Compared with untrained (UT) subjects, in trained (T) subjects the increased insulin sensitivity and decreased glucose induced insulin secretion would tend to promote health by decreasing glucose levels and insulin secretion whereas the increased food intake would tend to increase these variables. To study the net effect of training, blood was sampled from seven T and eight UT young men [VO2max: 76 +/- 2 (T) vs. 48 +/- 1 (UT) mL.kg-1.min-1] for 24 h during ordinary living conditions. Athletes exercised 204 +/- 20 min and ate 50% more calories and 130% more carbohydrate than UT subjects (P less than 0.05). However, 24-h integrated plasma concentrations of glucose, C-peptide, glucagon, free fatty acids, and glycerol as well as glycosylated hemoglobin levels were identical in T and UT subjects. Mean insulin concentration was 41% lower in T than in UT but levels differed significantly (P less than 0.05) only late during the night. Urinary excretion of pancreatic peptides paralleled plasma concentrations. In conclusion, during training adaptations in pancreas- and insulin-sensitive tissues allow the necessary increase in food intake without harmful hyperglycemia and overloading of beta-cells, but sparing of insulin secretion and reductions in glucose levels are only relative to food intake. However, training may be wholesome by increasing hepatic insulin extraction and thereby decreasing arterial insulin levels. Training-induced beta-cell adaptation is not caused by diminished average glucose levels. Finally, renal handling of insulin, C-peptide, and glucagon is not influenced by training.