Vitamin A--functions, dietary requirements and safety in humans

Abstract

In recent years, new physiological functions of vitamin A have been identified in addition to its role in vision, namely its role in immune defence reducing morbidity of measles, of respiratory and possibly HIV infections, in gene regulation, in cell differentiation and morphogenesis. With the discovery of nuclear receptors for retinoic acid additional functions are likely to be found. The recommended dietary allowances (RDA) for vitamin A, including provitamin A carotenoids, vary greatly between countries. This may be explained by difficulties in establishing needs: Homeostatic mechanisms tightly control absorption, storage, release and transport of vitamin A to target tissue, and plasma concentrations do not reflect status unless there is vitamin A deficiency or excess. In the United States RDAs were established on the basis of vitamin A depletion-repletion studies with radio-labelled retinol. Intake requirements were calculated to amount to 1,000 micrograms retinol equivalents (RE) for men, 800 micrograms RE for non-pregnant as well as pregnant women and 1,300 micrograms RE for lactating women. Dietary intake studies in different countries have shown that it is in principle possible to obtain these values from the diet either as preformed vitamin A or in the form of provitamin A carotenoids or both. Risk groups for inadequate intakes include low-income groups and younger persons following weight-reducing and other unbalanced diets. Since experience with recommended intake values in the United States has been excellent, attempts at reducing these levels are unjustified and should be resisted. At intakes of up to three RDA values (approximately 3,000 micrograms RE) no unwanted side-effects are to be expected. Even higher doses, if not taken chronically, have been well tolerated, e.g. in deprived populations with low liver reserves. The suggestion derived from a case-control study that vitamin A taken at supplemental doses of 2,400 micrograms RE may be teratogenic have not been confirmed by several other studies. But it is judicious to follow the recommendations of the American Pediatric Society that women should not exceed a total daily intake of 3,000 micrograms RE. In developing countries where acute and chronic vitamin A deficiency is endemic causing xerophthalmia and blindness and increasing the prevalence of infectious diseases and death in children, special efforts are being made by WHO/UNICEF to provide programs for the eradication of vitamin A deficiency by immediate treatment and by long-term changes in dietary practices.