The intake of nutrients is known to increase energy expenditure. Measured thermic effects of nutrient are 0-3% for fat, 5-10% for carbohydrates and 20-30% for proteins. Stimulation of adenosine triphosphate (ATP) hydrolysis during intestinal absorption, initial metabolic steps and nutrient storage are responsible for this food thermic effect. The autonomic nervous system modulates the thermic effect of nutrients. Parasympathetic muscarinic antagonists reduce the thermic effect of orally administered nutrients, most likely by delaying gastric emptying and decreasing the amount of nutrient storage. Antagonists of the beta-adrenoreceptors decrease the thermic effect of glucose. The part of glucose induced thermogenesis which is eliminated by beta-adrenergic antagonists has been called 'facultative thermogenesis' and takes place, at least in part, in skeletal muscle. Insulin-induced stimulation of muscle sympathetic nerve activity may be involved in this facultative thermogenesis. The thermic effect of food is reduced in obese, insulin-resistant patients. The effect of thermogenesis represents about 50-150 kcal/day in such patients, and can explain only a minor part of their excess body weight. Defective thermogenesis may, however, contribute to weight gain, or impair weight loss in such patients.