How much vitamin a is too

Vitamin a is a fat-soluble vitamins required for vision, growth, reproduction, cell growth and differentiation, and a healthy immune system. It is a common ingredient in multivitamins. Recent studies, however, suggest that excessive intake of vitamin a is associated with an increased risk of fracture.

A study published in the New England Journal of Medicine followed 2322 men in Sweden for 30 years. The researchers found that the fracture risk was higher in men with the highest levels of retinol blood, one of the more active forms of vitamin a.

Another study showed similar results. The nurses health study examined postmenopausal women 72,337 over a period of 18 years and found that women who consumed at least 3000 mcg a day as retinol equivalents were more likely to have a hip fracture by 48% from women whose daily was less than 1 250 mcg (4125 IU) per day.

Retinol can interfere with the activity of vitamin D, a vitamin which facilitates the absorption of calcium.

The Centers for Disease Control and Prevention (CDC), however, review of the data of the 5,800 people in the third national health and Nutrition Examination Survey (1988-1994) to determine if there is an association between bone mineral density and blood levels of vitamin A (in the form of retinyl esters). They found an association.

Retinol is the form of the vitamin found in most multivitamins and cod liver oil, liver, foods enriched and whole milk products. Beta-carotene, which is converted into vitamin a in the body (and another common multivitamin ingredient), was not associated with an increased risk of fracture. Beta carotene found in carrots, tomatoes, cantaloupe, spinach and other fruits and vegetables.

The recommended food allowance (RDA) of vitamin a for men over 18 years is 900 mcg (3000 IU) and for women, is 700 mcg (2 310 UI). This calculation should include all sources of retinol, including foods such as liver, dietary supplements such as multivitamins, vitamin supplements and cod liver oil has and enriched foods, such as cereals. The upper limit of retinol is 3000 mcg (10000) IU per day.

Until we know more, it would be wise to check your (especially your multivitamin) supplements and to consult your health care provider before taking vitamin A supplements, to avoid the possible negative effects.

Sources
Ballew C, D. Galuska, Gillespie v. High serum retinyl esters are not related to bone mineral density reduced in the third National Health and Nutrition Examination Survey, 1988-94. J Bone Miner Res 2001; 16: 2306-12.

Feskanich D, Singh V, Willett WC, Colditz GA.Vitamin a intake and hip fractures in postmenopausal women.JAMA 2002 Jan 2. 287 (1): 47-54.

Michaelsson K, Lithell H, Vessby B, Melhus h. serum retinol and the risk of fracture. New England Journal of Medicine. 2003; 348: 387-94.

Johansson S, Lind PM, HAKansson H, Oxlund H, rberg J, fragile bones cause hypervitaminosis A Melhus h. Subclinical rat. OS. 2002; 31: 685-9.

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