Our Current Dietary Reference Intakes for Vitamin A-Now 20 Years Old

Abstract

The DRI values for vitamin A were last reviewed and defined in 2001. At the time, there was very sparse data that could be used to set the DRI values for pregnancy, lactation, and infancy. In the subsequent 20 y since the last formal review, a number of findings relevant to the adequacy indicator of visual dark adaptation in pregnancy, the usual vitamin A content of breast milk across lactation stages, and vitamin A metabolism in women and children have been published. Furthermore, identification of genetic variables affecting the bioconversion of provitamin A carotenoids to vitamin A have provided an improved explanation for interindividual variability in responses to provitamin A carotenoids. The purpose of this collection of articles, introduced herein, is to review and apply recent findings about vitamin A status, address current gaps in knowledge, and suggest avenues for future research needed to refine the DRI values for pregnancy, lactation, and early life.

Keywords: Dietary Reference Intakes; Recommended Dietary Allowance; carotenoids; infants; lactation; pregnancy; vitamin A.

FIGURE 1

Schematic of the bioavailability of carotenoids to vitamin A as published in (1).

FIGURE 2

Schematic of intestinal β-carotene metabolism and its feedback regulation by retinoic acid. An enterocyte is illustrated. Newer information since the time of the 2001 report on vitamin A has identified brush border-associated transporters involved in carotene uptake, SCARB1; a mechanism for the enzymatic cleavage of β-carotene by BCO1; and a feedback mechanism involving the intestine-specific transcription factor, ISX, which is activated by RA, an active downstream product of vitamin A and β-carotene metabolism. When the RA concentration rises and the expression of the ISX gene is induced, then ISX protein serves to repress the transcription of the SCARB1 and BCO1 genes, thereby reducing the efficiency of cleavage of β-carotene. See refs (38–43) and text for discussion. Abbreviations: BCO1, ß-carotene oxygenase 1; CM, chylomicron; ISX, intestine-specific transcription factor; RA, retinoic acid; RARE, retinoic acid receptor response element; SCARB1, scavenger receptor class B type 1.