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           Vitamin A

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Vitamin A: Vitamin A refers to a group of structurally related fat-soluble substances. "Vitamin A" usually refers to retinol, found in animal-derived sources such as liver and dairy foods. The carotenoids, include alph-,beta-, gamma- and delta- carotene, zeaxanthin, lutein, and lycopene come from fruits and vegetables. One IU is equal to 0.30 micrograms of all-trans retinol or to 0.60 micrograms of all-trans beta carotene. Deficiency of Vitamin A may be caused by inadequate dietary intake, malabsorption syndromes (e.g. Crohn's disease, ulcerative colitis), pancreatic disease and chronic liver disease (e.g. cirrhosis).

Toxicity: Too much Vitamin A can result in dry, itchy skin, headache, fatigue, hair loss and liver damage. In pregnant women excess may induce birth defects (as can deficiency). Beta-carotene's conversion is very much slowed when there is sufficient Vitamin A in the body, and is therefore not felt to be a potential risk for toxicity. A recent clinical trial in Finnish smokers who supplemented their diet with 20 mg of beta-carotene had an increased risk of lung cancer, but there may be a relationship between alcohol because "only those men who consumed more than 11g/day of alcohol (approximately one drink/day) showed an adverse response...in the lung cancer trial".

Tolerable Upper Limits: Because of the potential risk the FDA does not recommend beta-carotene at this time. The TULs for Vitamin A are:

Pregnant and lactating women: 5000 IU

Men and women 11y and older:10,000 IU

Children 1-3 1000 IU; 4-8 1,333 IU; 9-13 2000 IU; >14 3000 IU.

Drug Interactions: Absorption of Vitamin A is decreased by cholestyramine and other drugs resulting in fat malabsorption (e.g. Olestra), colestipol, mineral oil, and neomycin. Oral contraceptives may increase serum retinol. Supplemental vitamin A may add to the toxicity of retinoid drugs (e.g. isotretinoin, acitretin).

 

 

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Last modified: June 04, 2005