Mechanisms Involved in the Intestinal Digestion and Absorption of Dietary Vitamin A

Abstract

Dear Editor: Harrison and Hussain (1) elegantly summarized the current knowledge concerning the metabolism of dietary vitamin A during digestion and absorption on the enteral level. They showed that absorption and enteral processing and chylomicron release into the body is highly regulated. They concluded that no lipoproteins other than chylomicrons and lipoproteins of the VLDL fraction can transport retinyl ester in plasma. This indeed is peculiar because other lipids and lipophilic components, such as triglycerides, cholesterol, phospholipids, vitamin E and carotenoids, are found in all lipoprotein fractions. We and others have shown that within the animal kingdom, retinyl esters occur physiologically at high concentrations in blood plasma not only in dogs but also in many other carnivorous species (2–4). In ferrets and dogs, the majority of retinyl esters (primarily retinyl stearate and palmitate) are associated with the HDL fraction (5,6). Although their plasma levels are dependent on the oral supplementation of vitamin A, substantial levels are still present when animals were fed a vitamin A-depleted diet for an extended period of time (7,8). The importance of retinyl esters in the pathogenesis of vitamin A intoxication is controversial (9,10). In humans with a two-point mutation in the DNA sequence of the retinol binding protein gene, no effects other than night blindness occur despite extremely low levels of retinol. This observation suggests that the usual cellular supply of retinol might be bypassed by retinyl esters and/or -carotene or retinoic acid (11). Results in carnivorous species might further support the hypothesis that retinyl esters are of additional importance for the supply of peripheral target tissues. It remains to be answered why only carnivores’ plasma retinyl esters behave similarly to triglycerides and cholesteryl esters but not in any other species in which mainly retinol is present in plasma. The molecular and cellular bases of the lipoprotein associated transport of retinyl esters in plasma as well as the excretion of substantial amounts of retinol and retinyl esters in the urine of carnivores (3) remain to be elucidated. In both cases, however, it seems obvious that plasma transport and urinary excretion are not only dependent on dietary supplementation but are highly regulated (7). This regulation might involve both the liver and the kidney. Carnivores might, thus, provide an interesting model to study the importance of the gut and the liver in the regulation of the incorporation of retinyl esters into lipoproteins other than chylomicrons and the kidney in the uptake and the regulated excretion of retinyl esters in the urine.