Nutrient intake and energy regulation in physical exercise.

Abstract

Rates of energy expenditure as well as total daily energy cost can be considerable during periods of exercise. In trained athletes, expenditure can be as high as 380 kJ/min during short-term maximal exercise. Training programmes of several hours' duration lead to a daily nutrient intake of 25-35 MJ in most Olympic sports. The mobilization of the energetic fuels of the body is modulated by the nature of the exercise. ATP and creatine phosphate stores in muscle cells are depleted within seconds during maximal work. Glycogen is the main fuel for heavy exercise of a few minutes' duration where performance capacity is limited by the degree of lactate accumulation and intracellular acidosis. Oxidation of both glucose and free fatty acids supplies the energy needed for exercise lasting more than two minutes, the relative contribution of lipids increasing with a longer duration or a lower intensity of the muscular work. Intramuscular stores of glycogen and triglycerides may be almost completely depleted in long-lasting exercise, e.g. a 100 km run. Under these conditions, glycogen stores in the liver and triglycerides in adipose tissue contribute approximately 70% of the energy need whereas 5-10% of the supply comes from oxidation of amino acids. Although adequate nutrition for exercise could be achieved through the intake of a well-balanced diet, the regulation of energy utilization can be influenced by the sources of food energy, by dietary modifications before exercise or by nutrient supplements during exercise. Intake before exercise of fructose or medium-chain triglycerides, both only weakly insulinogenic compared to glucose, leads to changes in blood substrates and metabolites. However, neither glycogen depletion in the working muscles nor performance capacity was influenced by a single meal containing this particular carbohydrate or lipid. Mobilization of free fatty acids in adipose tissue can be enhanced by caffeine or depressed by nicotinic acid. Since the rate of free fatty acid oxidation in skeletal muscle depends on the blood concentration of this substrate, energy regulation during exercise and work output are considerably influenced by the ingestion of such substances.