The effect of vitamin A on epithelial integrity

Vitamin A is the generic term for a variety of fat-soluble substances inclu ding retinol, retinyl palmitate and the provitamin A carotenoids such as al l-trans-p-carotene. Vitamin A is commonly known as the anti-infective vitam in and has an essential role in vision and cellular differentiation, the la tter providing a unique core mechanism helping to explain the influence of vitamin A on epithelial barriers. Alterations in the epithelial lining of v ital organs occur early in deficiency, suggesting a potentially important r ole for the barrier function. Vitamin A deficiency (VAD) is most commonly r ecognized in the eye. The conjunctival-impression cytology test detects the presence of larger irregular keratinized cells and the absence of mucous-s ecreting goblet cells, indicative of VAD. The method is simple, quick and s ensitive in populations where VAD is present. In the respiratory tract, obs ervational studies all show an association with VAD, although vitamin A sup plementation studies appear to have little effect on respiratory disease. O rgan-specific targeting may improve success rates. The dual-sugar intestina l-permeability test allows the effect of vitamin A supplementation to be mo nitored on the gastrointestinal tract. Two vitamin A supplementation studie s were carried out recently in Orissa State, India. Healthy infants of wean ing age were administered orally eight weekly doses of 5.0 mg retinol equiv alents and hospitalized infants received one large oral dose 60 mg retinol equivalents in the form of retinyl palmitate. Improvements in gut integrity and haematological status were observed in both studies. In summary, the r esponse of the eye to vitamin A supplementation is well established; the pr esent review highlights some of the more recent observations examining the effects of vitamin A.