Vitamin A

The first to be recognized as a vitamin per se, the term "vitamin A" actually refers to a group of lipid-soluble compounds (called retinoids) all having biological activity similar to all-trans-retinol. Vitamin A is essential for vision—both in low- and high-intensity light [44, 45]. It also plays a role in intercellular communication as a transcription factor in a variety of signaling pathways, including those important for cell cycle progression [46-48]. Because of its diverse functions, vitamin A deficiency can influence almost every physiologic system including the immune system and reproduction. Foods supplying the majority of vitamin A to the American diet include meat (especially liver), whole milk, butter, and dark-green or deep-yellow fruits, and vegetables (via provitamin A carotenoids).

During pregnancy, additional vitamin A is needed to support maternal metabolism and tissue growth as well as fetal growth and development. It is generally accepted that this increased need is relatively small and can be best met through diet (not supplements). In fact, there is likely more concern about vitamin A toxicity during pregnancy in healthy populations, as excess vitamin A from supplements has been shown to be teratogenic. For example, Rothman and colleagues found that consumption of >3,000 mcg/day vitamin A in the form of supplements during the first trimester was related to a fourfold increase in the risk for neural crest defects [49].

However, in areas of the world with endemic vitamin A deficiency, supplementation with vitamin A during pregnancy may be beneficial for both maternal and infant health. For example, vitamin A supplementation may enhance night vision, maternal immune function, and weight gain during pregnancy, especially in women with comorbidities [50-52]. Recently, the relationship between vitamin A status and transplacental transmission of HIV from mother to child has also been studied. However, in a systematic review of this literature, Wiysonge and colleagues concluded that, overall, there is no evidence that vitamin A supplementation decreases maternal-to-infant HIV transmission [53]. There is limited evidence, in fact, that vitamin A supplementation may increase transmission. Not surprisingly, this topic is one of great current intensity, and the reader is directed to Chap. 12, "HIV/AIDS in Pregnancy," which provides a more thorough discussion. Recommendations

In summary, vitamin A supplementation is likely not warranted or desired in otherwise healthy women. However, in areas of the world with chronic and endemic vitamin A deficiency, routine low-dose daily supplementation or weekly higher-dose supplementation may be recommended [54]. Because of the risk of fetal malformations, the IOM has established the UL for this nutrient to be 3,000 mcg/day during pregnancy [55].

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