Doctors have long known that factors such as advanced age, low body weight, and low calcium intake increase a woman's risk of hip fracture. Now, research supported in part by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) has revealed another potential risk factor - low blood levels of vitamin D, a vitamin that helps the body absorb calcium from food.

Vitamin D was measured in the initial blood samples of 400 women who subsequently had hip fractures and 400 matched controls. Women who later had hip fractures had lower blood levels of 25-hydroxy vitamin D (a form of vitamin D) compared to the controls. When the women in the lowest quartile of serum vitamin D were compared with the women in the highest quartile, there appeared to be about a 70 percent increased risk of hip fracture in those with the low vitamin D. The association of low vitamin D levels with increased risk of hip fracture was independent of falls and measures of fragility.

Although this observational study, which was published in the Annals of Internal Medicine , suggests an association between low vitamin D and hip fractures, interventional studies to date have largely failed to reduce the risk of hip fractures with vitamin D supplements alone. A Women's Health Initiative (WHI) clinical trial of calcium combined with vitamin D, however, showed a 29 percent reduction in the risk of fractures in women who consistently took both a calcium supplement and 400 international units (IU) of vitamin D daily.

Approximately 300,000 people-mostly women-are hospitalized for hip fractures each year. Many of them die from complications within a year following surgery; most will have difficulty living independently following the fracture, making the understanding of hip fracture risk and finding ways to reduce it critically important.

The WHI, led by the NIH's National Heart, Lung, and Blood Institute, is a major research program that addresses the most common causes of death, disability and poor quality of life in postmenopausal women. The Initiative consisted of a set of clinical trials and an observational study, which together involved 161,808 generally healthy postmenopausal women.

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Reference: Cauley JA, LaCroix AZ, Wu L, Horwitz M, Danielson ME, Bauer DC, Lee JS, Jackson RD, Robbins JA, Wu C, Stanczyk FZ, LeBoff MS, Wactawski-Wende J, Sarto G, Ockene J, Cummings SR. Serum 25-hydroxyvitamin D concentrations and risk for hip fractures. Ann Intern Med. 2008 Aug;149(4):242-50.

National Heart, Lung, and Blood Institute. "WHI Background and Overview," National Institutes of Health. http://www.nhlbi.nih.gov/whi/background.htm.