vitamin information

Vitamin A and osteoporosis

Osteoporosis, a disorder characterized by porous and weak bones, is a serious public health problem for more than 10 million Americans, 80% of whom are women. Another 18 million Americans have decreased bone density which precedes the development of osteoporosis. Many factors increase the risk for developing osteoporosis, including being female, thin, inactive, at advanced age, and having a family history of osteoporosis. An inadequate dietary intake of calcium, cigarette smoking, and excessive intake of alcohol also increase the risk.

Researchers are now examining a potential new risk factor for osteoporosis: an excess intake of vitamin A. Animal, human, and laboratory research suggests an association between greater vitamin A intake and weaker bones. Worldwide, the highest incidence of osteoporosis occurs in northern Europe, a population with a high intake of vitamin A. However, decreased biosynthesis of vitamin D associated with lower levels of sun exposure in this population may also contribute to this finding.

One small study of nine healthy individuals in Sweden found that the amount of vitamin A in one serving of liver may impair the ability of vitamin D to promote calcium absorption. To further test the association between excess dietary intakes of vitamin A and increased risk for hip fractures, researchers in Sweden compared bone mineral density and retinol intake in approximately 250 women with a first hip fracture to 875 age-matched controls. They found that a dietary retinol intake greater than 1,500 mcg/day (more than twice the recommended intake for women) was associated with reduced bone mineral density and increased risk of hip fracture as compared to women who consumed less than 500 mcg/day.

This issue was also examined by researchers with the Nurses Health Study, who looked at the association between vitamin A intake and hip fractures in over 72,000 postmenopausal women. Women who consumed the most vitamin A in foods and supplements (greater than or equal to 3,000 mcg/day as retinol equivalents, which is over three times the recommended intake) had a significantly increased risk of experiencing a hip fracture as compared to those consuming the least amount (less than 1,250 mcg/day). The effect was lessened by use of estrogens. These observations raise questions about the effect of retinol because retinol intakes greater than 2,000 mcg/day were associated with an increased risk of hip fracture as compared to intakes less than 500 mcg.

A longitudinal study in more than 2,000 Swedish men compared blood levels of retinol to the incidence of fractures in men. The investigators found that the risk of fractures was greatest in men with the highest blood levels of retinol (greater than 75 mcg per deciliter). Men with blood retinol levels in the 99^th percentile (greater than 103 mcg per dL) had an overall risk of fracture that exceeded the risk among men with lower levels of retinol by a factor of seven . However, high vitamin A intake does not necessarily equate to high blood levels of retinol. Age, gender, hormones, and genetics also influence these levels. Researchers did not find any association between blood levels of beta-carotene and risk of hip fracture. Researchers' findings, which are consistent with the results of animal, in vitro (laboratory), and epidemiologic studies, suggest that intakes above the UL, or approximately two times that of the RDA for vitamin A, may pose subtle risks to bone health that require further investigation.

The Centers for Disease Control and Prevention (CDC) reviewed data from NHANES III (1988-94) to determine whether there was any association between bone mineral density and fasting blood levels of retinyl esters, a form of vitamin A. No significant associations between blood levels of retinyl esters and bone mineral density in 5,800 subjects were found.

There is no evidence of an association between beta-carotene intake, especially from fruits and vegetables, and increased risk of osteoporosis. Current evidence points to a possible association with vitamin A as retinol only. If you have specific questions regarding your intake of vitamin A and risk of osteoporosis, discuss this information with your physician or other qualified healthcare practitioner to determine what's best for your personal health.