MECHANISMS BY WHICH WHEAT BRAN AND OAT BRAN INCREASE STOOL WEIGHT IN HUMANS

Generally, stool weight is significantly increased by adding sources o f insoluble fiber to the diet. Comparable amounts of fiber provided by wheat and oat brans have the same effect on daily stool output, even though >90% of wheat bran fiber but only 50-60% of oat bran fiber is i nsoluble. To determine the bases for these increases in stool weight, stool samples collected from 5 men in 2 constant diet studies that det ermined the effects of wheat and oat brans on large-bowel physiology w ere fractionated by using a physicochemical procedure into plant, bact erial, and soluble fractions, which were weighed and analyzed for suga r content and composition. Nitrogen, crude fat, and ash outputs were a lso determined. Wheat bran increased the fecal concentration of sugars and mass of plant material more than did oat bran, whereas oat bran i ncreased fecal bacterial mass more. Each fiber source increased nitrog en, ash, and fat excretion, but excretion of fat was greater with oat bran. The apparent digestibility of plant-derived neutral sugars decre ased significantly when wheat but not oat bran was consumed. The appar ent digestibility of neutral sugars provided by wheat bran was 56%; th e apparent digestibility of those provided by oat bran was 96%. We con clude that bacteria and lipids are major contributors to the increase in stool weight with oat bran consumption, whereas undigested plant fi ber is responsible for much of the increase in stool weight with wheat bran consumption. Results are consistent with the hypothesis that oat bran increases stool weight by providing rapidly fermented soluble fi ber in the proximal colon for bacterial growth, which is sustained unt il excretion by fermentation of the insoluble fiber.