Intestinal absorption of nutrients in animals dosed orally with retinoids: considerations for chemopreventive studies

Biochemical measurements demonstrate that, in some species, various retinoids administered in the diet, can reduce the tissue contents of several lipid-soluble compounds: vitamin K, alpha-tocopherol, cholesterol, and vitamin A itself. The effect is probably due to an interference with intestinal absorption, as indicated by use of radioactive compounds and administration of bile acids. In male rats, a vitamin K deficiency induced by retinoids in the diet can lead to fatal hemorrhaging. Although retinoids inhibit the uptake of dietary cholesterol, they do not change plasma and tissue concentrations of the endogenous compound. Conversely, cholesterol in the diet appears to interfere with the absorption of retinyl esters. alpha-Tocopherol deficiency caused by retinoids results in tissues with low antioxidant and high peroxide concentrations. Humans may be less susceptible to these effects of retinoids, which, however, still deserve consideration in the planning of clinical trials.