availability in both animals and humans has been shown to cause enhanced translation initiation and protein synthesis and the inhibition of insulin-stimulated glucose transport in skeletal muscle. Moreover, dietary interventions in animals fed proteins from various sources resulted in drastically different outcomes in terms of glucose metabolism and insulin signaling in skeletal muscles. Finally, amino acids, particularly leucine, were shown to modulate insulin action by specifically activating the mammalian target of rapamycin nutrient sensing pathway.

SUMMARY

Dietary proteins and amino acids are important modulators of glucose metabolism and insulin signaling via their ability, at least partly, to modulate the mammalian target of rapamycin pathway.