The older the woman, the more likely it is that consistent use of calcium and vitamin-D supplements will play a role in reducing her risk for osteoporosis, according to the results of a large national clinical trial conducted as part of the Women's Health Initiative (WHI).
In the final set of results answering the initiative's principal questions about women's health, study leaders say that even the slight benefits demonstrated by the trial involving more than 36,000 participants suggest calcium and vitamin-D supplementation provides an overall public health benefit to postmenopausal women.
The findings from the calcium and vitamin-D supplementation arm of the WHI are published in the Feb. 16 issue of the New England Journal of Medicine. A separate article about these supplements and colorectal cancer prevention is published in the same issue.
"Despite recommendations that women should ensure adequate calcium plus vitamin-D intake for postmenopausal bone health, the role of these supplements on reducing fractures has been conflicting. Now, based on the WHI, we have better data to address these issues," said Rebecca Jackson, lead author of the journal article about supplementation and fractures and Ohio State University Medical Center's principal investigator for the WHI. "The value of a study this large is that it does show, even if only on a small scale, that the intervention can be effective to lower the risk of osteoporosis within two to three years. A physician isn't needed to prescribe these supplements. All this means any supplementation of this kind is potentially beneficial, particularly in women over 60 years old. That's a huge finding."
The calcium-with-vitamin-D trial found a small but significant 1 percent higher hip bone density for women taking calcium combined with vitamin D compared to other women taking placebo. During the trial, 374 women who received supplements broke their hips, with a fracture rate of 14 per 10,000 cases per year, compared to 16 per 10,000 fractures per year in the placebo group - a 12 percent reduction, which was not statistically significant.
However, in analyses of subgroups of participants, researchers found that women who were most compliant about taking the supplements experienced a significant 29 percent decrease in hip fractures. And women 60 and older had a significant 21 percent reduction in broken hips.
The most common adverse effect of the supplementation was kidney stones, which were reported by 449 women in the supplement group, compared to 381 women in the placebo group.
"This all really points to the ability of women at highest risk for osteoporosis to make their own informed choices about supplements they take," said Jackson, associate professor of internal medicine and physical medicine at OSU Medical Center.
Half of the 36,282 participants in the supplementation trial received a daily dose of 1,000 milligrams of calcium carbonate combined with 400 international units (IUs) of vitamin D, while the other half received placebo. Study participants were followed for an average of seven years, with about 76 percent still taking their pills by the end of the study.
Researchers suggested a number of factors that could account for the limited benefits provided by the supplementation in the trial, including:
Higher doses of vitamin D, 600 IUs or higher, might be required to produce a statistically significant benefit to bone health.
The benefit may have existed only for women who took the supplements exactly as prescribed in the trial. Even though three-quarters of women were still taking study pills at the end of the study, only 59 percent were taking the intended dose.
Fewer hip fractures occurred among the population studied than researchers anticipated, lowering the power of the study to show a significant finding, Jackson said. This lower rate of hip breaks could have been attributed to a higher-than-expected average body-mass index of 29 among participants, because larger women have stronger bones; the inclusion of fewer women over 70 in the study than anticipated; and the fact that many participants were already using supplements or were on hormone therapy.
The supplements had no significant effect on spine or total fractures.
In patients with osteoporosis, the natural cycle of losing and adding minerals in healthy bone falls out of balance and the loss outpaces the gain, leading to low bone mass, structural deterioration of bone tissue, fragility and an increased susceptibility to fractures of the hip, spine and wrist. An estimated 44 million Americans either have or are at risk for osteoporosis, which contributes to about 300,000 broken hips in the United States each year.