Isomaltooligosaccharides (IMO), sweeteners derived from corn-starch, selectively promote the growth of bifidobacteria in the human intestine. The metabolic fate of IMO in healthy men was investigated. The expiration rates of excess (13)CO2 and hydrogen of six men were measured while sedentary and while taking physical exercise after the (13)C-labeled IMO intakes. The breath H2 excretion kept at a constant state after IMO ingestion in the sedentary test and increased in the exercise test. The serum glucose and serum insulin increased 30 min after IMO ingestion. The (13)CO2 recoveries were 28.7% in the sedentary test and 60.9% in the exercise test. These recoveries were 70-80% compared those of maltose. These results indicated that a part of IMO was digested and the residual IMO was fermented by intestinal flora. The energy value of IMO might be about 75% of that of maltose.