The effect of adding fiber to liquid formula diets on gastrointestinal transit is still controversial. Different fiber types (soluble vs insoluble) and different methodology of transit time measurements yielded variable results. Factors affecting transit include colonic fermentation, neural, and hormonal factors. We have therefore compared the effects of a standardized normal diet and two liquid formula diets with and without supplementation of a soluble fiber (21 g/L) on orocecal transit time measured by the hydrogen lactulose breath test, colonic transit time measured by radiopaque markers with an abdominal x-ray, bowel movements, stool consistency, and cholecystokinin release in 12 healthy male volunteers. The diets were consumed in a randomized order, each one for 7 days. The addition of soluble fiber did not affect orocecal transit time. Colonic transit time, however, was significantly prolonged (55 h) with fiber supplementation compared with the liquid diet (39 h) and the self-selected diet (30 h) (p < .01). Stool frequency and consistency was not significantly affected. During administration of both liquid diets, fasting cholecystokinin concentrations were significantly elevated compared with the concentrations found with a self-selected diet (p < .05). The fasting cholecystokinin concentration correlated significantly with the increase of segmental (right colon) colonic transit time (p = .02). The prolongation of colonic transit time in liquid diet-fed volunteers might be caused by the combined effect of increased colonic fermentation and high basal cholecystokinin concentrations.