Dietary fiber plays a role in maintaining colonic structure and function. Recently, fiber-supplemented liquid diets containing primarily soy polysaccharide have been marketed. However, the effects of these diets on the colon, particularly absorptive function, are not well documented. We compared Jevity (containing 1.4% soy polysaccharide) to fiber-free Osmolite. In addition, we were interested in comparing the effects of soy polysaccharide to that of pectin, which has previously been shown to facilitate adaptation. A 1% pectin-supplemented rat liquid diet was compared to the liquid diet alone. After 2 weeks of dietary treatment, rats were anesthetized and in vivo colonic absorption measured. Both Jevity and the pectin-containing diets led to a significantly greater (p less than 0.01) basal and butyrate stimulated water absorption as well as 20 mM butyrate absorption compared to their fiber-free fed controls. After perfusion, rats were killed and parameters of colonic mass measured including mucosal weight, protein, and DNA. The fiber-containing diets produced a modest, nonsignificantly greater colonic mass compared to their fiber-free controls. A commercial diet containing soy polysaccharide seems to be equally effective as pectin in maintaining colonic absorptive function.