Anton J. M. Wagenmakers

The effects of increasing exercise intensity on muscle fuel utilisation in humans

1 Contemporary stable isotope methodology was applied in combination with muscle biopsy sampling to accurately quantify substrate utilisation and study the regulation of muscle fuel selection during exercise. 2 Eight cyclists were studied at rest and during three consecutive 30 min stages of exercise at intensities of 40, 55 and 75 % maximal workload (Wmax). A continuous infusion of [U‐13C]palmitate and [6,6‐2H2]glucose was administered to determine plasma free fatty acid (FFA) oxidation and estimate plasma glucose oxidation, respectively. Biopsy samples were collected before and after each exercise stage. 3 Muscle glycogen and plasma glucose oxidation rates increased with every increment in exercise intensity. Whole‐body fat oxidation increased to 32 ± 2 kJ min−1 at 55 % Wmax, but declined at 75 % Wmax (19 ± 2 kJ min−1). This decline involved a decrease in the oxidation rate of both plasma FFA and triacylglycerol fat sources (sum of intramuscular plus lipoprotein‐derived triacylglycerol), and was accompanied by increases in muscle pyruvate dehydrogenase complex activation and acetylation of the carnitine pool, resulting in a decline in muscle free carnitine concentration. 4 We conclude that the most likely mechanism for the reduction in fat oxidation during high‐intensity exercise is a downregulation of carnitine palmitoyltransferase I, either by this marked decline in free carnitine availability or by a decrease in intracellular pH.

Preferential Uptake of Dietary Fatty Acids in Adipose Tissue and Muscle in the Postprandial Period

Despite consistent evidence that abnormalities of fatty acid delivery and storage underlie the metabolic defects of insulin resistance, physiological pathways by which fat is stored in adipose tissue and skeletal muscle are not clear. We used a combination of stable isotope labeling and arteriovenous difference measurements to elucidate pathways of postprandial fat deposition in adipose tissue and skeletal muscle in healthy humans. A test meal containing [U-13C]palmitate was combined with intravenous infusion of [2H2]palmitate to label plasma fatty acids and VLDL-triglyceride. Both dietary (chylomicron) and VLDL-triglyceride were cleared across adipose tissue and muscle, though with greater fractional extraction of the chylomicron-triglyceride. In adipose tissue there was significant uptake of plasma nonesterified fatty acids (NEFAs) in the postprandial but not the fasting state. However, this was minor in comparison with chylomicron-triglyceride fatty acids. We modeled the fate of fatty acids released by lipoprotein lipase (LPL). There was clear preferential uptake of these fatty acids compared with plasma NEFAs. In muscle, there was unexpected evidence for release of LPL-derived fatty acids into the plasma. With this integrative physiological approach, we have revealed hidden complexities in pathways of fatty acid uptake in adipose tissue and skeletal muscle.

Muscle amino acid metabolism at rest and during exercise: role in human physiology and metabolism.

Six amino acids are metabolized in resting muscle. They are leucine, isoleucine, valine, asparagine, aspartate, and glutamate. These amino acids provide the amino groups and probably the ammonia required for synthesis of glutamine and alanine, which are released in excessive amounts in the postabsorptive state and during ingestion of a protein-containing meal. Only leucine and part of the isolecine molecule can be oxidized in muscle as they are converted to acetyl-CoA. The other carbon skeletons are used solely for de novo synthesis of TCA-cycle intermediates and glutamine. The carbon atoms of the released alanine originate primarily from glycolysis of blood glucose and from muscle glycogen (about half each in resting conditions). After consumption of a protein-containing meal, BCAA and glutamate are taken up by muscle and their carbon skeletons are used for de novo synthesis of glutamine. About half of the glutamine released from muscle originates from glutamate taken up from the blood, both after overnight starvation, after prolonged starvation, and after consumption of a mixed meal. Glutamine produced by muscle is an important fuel and regulator of DNA and RNA synthesis in mucosal cells and immune system cells, and fulfils several other important functions in human metabolism. The alanine aminotransferase reaction functions to establish and maintain high concentrations of TCA-cycle intermediates in muscle during the first 10 min of exercise. The increase in concentration of TCA-cycle intermediates probably is needed to increase the flux of the TCA-cycle and meet the increased energy demand of exercise. A gradual increase in leucine oxidation subsequently leads to a carbon drain on the TCA-cycle in glycogen-depleted muscles, and may thus reduce the maximal flux in the TCA-cycle and lead to fatigue. Deamination of amino acids and glutamine synthesis present alternative anaplerotic mechanisms in glycogen-depleted muscles, but only allow exercise at 40-50% of Wmax. One-leg exercise leads to the net breakdown of muscle protein. The liberated amino acids are used for synthesis of TCA-cycle intermediates and glutamine. Today, the importance of this process in endurance exercise in the field (running or cycling) in athletes who ingest carbohydrates is not clear. It is proposed that the maximal flux in the TCA-cycle is reduced in glycogen-depleted muscles due to insufficient TCA-cycle anaplerosis, and that this presents a limitation for the maximal rate of fatty acid oxidation. Interactions between the amino acid pool and the TCA-cycle are suggested to play a central role in the energy metabolism of the exercising muscle.

Ingestion of branched-chain amino acids and tryptophan during sustained exercise in man: failure to affect performance.

1. An increased uptake of tryptophan in the brain may increase serotoninergic activity and recently has been suggested to be a cause of fatigue during prolonged exercise. The present study, therefore, investigates whether ingestion of tryptophan or the competing branched‐chain amino acids (BCAAs) affect performance. Ten endurance‐trained male athletes were studied during cycle exercise at 70‐75% maximal power output, while ingesting, ad random and double‐blind, drinks that contained 6% sucrose (control) or 6% sucrose supplemented with (1) tryptophan (3 g l‐1), (2) a low dose of BCAA (6 g l‐1) or (3) a high dose of BCAA (18 g l‐1). 2. These treatments greatly increased the plasma concentration of the respective amino acids. Using the kinetic parameters of transport of human brain capillaries, BCAA supplements were estimated to reduce brain tryptophan uptake at exhaustion by 8‐12%, while tryptophan ingestion caused a 7‐ to 20‐fold increase. Exercise time to exhaustion was not different between treatments (122 +/‐ 3 min). 3. The data suggest that manipulation of tryptophan supply to the brain either has no additional effect upon serotoninergic activity during prolonged exhaustive exercise or that manipulation of serotoninergic activity functionally does not contribute to mechanisms of fatigue.

Glucose kinetics during prolonged exercise in highly trained human subjects: effect of glucose ingestion

1 The objectives of this study were (1) to investigate whether glucose ingestion during prolonged exercise reduces whole body muscle glycogen oxidation, (2) to determine the extent to which glucose disappearing from the plasma is oxidized during exercise with and without carbohydrate ingestion and (3) to obtain an estimate of gluconeogenesis. 2 After an overnight fast, six well‐trained cyclists exercised on three occasions for 120 min on a bicycle ergometer at 50% maximum velocity of O2 uptake and ingested either water (Fast), or a 4% glucose solution (Lo‐Glu) or a 22% glucose solution (Hi‐Glu) during exercise. 3 Dual tracer infusion of [U‐13C]‐glucose and [6,6‐2H2]‐glucose was given to measure the rate of appearance (Ra) of glucose, muscle glycogen oxidation, glucose carbon recycling, metabolic clearance rate (MCR) and non‐oxidative disposal of glucose. 4 Glucose ingestion markedly increased total Ra especially with Hi‐Glu. After 120 min Ra and rate of disappearance (Rd) of glucose were 51‐52 μmol kg−1 min−1 during Fast, 73‐74 μmol kg−1 min−1 during Lo‐Glu and 117–119 μmol kg−1 min−1 during Hi‐Glu. The percentage of Rd oxidized was between 96 and 100% in all trials. 5 Glycogen oxidation during exercise was not reduced by glucose ingestion. The vast majority of glucose disappearing from the plasma is oxidized and MCR increased markedly with glucose ingestion. Glucose carbon recycling was minimal suggesting that gluconeogenesis in these conditions is negligible.

Carbohydrate ingestion can completely suppress endogenous glucose production during exercise

The purposes of this study were 1) to investigate the effect of carbohydrate (CHO) ingestion on endogenous glucose production (EGP) during prolonged exercise, 2) to study whether glucose appearance in the circulation could be a limiting factor for exogenous CHO oxidation, and 3) to investigate whether large CHO feedings can reduce muscle glycogen oxidation during exercise. Six well-trained subjects exercised three times for 120 min at 50% maximum workload while ingesting water (FAST), a 4% glucose solution (LO-Glc), or a 22% glucose solution (HI-Glc). A primed continuous intravenous [6,6-2H2]glucose infusion was given, and the ingested glucose was enriched with [U-13C]glucose. Glucose ingestion significantly elevated CHO oxidation as well as the rates of appearance (Ra) and disappearance. Ra glucose equaled Ra of glucose in gut (Ra gut) during HI-Glc, whereas EGP was completely suppressed. During LO-Glc, EGP was partially suppressed, whereas Ra gut provided most of the total glucose Ra. We conclude that 1) high rates of CHO ingestion can completely block EGP, 2) Ra gut may be a limiting factor for exogenous CHO oxidation, and 3) muscle glycogen oxidation was not reduced by large glucose feedings.The purposes of this study were 1) to investigate the effect of carbohydrate (CHO) ingestion on endogenous glucose production (EGP) during prolonged exercise, 2) to study whether glucose appearance in the circulation could be a limiting factor for exogenous CHO oxidation, and 3) to investigate whether large CHO feedings can reduce muscle glycogen oxidation during exercise. Six well-trained subjects exercised three times for 120 min at 50% maximum workload while ingesting water (FAST), a 4% glucose solution (LO-Glc), or a 22% glucose solution (HI-Glc). A primed continuous intravenous [6, 6-2H2]glucose infusion was given, and the ingested glucose was enriched with [U-13C]glucose. Glucose ingestion significantly elevated CHO oxidation as well as the rates of appearance (Ra) and disappearance. Ra glucose equaled Ra of glucose in gut (Ra gut) during HI-Glc, whereas EGP was completely suppressed. During LO-Glc, EGP was partially suppressed, whereas Ra gut provided most of the total glucose Ra. We conclude that 1) high rates of CHO ingestion can completely block EGP, 2) Ra gut may be a limiting factor for exogenous CHO oxidation, and 3) muscle glycogen oxidation was not reduced by large glucose feedings.

Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance‐trained males in a fasted state

Both stable isotope methodology and fluorescence microscopy were applied to define the use of intramuscular triglyceride (IMTG) stores as a substrate source during exercise on a whole‐body as well as on a fibre type‐specific intramyocellular level in trained male cyclists. Following an overnight fast, eight subjects were studied at rest, during 120 min of moderate intensity exercise (60 % maximal oxygen uptake capacity (V̇O2,max)) and 120 min of post‐exercise recovery. Continuous infusions of [U‐13C]palmitate and [6,6‐2H2]glucose were administered at rest and during subsequent exercise to quantify whole‐body plasma free fatty acid (FFA) and glucose oxidation rates and the contribution of other fat sources (sum of muscle‐ plus lipoprotein‐derived TG) and muscle glycogen to total energy expenditure. Fibre type‐specific intramyocellular lipid content was determined in muscle biopsy samples collected before, immediately after and 2 h after exercise. At rest, fat oxidation provided 66 ± 5 % of total energy expenditure, with FFA and other fat sources contributing 48 ± 6 and 17 ± 3 %, respectively. FFA oxidation rates increased during exercise, and correlated well with the change in plasma FFA concentrations. Both the use of other fat sources and muscle glycogen declined with the duration of exercise, whereas plasma glucose production and utilisation increased (P < 0.001). On average, FFA, other fat sources, plasma glucose and muscle glycogen contributed 28 ± 3, 15 ± 2, 12 ± 1 and 45 ± 4 % to total energy expenditure during exercise, respectively. Fluorescence microscopy revealed a 62 ± 7 % net decline in muscle lipid content following exercise in the type I fibres only, with no subsequent change during recovery. We conclude that IMTG stores form an important substrate source during moderate intensity exercise in endurance‐trained male athletes following an overnight fast, with the oxidation rate of muscle‐ plus lipoprotein‐derived TG being decreased with the duration of exercise.

Exercise induced activation of the branched-chain 2-oxo acid dehydrogenase in human muscle

SummaryThe present study was conducted to investigate the metabolic regulation of the oxidation of branched-chain amino acids (BCAA) by exercise in human skeletal muscle. Five trained male volunteers were exercised on a cycle ergometer at 70%±10% (mean±SD) of their maximal oxygen consumption % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Jf9qqqrpepC0xf9qiW7rqqrFfpeea0xe9LqFf0x% c9q8qqaqFn0dXdir-xcvk9pIe9q8qqaq-xir-f0-yqaqVeLsFr0-vr% 0-vr0xc8meaabaqaciGacaGaaeqabaWaaeaaeaaakeaacaGGOaGabm% OvayaacaWaaSbaaSqaaGqaaiacCb4FpbWaaSbaaWqaaiaa-jdacaWF% TbGaa8xyaiaa-HhaaeqaaaWcbeaakiaacMcaaaa!41C5!

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

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