Calcium Plus Vitamin D Supplementation and the Risk of Fractures

What is the problem and what is known about it so far?
Osteoporosis (loss of bone) and resultant fractures are a major health problem, particularly in postmenopausal women. Over 300,000 hip fractures occur in the United States annually, often having a significant impact on patients’ lives. Since calcium and vitamin D are both necessary for bone growth, supplements of both have been recommended to help prevent osteoporosis and fractures. Studies thus far have not clearly shown that taking calcium and vitamin D supplements reduce fractures in postmenopausal women.

Why did the researchers do this particular study?
This research was done to determine whether or not taking calcium with vitamin D supplements can reduce the incidence of hip fractures in postmenopausal women. They also wanted to see if all fractures, including those at other sites, were reduced.

Who was studied?
The study followed 36,282 postmenopausal women. These women were 50 to 79 years of age and were all participants in a large clinical trial called the Women’s Health Initiative (WHI). The trial was conducted at 40 locations in 23 states across the US. The average age was 62. Eighty three percent were white, 9% black, 4% Hispanic, and 4% Asian or other. Women were excluded from the study if they had hypercalcemia (high calcium in the blood), kidney stones, if they were taking cortisone medication, or already taking Vitamin D. They were allowed to take their own calcium supplements if they were already doing so at the start of the study.

How was the study done?
Half of the women were given 500mg of calcium and 200 IU of vitamin D twice daily while the other half of the women were given a placebo. The women were seen every six months to monitor for fractures, compliance with study medications, and side effects. The women were followed for an average of seven years. A subset of 2431 of the women had their bone densities measured periodically.

What did the researchers find?
At first glance the study appeared to show that the risk of fracture was not significantly lower for the women taking calcium with vitamin D than for those taking the placebo, because the 12% reduction in incidence of hip fracture between the groups was not statistically significant. However, when the researchers looked more closely they found that among the women who were assigned to take calcium with vitamin D, some adhered more strictly to taking the recommended dose than others. These women showed a 29% reduction in the incidence of hip fracture compared to the placebo group, which was statistically significant. Fractures in non-hip sites were not reduced with the supplements. Bone density at the hip was better preserved in women taking calcium and vitamin D, but there was no difference noted in bone density of the spine, or whole body. There was however a 17% higher risk of kidney stones in the women who took supplements.

What were the limitations of the study?
Only 60% of women were fully compliant with their medications. In addition, participants were not limited from taking calcium and vitamin D in their diets or as supplements on their own. This could have potentially made it difficult to show a difference between the groups and thus the benefit of the study supplements.

What were the implications of the study?
This study shows that calcium and vitamin D supplementation reduces hip fracture in postmenopausal women by 29 percent. The benefit is only seen if women adhere to the regimen, and comes at the price of an increase in the risk of kidney stones.