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.