The lack in control of insulin release combined with an inadequate carbohydrate (CHO) ingestion accounts for the occurrence of frequent metabolic unbalances during exercise in type 1 diabetic patients. The aim of the study was to quantify, in these patients, the CHO requirement to prevent hypoglycemia during moderate exercise performed at different time intervals after morning subcutaneous insulin injection. Twelve type 1 diabetic patients and 12 well-matched healthy subjects cycled 4 times for 1 hour at a constant workload. The rate of glucose oxidation was calculated continuously by indirect calorimetry throughout the exercise, while blood parameters were assessed periodically and orally given CHO were checked. CHO needed by the patients to prevent hypoglycemia decreased as the time elapsed from insulin administration increased, amounting to 0.63 +/- 0.30, 0.44 +/- 0.32, 0.28 +/- 0.24, and 0.14 +/- 0.18 g/kg after 1, 2.5, 4, and 5.5 hours, respectively. Total glucose requirement during moderate exercise (sum of alimentary and extracellular source) was correlated (r = 0.739, P <.001) to plasma insulin concentration, but not with fitness level. Time elapsed from last insulin dose is not a factor influencing the risk of hypoglycemia during exercise when a proportional, appropriate amount of CHO is ingested.