OBJECTIVE: There is limited information on the effect of exercise on colonic function. Beneficial effects have been described, including prevention of colon cancer. In the present study, we demonstrate that short duration exercise results in enhancement of breath hydrogen excretion after consumption of lactulose. METHODS: Twelve normal volunteers who performed regular exercise were recruited. Each study subject underwent four study sessions (two resting and two exercise) after consumption of 10 g of crystalline lactulose. Colonic hydrogen production was measured in mid-expiratory breath samples obtained at baseline and frequent intervals to 420 min. Exercise sessions consisted of 5 min on a treadmill at a 20% incline at 10 km/h. This was performed 180 min after lactulose ingestion in the two exercise sessions. RESULTS: A characteristic pattern in the hydrogen concentration versus time curves was seen after exercises, consisting of an initial decrease then an increase in concentration above baseline for up to 3 h. Mean area under the curve from 0 to 420 min for resting studies was 5,156 +/- 2,621 ppm/min and was 7,051 +/- 2,447 ppm/min for exercise studies, p < 0.05 (37% increase). Mean area under the curve from 180 to 420 min was 2,808 +/- 1,592 ppm/min for resting studies and 4,543 +/- 1,729 for exercise studies, p < 0.005 (62% increase). CONCLUSION: This study demonstrates that exercise potentially enhances the metabolism of lactulose by colonic bacteria. The authors postulate that this effect is due to stirring of the colonic contents. The described phenomenon may explain, in part, the beneficial effects of exercise on colonic mucosa.