The effect of exercise on intestinal absorption of fructose was evaluated in 10 subjects after they consumed four beverages, each containing a total of 50 g carbohydrate: 100% fructose (100F), 95% fructose and 5% glucose (95F), 70% fructose and 30% glucose (70F), and 100% glucose (100G), as well as a water placebo. With 100F and 95F, breath hydrogen, which is an index of incomplete absorption, increased significantly in all subjects. In contrast, hydrogen excretion did not increase in any subject after consumption of 100G or water, or in five of seven subjects who consumed 70F. The rapid increase in hydrogen excretion observed when consumption of 100F was followed by exercise was not noted during a comparable nonexercise trial. These data suggest that intestinal capacity for absorption of fructose is readily saturated after ingestion of amounts as small as 50 g and that exercise, which reduces intestinal transit time, can cause incomplete absorption of fructose.