Evidence in man for different specialized intestinal transport mechanisms for riboflavin and thiamin.

Man has a limited capacity to absorb riboflavin (1-3) and thiamin (4, 5). The gastrointestinal absorption of these vitamins is describable by Michaelis-Menten type kinetics, characteristic of saturable, specialized absorption processes (6). Both vitamins are phosphorylated in the intestinal mucosa during absorption (7-9) and there is evidence suggesting that this process is the basic mechanism for the intestinal transport of thiamin (9). The absorption of riboflavin-5’phosphate (FMN) in man exhibits the same saturation kinetics as does riboflavin (2), but this is probably due to the rapid dephosphorylation and subsequent rephosphorylation of this vitamin during absorption (2). The available evidence suggests that phosphorylation may be involved in the specialized transport of riboflavin and thiamin, and that both vitamins may share the same transport process. This is not only of mechanistic but of practical interest; concomitant oral administration of large (“therapeutic”) doses of riboflavin and thiamin could result in mutual inhibition of absorption if both vitamins must compete for the same transport process. In the investigation to be described here, the effects of a large dose of riboflavin on the absorption of a large dose of thiamin, and vice versa, have been determined in healthy adult volunteers. It has been shown previously that the absorption of large doses of riboflavin is increased substantially if the vitamin is taken after breakfast rather than on an empty stomach (1, 2). A similar effect may be anticipated in the case of thiamin if this vitamin is absorbed by the same process. Therefore, the absorption of riboflavin and thiamin was determined upon concomitant administration of both vitamins on an empty stomach and after a standard breakfast. Material and methods