Demonstration of Saturation Kinetics in the Intestinal Absorption of Vitamin C in Man and the Guinea Pig

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Abstract
Intestinal absorption of ascorbic acid is believed to be mediated through a Na-dependent active transport process in man and in the guinea pig, both species having a nutritional requirement for the vitamin. Vitamin C transport was studied in man and in the guinea pig by in vivo intestinal perfusion of concentrations of vitamin C ranging from physiologic to clearly pharmacologic levels. Triple lumen intestinal perfusion of 7 human volunteers with vitamin C concentrations ranging from 0.85 to 11.36 mM demonstrated saturation kinetics of absorption with a Km = 5.44 mM. Net secretion of H2O was observed in 3 of 7 humans with the highest (11.36 mM) concentration of vitamin C. Perfusion of isolated segments of guinea pig intestines with intact blood supply also revealed saturation kinetics (Km = 5.54 mM) in the range of 1.42-56.8 mM vitamin C but linear absorption below this range. The phenomenon of decreased H2O absorption noted with incremental vitamin C dose in human volunteers could not be reproduced in the guinea pig, nor were the intestinal tissue levels of cyclic AMP and GMP increased by high-dose vitamin C in this species. Megavitamin doses of vitamin C (> 1 gm) are probably not as efficiently absorbed as smaller multiple doses of the vitamin. Intestinal secretion of H2O may contribute to the diarrhea which is the most common side effect of large doses of vitamin C. The guinea pig is a useful but limited model for vitamin C absorption in man.