Diet-induced thermogenesis (DIT) denotes the increase in energy expenditure that occurs in response to food ingestion. The purpose of the present study was to examine the possible influence of age, training state and sympatho-adrenal activity on the early phase of DIT in healthy individuals and further to study whether the magnitude of DIT is reduced in human obesity and, if so, to what extent DIT is influenced by weight reduction induced by surgical treatment, i.e. gastric banding or vertical banded gastroplasty. In addition, the effect of an artificial abdominal insulation on the DIT reaction was examined in healthy subjects in order to find out if the spontaneously enhanced thermal insulation of the body in obese individuals may be accompanied by a reduced DIT. The subjects were studied in the basal state and during 2-3 hours after a mixed meal. The energy expenditure was determined by indirect calorimetry. Blood temperature and blood flow in the hepatic vein were measured and splanchnic oxygen uptake and blood-drained heat from the splanchnic region were calculated. The meal was in liquid form, consisting of 17% kJ protein, 28% kJ lipids and 55% kJ carbohydrates, corresponding to either 60% of the individually measured 24-h resting energy expenditure or to 40% of the individually predicted basal metabolic rate. DIT was expressed as the average increase in energy expenditure above the basal level (means +/- SEM). After a 60% meal it was less (21 +/- 3%, P less than 0.01) in 8 elderly (70 +/- 1 years) and 7 middle-aged (51 +/- 3 yrs) individuals (24 +/- 2%, P less than 0.05) than in 10 young (27 +/- 1 yrs) men (29 +/- 2%). Its magnitude was similar (n.s.) in 7 well-trained men with a higher (58 +/- 2 ml/min/kg BW) maximal oxygen uptake (25 +/- 2%) and 7 sedentary individuals with a lower (39 +/- 2 ml/min/kg BW) aerobic capacity (29 +/- 2%). An intravenous pharmacological inhibition of the beta-adrenergic receptor function failed to influence the DIT in 10 men, irrespective of whether the beta-blockade was instituted by a selective-beta-1 antagonist (atenolol) or a non-selective blocker (propranolol). The DIT was 29 +/- 1% with and 29 +/- 2% (n.s.) without a beta-blockade and it was 29% in 2 subjects after 1 week of oral propranolol medication.(ABSTRACT TRUNCATED AT 400 WORDS)