Vitamin A is used to classify a group of fat-soluble substances that all possess qualities of retinol activity and structure. Retinol is the parent of these substances which not only includes vitamin A but also various carotenes (i.e. beta-carotene). Vitamin A is essential to vision, the immune system, growth and development of the brain and bones, reproduction, and epithelial differentiation. Vitamin A is connected to the cornea and helps to prevent damage that may cause blindness. Vitamin A also displays antioxidant and anticarcinogenic activities. Animal liver provides the richest concentrations of vitamin A but high concentrations can also be found in eggs, whole milk, fish liver oils, butter and cream.
Vitamin A may increase the risk of bleeding when taken with warfarin drugs; avoid this combination. People taking retinoid and tetracycline should avoid taking vitamin A as well, as toxic effects may occur. Orlistat, mineral oil, estrogens, oral contraceptives, aminoglycosides, and bile acid sequestrants will decrease the absorption and effectiveness of vitamin A. Contrary to this, dietary fat helps to increase the absorption of vitamin A by the body. As with all drugs or supplements, consult a health care professional before use.
For the most part, vitamin A is well tolerated at the recommended dosage; for adults this amount is up to 3,000 mcg per day. When higher doses are taken, especially upwards of 15,000 mcg daily, symptoms of acute toxicity may occur. These symptoms include nausea, vomiting, headache, vertigo, impaired vision, lack of muscle coordination, and a rise in cerebrospinal fluid pressure.
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