Peter W. Watt

An obesity-associated FTO gene variant and increased energy intake in children

BACKGROUND Variation in the fat mass and obesity-associated (FTO) gene has provided the most robust associations with common obesity to date. However, the role of FTO variants in modulating specific components of energy balance is unknown. METHODS We studied 2726 Scottish children, 4 to 10 years of age, who underwent genotyping for FTO variant rs9939609 and were measured for height and weight. A subsample of 97 children was examined for possible association of the FTO variant with adiposity, energy expenditure, and food intake. RESULTS In the total study group and the subsample, the A allele of rs9939609 was associated with increased weight (P=0.003 and P=0.049, respectively) and body-mass index (P=0.003 and P=0.03, respectively). In the intensively phenotyped subsample, the A allele was also associated with increased fat mass (P=0.01) but not with lean mass. Although total and resting energy expenditures were increased in children with the A allele (P=0.009 and P=0.03, respectively), resting energy expenditure was identical to that predicted for the age and weight of the child, indicating that there is no defect in metabolic adaptation to obesity in persons bearing the risk-associated allele. The A allele was associated with increased energy intake (P=0.006) independently of body weight. In contrast, the weight of food ingested by children who had the allele was similar to that in children who did not have the allele (P=0.82). CONCLUSIONS The FTO variant that confers a predisposition to obesity does not appear to be involved in the regulation of energy expenditure but may have a role in the control of food intake and food choice, suggesting a link to a hyperphagic phenotype or a preference for energy-dense foods.

Muscle full effect after oral protein: time-dependent concordance and discordance between human muscle protein synthesis and mTORC1 signaling

BACKGROUND We previously showed that human muscle protein synthesis (MPS) increased during infusion of amino acids (AAs) and peaked at ≈120 min before returning to baseline rates, despite elevated plasma AA concentrations. OBJECTIVE We tested whether a protein meal elicited a similar response and whether signaling responses that regulate messenger RNA translation matched MPS changes. DESIGN Eight postabsorptive healthy men (≈21 y of age) were studied during 8.5 h of primed continuous infusion of [1,2-¹³C₂]leucine with intermittent quadriceps biopsies for determination of MPS and anabolic signaling. After 2.5 h, subjects consumed 48 g whey protein. RESULTS At 45-90 min after oral protein bolus, mean (± SEM) myofibrillar protein synthesis increased from 0.03 ± 0.003% to 0.10 ± 0.01%/h; thereafter, myofibrillar protein synthesis returned to baseline rates even though plasma essential AA (EAA) concentrations remained elevated (+130% at 120 min, +80% at 180 min). The activity of protein kinase B (PKB) and phosphorylation of eukaryotic initiation factor 4G preceded the rise of MPS and increases in phosphorylation of ribosomal protein kinase S6 (S6K1), and 4E-binding protein 1 (4EBP1) was superimposable with MPS responses until 90 min. However, although MPS decreased thereafter, all signals, with the exception of PKB activity (which mirrored insulin responses), remained elevated, which echoed the slowly declining plasma EAA profile. The phosphorylation of eukaryotic initiation factor 2α increased only at 180 min. Thus, discordance existed between MPS and the mammalian target of rapamycin complex 1 (mTORC1) and signaling (ie, S6K1 and 4EBP1 phosphorylation). CONCLUSIONS We confirm our previous findings that MPS responses to AAs are transient, even with oral protein bolus. However, changes in MPS only reflect elevated mTORC1 signaling during the upswing in MPS.

Lactate--a signal coordinating cell and systemic function.

SUMMARY Since its first documented observation in exhausted animal muscle in the early 19th century, the role of lactate (lactic acid) has fascinated muscle physiologists and biochemists. Initial interpretation was that lactate appeared as a waste product and was responsible in some way for exhaustion during exercise. Recent evidence, and new lines of investigation, now place lactate as an active metabolite, capable of moving between cells, tissues and organs, where it may be oxidised as a fuel or reconverted to form pyruvate or glucose. The questions now to be asked concern the effects of lactate at the systemic and cellular level on metabolic processes. Does lactate act as a metabolic signal to specific tissues, becoming a metabolite pseudo-hormone? Does lactate have a role in whole-body coordination of sympathetic/parasympathetic nerve system control? And, finally, does lactate play a role in maintaining muscle excitability during intense muscle contraction? The concept of lactate acting as a signalling compound is a relatively new hypothesis stemming from a combination of comparative, cell and whole-organism investigations. It has been clearly demonstrated that lactate is capable of entering cells via the monocarboxylate transporter (MCT) protein shuttle system and that conversion of lactate to and from pyruvate is governed by specific lactate dehydrogenase isoforms, thereby forming a highly adaptable metabolic intermediate system. This review is structured in three sections, the first covering pertinent topics in lactates history that led to the model of lactate as a waste product. The second section will discuss the potential of lactate as a signalling compound, and the third section will identify ways in which such a hypothesis might be investigated. In examining the history of lactate research, it appears that periods have occurred when advances in scientific techniques allowed investigation of this metabolite to expand. Similar to developments made first in the 1920s and then in the 1980s, contemporary advances in stable isotope, gene microarray and RNA interference technologies may allow the next stage of understanding of the role of this compound, so that, finally, the fundamental questions of lactates role in whole-body and localised muscle function may be answered.

Insulin activates protein kinase B, inhibits glycogen synthase kinase‐3 and activates glycogen synthase by rapamycin‐insensitive pathways in skeletal muscle and adipose tissue

Insulin stimulated protein kinase Bα (PKBα) more than 10‐fold and decreased glycogen synthase kinase‐3 (GSK3) activity by 50±10% in skeletal muscle and adipocytes. Rapamycin did not prevent the activation of PKB, inhibition of GSK3 or stimulation of glycogen synthase up to 5 min. Thus rapamycin‐insensitive pathways mediate the acute effect of insulin on glycogen synthase in the major insulin‐responsive tissues. The small and very transient effects of EGF on phosphatidylinositol (3,4,5)P3 PKBα and GSK3 in adipocytes, compared to the strong and sustained effects of insulin, explains why EGF does not stimulate glucose uptake or glycogen synthesis in adipocytes

CHARACTERISTICS OF A GLUTAMINE CARRIER IN SKELETAL MUSCLE HAVE IMPORTANT CONSEQUENCES FOR NITROGEN LOSS IN INJURY, INFECTION, AND CHRONIC DISEASE

A carrier for glutamine, identified in rat muscle, has properties in terms of kinetics, ion dependence and hormone sensitivity, and effects of endotoxin and branched-chain aminoacids that point to an important function in the control of whole-body aminoacid metabolism. The existence of a link between the size of the glutamine pool in muscle and the rate of muscle protein synthesis raises possibilities for therapeutic interventions to limit protein loss in injury, sepsis, and chronic disease.

Fat utilization during exercise: adaptation to a fat-rich diet increases utilization of plasma fatty acids and very low density lipoprotein-triacylglycerol in humans

1 This study was carried out to test the hypothesis that the greater fat oxidation observed during exercise after adaptation to a high‐fat diet is due to an increased uptake of fat originating from the bloodstream. 2 Of 13 male untrained subjects, seven consumed a fat‐rich diet (62% fat, 21% carbohydrate) and six consumed a carbohydrate‐rich diet (20% fat, 65% carbohydrate). After 7 weeks of training and diet, 60 min of bicycle exercise was performed at 68 ± 1% of maximum oxygen uptake. During exercise [1‐13C]palmitate was infused, arterial and venous femoral blood samples were collected, and blood flow was determined by the thermodilution technique. Muscle biopsy samples were taken from the vastus lateralis muscle before and after exercise. 3 During exercise, the respiratory exchange ratio was significantly lower in subjects consuming the fat‐rich diet (0.86 ± 0.01, mean ±s.e.m.) than in those consuming the carbohydrate‐rich diet (0.93 ± 0.02). The leg fatty acid (FA) uptake (183 ± 37 vs. 105 ± 28 μmol min−1) and very low density lipoprotein‐triacylglycerol (VLDL‐TG) uptake (132 ± 26 vs. 16 ± 21 μmol min−1) were both higher (each P < 0.05) in the subjects consuming the fat‐rich diet. Whole‐body plasma FA oxidation (determined by comparison of 13CO2 production and blood palmitate labelling) was 55‐65% of total lipid oxidation, and was higher after the fat‐rich diet than after the carbohydrate‐rich diet (13.5 ± 1.2 vs. 8.9 ± 1.1 μmol min−1 kg−1; P < 0.05). Muscle glycogen breakdown was significantly lower in the subjects taking the fat‐rich diet than those taking the carbohydrate‐rich diet (2.6 ± 0.5 vs. 4.8 ± 0.5 mmol (kg dry weight)−1 min−1, respectively; P < 0.05), whereas leg glucose uptake was similar (1.07 ± 0.13 vs. 1.15 ± 0.13 mmol min−1). 4 In conclusion, plasma VLDL‐TG appears to be an important substrate source during aerobic exercise, and in combination with the higher plasma FA uptake it accounts for the increased fat oxidation observed during exercise after fat diet adaptation. The decreased carbohydrate oxidation was apparently due to muscle glycogen sparing and not to diminished plasma glucose uptake.

Proteomic sensitivity to dietary manipulations in rainbow trout

Changes in dietary protein sources due to substitution of fish meal by other protein sources can have metabolic consequences in farmed fish. A proteomics approach was used to study the protein profiles of livers of rainbow trout that have been fed two diets containing different proportions of plant ingredients. Both diets control (C) and soy (S) contained fish meal and plant ingredients and synthetic amino acids, but diet S had a greater proportion of soybean meal. A feeding trial was performed for 12 weeks at the end of which, growth and protein metabolism parameters were measured. Protein growth rates were not different in fish fed different diets; however, protein consumption and protein synthesis rates were higher in the fish fed the diet S. Fish fed diet S had lower efficiency of retention of synthesised protein. Ammonia excretion was increased as well as the activities of hepatic glutamate dehydrogenase and aspartate amino transferase (ASAT). No differences were found in free amino acid pools in either liver or muscle between diets. Protein extraction followed by high-resolution two-dimensional electrophoresis, coupled with gel image analysis, allowed identification and expression of hundreds of protein. Individual proteins of interest were then subjected to further analysis leading to protein identification by trypsin digest fingerprinting. During this study, approximately 800 liver proteins were analysed for expression pattern, of which 33 were found to be differentially expressed between diets C and S. Seventeen proteins were positively identified after database searching. Proteins were identified from diverse metabolic pathways, demonstrating the complex nature of gene expression responses to dietary manipulation revealed by proteomic characterisation.

Inhibition of protein breakdown by glutamine in perfused rat skeletal muscle

We have assessed the effects of glutamine (Gln) availability on protein breakdown in perfused rat hindlimb by measuring net phenylalanine (Phe) production (an index of protein balance), the dilution of [15N]Phe labelling (an index of mixed protein breakdown) and rate of production of 3‐methylhistidine (3‐MeH) (an index of myofibrillar breakdown). 15 mM Gln significantly inhibited net protein loss and protein breakdown compared to rates obtained in its absence (net protein loss, 200±230 vs 2080±200 nmol Phe/hindlimb per h; protein breakdown, 4566±480 vs 1614±180 nmol Phe/hindlimb per h; both p<0.01). Insulin (100 μU/ml) inhibited protein breakdown but less than Gln. The effects on protein breakdown of Gln and insulin together were not additive, suggesting a common mode of action. Production of 3‐MeH (mean 20.3±2.8 nmol/hindlimb per h) was unaffected by Gln or insulin. Gln appears to inhibit protein breakdown of soluble rather than myofibrillar protein in muscle.

Skeletal muscle glutamine transport, intramuscular glutamine concentration, and muscle-protein turnover

This article reviews work we have carried out to investigate (1) the transport mechanisms responsible for the high distribution ratio of free glutamine commonly observed in skeletal muscle; (2) the fall in the distribution ratio that accompanies starvation, injury and chronic disease, whether directly involving muscle or not; and (3) the effect of modulation of intracellular free-glutamine concentration on protein synthesis and breakdown in skeletal muscle. We suggest that the results are consistent with the controlling role of the muscle membrane glutamine-sodium cotransporter in the regulation of the intracellular glutamine pool, the existence of pathophysiological mechanisms for the modulation of intramuscular glutamine and anabolic effects of glutamine in promoting protein synthesis, with a smaller effect in reducing protein breakdown. The mechanisms by which glutamine affects skeletal muscle protein turnover, and thus muscle protein balance, and the extent of the net flow of amino acids between the periphery and the viscera are unknown as yet, but the results suggest that modulation of transporter activity may offer the possibility of therapeutic intervention to reduce muscle wasting associated with injury and disease.

Ammonia metabolism, the brain and fatigue; revisiting the link

This review addresses the ammonia fatigue theory in light of new evidence from exercise and disease studies and aims to provide a view of the role of ammonia during exercise. Hyperammonemia is a condition common to pathological liver disorders and intense or exhausting exercise. In pathology, hyperammonemia is linked to impairment of normal brain function and the onset of the neurological condition, hepatic encephalopathy. Elevated blood ammonia concentrations arise due to a diminished capacity for removal via the liver and lead to increased exposure of organs, such as the brain, to the toxic effects of ammonia. High levels of brain ammonia can lead to deleterious alterations in astrocyte morphology, cerebral energy metabolism and neurotransmission, which may in turn impact on the functioning of important signalling pathways within the neuron. Such changes are believed to contribute to the disturbances in neuropsychological function, in particular the learning, memory, and motor control deficits observed in animal models of liver disease and also patients with cirrhosis. Hyperammonemia in exercise occurs as a result of an increased production by contracting muscle, through adenosine monophosphate (AMP) deamination (the purine nucleotide cycle) and branched chain amino acid (BCAA) deamination prior to oxidation. Plasma concentrations of ammonia during exercise often achieve or exceed those measured in liver disease patients, resulting in increased cerebral uptake. In this article we propose that exercise-induced hyperammonemia may lead to concomitant disturbances in brain function, potentially through similar mechanisms underpinning pathology, which may impact on performance as fatigue or reduced function, especially during extreme exercise.