nutritional disorders
Vitamin A Toxicity
Introduction
Vitamin Atoxicity can be acute (usually due to accidental ingestion by children) or chronic. Both types usually cause headache and increased intracranial pressure. Acute toxicity causes nausea and vomiting. Chronic toxicity causes changes in skin, hair, and nails; abnormal liver test results; and, in a fetus, birth defects. Diagnosis is usually clinical. Unless birth defects are present, adjusting the dose almost always leads to complete recovery. 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. 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. 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). 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. When taken as a supplement, beta-carotene has been associated with increased cancer and cardiovascular risk; risk does not seem to increase when carotenoids are consumed in fruits and vegetables. (See also Overview of Vitamins .) 