Vitamin A encompasses a number of different retinoid compounds that include retinol and retinal. These are termed preformed vitamin A. Carotenoids, or beta-carotenes, can be used to make provitamin A. Retinoids and beta-carotenes are referred to as precursers, compounds needed to produce another compound. Vitamin A is a fat-soluble vitamin that is found in mango, spinach, pumpkin, cantaloupe, squash, sweet potato and carrots. Consult with your doctor before taking any supplements.
Retinoids
Retinoids include retinoic acid, retinyl, retinol and retinal. Retinol forms the active part of vitamin A and is not often found in the foods you eat. Instead, your body must synthesize retinol from precursor compounds such as fatty acid retinyl ester, which is found in liver, fish oil, egg yolks, whole milk, butter, margarine and some foods and breakfast cereals fortified with vitamin A. Toxicity from too much preformed vitamin A, retinol, is easy to get. Up to 3,000 micrograms of preformed vitamin A is about the limit of safety, according to the Harvard School of Public Health. Too much preformed vitamin A my increase the risk of hip fractures and birth defects and may interfere with the positive effects of vitamin D.
Beta-carotene
Beta-carotene is a carotenoid that is used as a precursor to vitamin A, called provitamin A. Beta-carotene is an antioxidant and antiaging nutrient of the same caliber as vitamins C and E. Another carotenoid called alpha-carotene can also be converted to vitamin A, but beta-carotene is the most bountiful, found in foods such as sweet potatoes, mangoes, carrots, tomatoes, cantaloupe, apricots and watermelon. You are unlikely to develop toxicity from provitamin A.
Function of Vitamin A
Vitamin A is necessary for strong bones, healthy skin and maintaining soft tissues and mucous membranes. Through a complex chemical process, vitamin A, in the form of retinol, is important for night vision. As retinoic acid, it influences the expression of DNA coded in your cells. Vitamin A is referred to as the antiinfective vitamin, as it keeps your mucosal linings healthy. It also plays a role in the production of red blood cells.
Vitamin A Deficiency
Diets lacking in vitamin A may result in serious health issues. Vision is directly affected by vitamin A deficiency, beginning with the inability to see at night and leading to blindness if not corrected. Even mild deficiencies of vitamin A may cause an increased risk of upper respiratory infections and diarrhea, especially in children. The recommended dietary allowance, RDA, for children aged 4 to 8 years is 1,333 IU per day. For adults, the RDA is 3,000 IU per day.
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