The solubility of a mineral is a principal criterion for its bioavailability. This property within the small intestinal tract may be markedly affected by a dietary protein source which forms a small amount of macropeptides; the latter interact with minerals having different strength of affinity. Such ligand formation between peptide and mineral cations usually lowers the absorption of minerals. However, in some instances-casein phosphopeptide (CPP) for example-a ligand with cation rather enhances absorption. The relationship in the affinity of mineral ions between ligand and the epithelial mucosal membrane is very important in considering the intestinal mineral absorption.

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Acetic, propionic, and n-butyric acids, called short-chain fatty acids (SCFA), are the major end-products of the hindgut fermentation. SCFA are rapidly absorbedacross the colonic mucosa and used as the nutrient. Luminal SCFA enhance the absorption of sodium and water. Bicarbonate accumulates in the hindgut lumen in proportion to the amount of SCFA absorbed in various animals. The buffering system of the bicarbonate accumulated into the lumen in response to SCFA absorption maystabilize the luminal pH. Propionic and n-butyric acids stimulate the colonic motility with an increased peristaltic propulsion of perfusate in the rat in vivo. Propionic, n-butyric, and n-valeric acids stimulate contractile response of the isolated segments of the rat colon in a dose-dependent manner. The contractile effect of SCFA on the rat colon does not act directly on the smooth muscle. The enteric nervous system may mediate the effect of SCFA, since the contractile responses were abolished by tetrodotoxin and atropine. The acute and chronic administration of SCFA (acetic, propionic, and n-butyric acid) into the hindgut of the rat enhanced the proliferation of the epithelium of the digestive tract in vivo.The stimulatory effect of SCFA is dose-dependent and is strong in order of n-butyric>propionic>acetic acids. Thus, SCFA are important not only as the nutrient but also as the modulator of electrolyte transport, motility, and epithelialproliferation of the digestive tract.

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Rearing rats on low- and high-protein diets yielded no difference in the level of hepatic ammonia while urea production was assessed at a higher level in the high-protein diet group. The appearance of a large number of ammonia-producing nonsporeforming anaerobic bacteria in the jejunum of the rats reared on a high-protein diet seems to imply that the hepatic urea cycle was stimulated by the bacteria enhancing the generative reaction from ammonia to urea. These results are significant when viewed as the probable indication of the integral ecological function of intestinal flora in the homeostatic regulation mechanism, taking the regulation of the urea cycle as one example.

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In order to examine the effect of Streptococcus lactis (ATCC 19435) cell preparation on the retarding of senile changes in Wistar and SHR rats, a comparatively large dose, namely 1.2×10¹¹cells or 120 mg in dry weight, was fed per capitafor life. Striking differences in appearance of Wistar rats at 27 months of age after weaning were observed between the control group which was fedcommercial pelletized feed and the study group which was fed the same plus 1% of S. lactis preparation; remarkable kephosis and alopecia developed in the former, none in the latter. Other age-related changes significantly suppressed by such extended supplementation of the culture were fibrosis and calcification of cartilage matrix; aortic endothelial damage, segmental thickening, calcification and chondroid cells in intima; hypertension in aging SHR; disarray of hepatic cords and portal fibrosis; atrophy of epidermis and appendages, and increase in amorphous dermal matrix in Wistar; waxy degeneration and atrophy of psoas muscles. These morphological and histological changes in tissues on aging could be reflected in such biochemical changes as an increase in lipoperoxides in the serum, liver, and brain; increase in hydroxyproline content in aorta, mesenteric artery, and liver; reduction in cyclic AMP and GMP in the serum. The S. lactis preparation also afforded mice protection against the X-ray irradiation (300 rad) as shownin primary antibody response and leukopenia. The present preparation of S. lactis exhibited superoxide dismutase activity and presence of glutathione, and also was found digestible in vivo and immediately phagocitized by Paneth's cells. Themechanism for the observed anti-aging effect of S. lactis was discussed.

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