Resistant starch exerts a lowering effect on plasma urea by enhancing urea N transfer into the large intestine

Abstract

Fermentable polysaccharides may have interesting effects in the large intestine, on urea N disposal and N fecal excretion. To further investigate this domain, rats were adapted to diets containing only digestible starch (DS) or in which a part of the starch was replaced by resistant starch (RS) (crude potato starch, 250g/Kg), the dietary protein level being either 130 or 260g casein/Kg. Feeding RS led to an hypertrophy of the cecum and a high concentration of short-chain fatty acids (SCFA) at acidic pH. 25% RS in the diet resulted in an elevated rate of urea N transfer and retention in the cecum. Plasma urea influenced the quantity of urea N entering the cecum and the extent of ammonia N cycling; however, the quantities of N retained in cecum were not significantly increased. In rats fed RS diets, the cecal ammonia remained at low concentrations, whereas the total cecal nitrogen was 3-fold enhanced. The increase of fecal N excretion by RS diets was probably the result of an accelerated rate of bacterial protein synthesis. The shift of N elimination towards urea N transfer into the large intestine was particularly effective in rats fed 130g casein/Kg diet: the N excretion by the fecal route being then of the same magnitude as urinary excretion. The urea lowering effect of RS was more pronounced when rats were fed a moderate casein level; thus, by manipulating dietary supply of protein and fermentable carbohydrates it could be possible to noticeably depress the quantities of N to be eliminated by the kidneys.