Fat metabolism during exercise: a review--part II: regulation of metabolism and the effects of training.

Department of Human Biology, Maastricht University, The Netherlands. This part discusses the complex regulation of fat metabolism. Catecholamines as a stimulator of lipolysis and insulin as a suppressor play very important roles in the regulation of fat oxidation. The interaction of carbohydrate and fat metabolism has been extensively studied in the past decennia but the understanding of this multifactorial regulation is complex and still incompletely understood. In 1963, Randle et al. proposed the glucose-fatty acid cycle as a possible mechanism, and more recently, regulation through malonyl-CoA has been put forward as a possible way to explain shifts in carbohydrate and fat metabolism at rest and during exercise. The exercise intensity affects fat oxidation mainly by increasing lipolysis and fatty acid availability during exercise of low to moderate intensity. At high exercise intensities, both a reduction in fatty acid availability (decreased RaFa) and intramuscular factors reduce fat oxidation. These intramuscular factors are largely unknown. The increased mitochondrial density after training and increased oxidative enzymes may partly explain the increased fatty acid oxidation during exercise as observed after training. However, also supply of fatty acids to the mitochondria may be important. The available evidence suggests that the additional fatty acids oxidized after training are primarily derived from intramuscular triacylglycerols and not from adipose tissue derived fatty acids or circulating triacylglycerols. Publication Types: Review Review, Tutoria

Fat metabolism during exercise: a review--part III: effects of nutritional interventions.

Nutrition Research Center, Department of Human Biology, University Maastricht, The Netherlands. By changes in nutrition it is possible to manipulate fat oxidation. It is often theorized that increasing fat oxidation may reduce glycogen breakdown and thus enhance performance. Therefore, the effects of acute, short-term and long-term fat feeding have been subjects of investigation for many years. Ingestion of long-chain triacylglycerols (LCT) during exercise may reduce the gastric emptying rate and LCT will appear in the plasma only slowly. Medium-chain triacylglycerols (MCT) do not have these disadvantages and they are rapidly oxidized. However, the contribution of MCT to energy expenditure is only small because they can only be ingested in small amounts without causing gastrointestinal distress. So at present, fat supplementation in the hours preceding to or during exercise (either long chain or medium chain triacylglycerols) cannot be recommended. High-fat diets and fasting have been suggested to increase fatty acid availability and spare muscle glycogen resulting in improved performance. Both fasting and short term high-fat diets will decrease muscle glycogen content and reduce fatigue resistance. Chronic high-fat diets may provoke adaptive responses preventing the decremental effects on exercise performance. However, at present, there is little evidence to support this hypothesis. Also from a health perspective, caution should be exercised when recommending high-fat diets to athletes. Publication Types: Review Review, Tutoria

Reduced oxidation rates of ingested glucose during prolonged exercise with low endogenous CHO availability.

Reduced oxidation rates of ingested glucose during prolonged exercise with low endogenous CHO availability. Jeukendrup AE, Borghouts LB, Saris WH, Wagenmakers AJ. Department of Human Biology, University Maastricht, The Netherlands. [email protected] This study investigated the effect of endogenous carbohydrate (CHO) availability on oxidation rates of ingested glucose during moderate-intensity exercise. Seven well-trained cyclists performed two trials of 120 min of cycling exercise in random order at 57% maximal O2 consumption. Preexercise glycogen concentrations were manipulated by glycogen-lowering exercise in combination with CHO restriction [low-glycogen (LG) trial] or CHO loading [moderate-to-high-glycogen (HG) trial]. In the LG and HG trials, subjects ingested 4 ml/kg body wt of an 8% corn-derived glucose solution of high natural 13C abundance at the start, followed by boluses of 2 ml/kg every 15 min. The third trial, in which potato-derived glucose was ingested, served as a control test for background correction. Exogenous glucose oxidation rates were calculated from the 13C enrichment of the ingested glucose and of the breath CO2. Total CHO oxidation was lower in the LG trial than in the HG trial during 60-120 min of exercise [84 +/- 7 (SE) vs. 116 +/- 8 g; P < 0.05]. Exogenous CHO oxidation in this period was 28% lower in the LG trial compared with the HG trial. Maximal exogenous oxidation rates were also lower (P < 0.05) in the LG trial (0.64 +/- 0.05 g/min) than in the HG trial (0.88 +/- 0.04 g/min). This decreased utilization of exogenous glucose was accompanied by increased plasma free fatty acid levels (2-3 times higher) and lower insulin concentrations. It is concluded that glycogen-lowering exercise, performed the evening before an exercise bout, in combination with CHO restriction leads to a reduction of the oxidation rate of ingested glucose during moderate-intensity exercise. Publication Types: Clinical Trial Randomized Controlled Tria

Exogenous glucose oxidation during exercise in endurance-trained and untrained subjects.

Department of Human Biology, Maastricht University, The Netherlands. [email protected] To investigate the effect of training status on the fuel mixture used during exercise with glucose ingestion, seven endurance-trained cyclists (Tr; maximum O2 uptake 67 +/- 2.3 ml.kg-1.min-1) and eight untrained subjects (UTr; 48 +/- 2 ml.kg-1.min-1) were studied during 120 min of exercise at approximately 60% maximum O2 uptake. At the onset of exercise, 8 ml.kg-1.min-1 of an 8% naturally enriched [13C]glucose solution was ingested and 2 ml/kg every 15 min thereafter. Energy expenditure was higher in Tr subjects compared with UTr subjects (3,404 vs. 2,630 kJ; P < 0.01). During the second hour, fat oxidation was higher in Tr subjects (37 +/- 2 g) compared with UTr subjects (23 +/- 1 g), whereas carbohydrate oxidation was similar (116 +/- 8 g in Tr subjects vs. 114 +/- 4 g in UTr subjects). No differences were observed in exogenous glucose oxidation (50 +/- 2 g in Tr subjects and 45 +/- 3 g in UTr subjects, respectively). Peak exogenous glucose oxidation rates were similar in the two groups (0.95 +/- 0.07 g/min in Tr subjects and 0.96 +/- 0.03 g/min in UTr subjects). It is concluded that the higher energy expenditure in Tr subjects during exercise at the same relative exercise intensity is entirely met by a higher rate of fat oxidation without changes in the rates of exogenous and endogenous carbohydrates