nutritional disorders
Vitamin A Deficiency
Introduction
Vitamin Adeficiency can result from inadequate intake, fat malabsorption, or liver disorders. Deficiency impairs immunity and hematopoiesis and causes rashes and typical ocular effects (eg, xerophthalmia, night blindness). Diagnosis is based on typical ocular findings and lowvitamin Alevels. Treatment consists ofvitamin Agiven orally or, if symptoms are severe or malabsorption is the cause, parenterally. Vitamin Ais required for the formation of rhodopsin, a photoreceptor pigment in the retina (see table).Vitamin Ahelps maintain epithelial tissues and is important for lysosome stability and glycoprotein synthesis. Dietary sources of preformedvitamin Ainclude fish liver oils, liver, egg yolks, butter, andvitamin A–fortified dairy products. Beta-carotene and other provitamin carotenoids, contained in green leafy and yellow vegetables, carrots, and deep- or bright-colored fruits, are converted tovitamin A. Carotenoids are absorbed better from vegetables when they are cooked or homogenized and served with some fat (eg, oils). Normally, the liver stores 80 to 90% of the body’svitamin A. To usevitamin A, the body releases it into the circulation bound to prealbumin (transthyretin) and retinol-binding protein. Retinol activity equivalents (RAE) were developed because provitamin A carotenoids have lessvitamin Aactivity than preformedvitamin A; 1 mcg retinol = 3.33 units. Synthetic vitamin analogs (retinoids) are being used increasingly in dermatology. The possible protective role of beta-carotene and retinoids against some epithelial cancers is under study. However, risk of certain cancers may be increased after beta-carotene supplementation. (See also Overview of Vitamins .) 