Vitamin A (Retinol): introduction
Vitamin A and its metabolites play diverse roles in physiology, ranging from incorporation into vision pigments to controlling transcription of a host of important genes. Health depends on maintaining vitamin A levels within a normal range, as either too little or too much of this vitamin lead to serious disease.Physiologic Effects of Vitamin A
Vitamin A and its metabolites retinal and retinoic acid appear to serve a number of critical roles in physiology, as evidenced by the myriad of disorders that accompany deficiency or excess states. In many cases, precise mechanisms are poorly understood. Some of the well-characterized effects of vitamin A include:
- Vision: Retinal is a necessary structural component of rhodopsin or visual purple, the light sensitive pigment within rod and cone cells of the retina. If inadequate quantities of vitamin A are present, vision is impaired.
- Resistance to infectious disease: In almost every infectious disease studied, vitamin A deficiency has been shown to increase the frequency and severity of disease. Several large trials with malnourished children have demonstrated dramatic reductions in mortality from diseases such as measles by the simple and inexpensive procedure of providing vitamin A supplementation. This "anti-infective" effect is undoubtedly complex, but is due, in part, to the necessity for vitamin A in normal immune responses. Additionally, many infections are associated with inflammatory reactions that lead to reduced synthesis of retinol-binding protein and thus, reduced circulating levels of retinol.
- Epithelial cell "integrity": Many epithelial cells appear to require vitamin A for proper differentiation and maintenance. Lack of vitamin A leads to dysfunction of many epithelia - the skin becomes keratinized and scaly, and mucus secretion is suppressed. It seems likely that many of these effects are due to impaired transcriptional regulation due to deficits in retinoic acid signalling.
- Bone remodeling: Normal functioning of osteoblasts and osteoclasts is dependent upon vitamin A.
- Reproduction: Normal levels of vitamin A are required for sperm production, reflecting a requirement for vitamin A by spermatogenic epithelial (Sertoli) cells. Similarly, normal reproductive cycles in females require adequate availability of vitamin A.
Sources of Vitamin A
Vitamin A is present in many animal tissues, and is readily absorbed from such dietary sources in the terminal small intestine. Liver is clearly the richest dietary source of vitamin A.Plants do not contain vitamin A, but many dark-green or dark-yellow plants (including the famous carrot) contain carotenoids such as beta-carotene that serve as provitamins because they are converted within the intestinal mucosa to retinol during absorption.
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