Vitamin A is a fat-soluble vitamin required for vision, growth, reproduction, cell growth and differentiation, and a healthy immune system. It's a common ingredient in multivitamins. Recent studies, however, suggest that excessive vitamin A intake 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. Researchers found that the fracture risk was greatest in men with the highest blood levels of retinol, one of the most active forms of vitamin A.
Another study found similar results. The Nurses Health Study looked at 72,337 postmenopausal women over an 18 year period and found that women who consumed at least 3000 mcg a day as retinol equivalents were 48% more likely to have a hip fracture compared to women whose daily intake was less than 1250 mcg (4125 IU) per day.
Retinol may interfere with the activity of vitamin D, a vitamin that facilitates the absorption of calcium.
The Centers for Disease Control and Prevention (CDC), however, reviewed data from the 5,800 people in the third National Health and Nutrition Examination Survey (1988-94) to determine whether there was an association between bone mineral density and blood levels of vitamin A (in the form of retinyl esters). They didn't find an association.
Retinol is the form of vitamin A found in most multivitamins and in cod liver oil, liver, fortified foods, and whole milk products. Beta carotene, which is converted to vitamin A in the body (and another common multivitamin ingredient), was not associated with an increased risk of fracture. Beta carotene is found in carrots, tomatoes, cantaloupe, spinach and other fruits and vegetables.
The recommended dietary allowance (RDA) of vitamin A for men over 18 is 900 mcg (3000 IU) and for women it's 700 mcg (2,310 IU). This calculation should include all sources of retinol, including foods such as liver, dietary supplements such as multivitamins, cod liver oil, and vitamin A supplements, and fortified foods such as cereal. The upper limit for retinol is 3000 mcg (10000) IU per day.
Until we know more, it would be wise to check your supplements (especially your multivitamin), and consult your healthcare provider before taking vitamin A supplements to avoid possible adverse effects.
Ballew C, Galuska D, Gillespie C. High serum retinyl esters are not associated with reduced bone mineral density 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 among postmenopausal women.JAMA 2002 Jan 2;287(1):47-54.
Michaelsson K, Lithell H, Vessby B, Melhus H. Serum retinol levels and the risk of fracture. New England Journal of Medicine. 2003;348:387-94.
Johansson S, Lind PM, HAKansson H, Oxlund H, rberg J, Melhus H. Subclinical hypervitaminosis A causes fragile bones in rats. Bone. 2002;31:685-9.