RESISTANT STARCH EXERTS A LOWERING EFFECT ON PLASMA UREA-BY ENHANCINGUREA N TRANSFER INTO THE LARGE-INTESTINE

Fermentable polysaccharides may have interesting effects in the large intestine, on urea N disposal and N fecal excretion. To further invest igate this domain, rats were adapted to diets containing only digestib le starch (DS) or in which a part of the starch was replaced by resist ant starch (RS) (crude potato starch, 250g/Kg), the dietary protein le vel being either 130 or 260g casein/Kg. Feeding RS led to an hypertrop hy of the cecum and a high concentration of short-chain fatty acids (S CFA) 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 cyc ling; however, the quantities of N retained in cecum were not signific antly increased. In rats fed RS diets, the cecal ammonia remained at l ow concentrations, whereas the total cecal nitrogen was 3-fold enhance d. The increase of fecal N excretion by RS diets was probably the resu lt of an accelerated rate of bacterial protein synthesis. The shift of N elimination towards urea N transfer into the large intestine was pa rticularly effective in rats fed 130g casein/Kg diet: the N excretion by the fecal route being then of the same magnitude as urinary excreti on. The urea lowering effect of RS was more pronounced when rats were fed a moderate casein level; thus, by manipulating dietary supply of p rotein and fermentable carbohydrates it could be possible to noticeabl y depress the quantities of N to be eliminated by the kidneys.