When dietary fiber intakes are increased by supplementing diets with bran and whole wheat products, then fecal fat, nitrogen, energy, and mineral excretion rise. These changes suggest that fiber may be altering normal digestive and absorptive function. Recent studies have confirmed this and have also shown that fiber of different composition and from contrasting sources produces different physiological effects. The gel-forming polysaccharides such as guar gum and pectin alter the pattern of glucose absorption and are hypocholesterolemic; fiber from cereals is not hypocholesterolemic but exerts a pronounced effect on the large gut. Dietary fiber is largely digested in the colon by the microflora and so influences colonic function, fecal weight, and composition. The significance of the changes in fat, nitrogen, and energy output remains to be evaluated, but the impairment of mineral absorption--particularly of calcium, zinc, and iron--by fiber gives cause for concern. Fiber must now be considered with other dietary constituents in all nutritional studies.