The study, published in The New England Journal of Medicine, followed more than 36,000 healthy, postmenopausal women for an average of seven years.
Women who were assigned to take calcium and vitamin D twice daily -- and did so -- were 29% less likely to fracture their hips than those who got fake supplements (placebo). Women not adhering to the daily vitamins had only a slightly lower risk of hip fractures compared with placebo.
"This all really points to the ability of women at highest risk of osteoporosis to make their own informed choices about the supplements they take," Jackson says in a news release.
The study was part of the Women's Health Initiative, which has covered a wide range of women's health issues.
Bone Health Debate
A journal editorial takes a slightly different view.
"Unfortunately, although the trial was well conducted, the results ... leave many questions unanswered," writes editorialist Joel Finkelstein, MD, of Boston's Massachusetts General Hospital.
"It seems reasonable to recommend that women consume the recommended daily levels of calcium and vitamin D through diet, supplements, or both," Finkelstein writes. "But one message is clear: Calcium with vitamin D supplementation by itself is not enough to ensure optimal bone health."
"Calcium with vitamin D supplementation is akin to the ante for a poker game: It is where everyone starts," he continues. A woman at significant risk of fracture "probably needs something more."
Real Supplements vs. Fakes
The researchers gave half the women calcium and vitamin D supplements. The other half got fake supplements (placebo). The women didn't know which pills they'd gotten.
The women took one pill twice daily at meals. The real supplements each contained 500 milligrams of calcium and 200 international units of vitamin D.
Many women already had several factors in their bones' favor before the study started, Jackson's team notes.
For one thing, the women, even in the placebo group, were getting lots of calcium in their diets at the start of the study.
Also, more than half the women in both treatment groups were taking hormone replacement therapy, which can help bone strength. Plus, the women's average BMI was 29 (overweight but not obese). Larger people tend to have stronger bones than skinny people.
All of these factors may have weakened the findings of the study. While vitamin D and calcium supplements may help prevent bone fractures, the benefits were small.
During the study, 374 women fractured their hips. That's fewer fractures than the researchers expected, which may have made it harder to draw conclusions.
At the end of the study the results revealed that women taking the real supplements as directed were also those that were the least likely to have hip fractures.
The real supplements were also linked to a "small but significant" 1% increase in bone density, note Jackson and colleagues. Possibly, higher doses of vitamin D would have helped, the researchers note.
Kidney stones -- tiny chunks of minerals including calcium -- were 17% more common among women taking the real supplements, the study shows.