EFFECTS OF SOYBEAN FIBER ON CECAL DIGESTION IN RATS PREVIOUSLY ADAPTED TO A FIBER-FREE DIET

This study was conducted to study digestion in rats of a diet containing soybean fiber (chiefly hemicellulose). The animals were first fed a fiber-free high starch diet for 7 d before receiving a 30% soybean fiber diet. There was a progressive change in the cecal pool of volatile fatty acids (VFA), up to approximately 1000-mu-mol, due to the enlargement of the cecum in conjunction with the rise of the VFA concentration, and the decrease in the cecal pH to 6. In rats fed the soybean fiber diet, the cecal concentrations of acetate and propionate increased, with propionate reaching a very high value (approximately 50 mmol/L) after 22 d of dietary treatment. Butyrate concentrations first increased, then declined to about 10 mmol/L. Absorption of VFA was enhanced fivefold between d 2 and d 21 and was then 10-fold higher than in rats fed the fiber-free diet. The transfer of blood urea to the cecum increased in parallel with the enlargement of the cecum and there was a large rise of ammonia absorption after 8 d. There was a rapid increase in the cecal pool of phosphorus, whereas the cecal pools of calcium and magnesium significantly increased only after 8 d. The cecal absorption of calcium and magnesium increased very early, even when the cecal pool of these cations was not enhanced. The soybean fiber diet also elicited a strong induction of ornithine decarboxylase (ODC) in cecal mucosa, which was attenuated after 21 d of dietary treatment. Thymidine kinase was also enhanced, although the relative induction was smaller than for ODC. It thus appears that the soybean fiber effectively increases microbial digestion in the large intestine, especially enhancing propionic acid production. Furthermore, the induction of ODC and thymidine kinase in the cecal mucosa seems transitory, in contrast to the effect of some other fermentable carbohydrates.