Vitamin D-3 intestinal absorption in vivo: influence of fatty acids, bile salts, and perfusate pH

Intestinal absorption of vitamin D-3 in physiological concentrations was studied in the live unanaesthetised rat. In both the jejunum and the ileum a linear relationship was found between the absorption rate of the vitamin and its intraluminal concentration. Increasing the sodium taurocholate concentration in the perfusate above 5 mM did not change ileal absorption rate but did decrease jejunal absorption rate. The vitamin's rate of absorption was raised by increases in either the hydrogen ion concentration or the perfusate's flow rate. Addition of 2-5 mM fatty acids of varying chain length and degrees of saturation resulted in a decrease in the rate of vitamin D-3 absorption. These experiments indicate that vitamin D-3 absorption in vivo is mediated by passive diffusion. The rate of absorption of the vitamin is influenced by the composition of the perfusate and the thickness of the unstirred layer.