Calcium and vitamin D supplements in healthy postmenopausal women provide a slight benefit in preserving bone mass and preventing hip fractures, but do not prevent other types of fractures or colorectal cancer, according to a Women's Health Initiative (WHI) study involving more than 36,000 women. Certain groups, such as older women, had greater benefit for preventing hip fracture.
The study results were published in two papers in the February 16 issue of The New England Journal of Medicine. The Public Health Sciences Division's Dr. Andrea LaCroix co-authored both papers. LaCroix is a co-principal investigator of the WHI Coordinating Center, which is housed at the Hutchinson Center.
The calcium and vitamin D study followed 36,282 women ages 50 to 79 for seven years. Half of the participants were randomly selected to take 1,000 milligrams of calcium and 400 international units of vitamin D daily, while the other half took a placebo. Participants in this study had previously enrolled in one or both of the WHI hormone — therapy or dietary — modification trials. At the study's end, women who took the supplements had a modest, 1 percent increase in bone-mineral density and a statistically insignificant 12 percent overall reduction in hip fractures. There was no reduction in other types of fractures. However, the effect of calcium and vitamin D was significant among women over 60, who had a 21 percent reduction in hip fractures. The women who took their supplements as often as directed also had a 29 percent reduced risk of hip fractures.
No cancer protection
LaCroix said the study participants were already at low risk for fracture since they were not calcium-deficient. "Still, rates of hip fracture were further reduced by taking calcium and vitamin D supplements," she said. "These results are encouraging, especially for women over 60 who are looking for simple, non-prescription, preventive actions to prevent hip fracture."
Calcium/vitamin D supplements had no effect on the incidence of colorectal cancer. "We were surprised that the trial showed absolutely no difference in colorectal cancer between the treatment and control groups, since a number of other studies supported that hypothesis," LaCroix said.
While generally well-tolerated, the supplements were associated with a 17 percent increase in kidney stones. LaCroix said the risk of kidney stones was unrelated to how much calcium the women reported taking prior to the study.
"We really have to look further to see if we can better understand why certain women got the kidney stones," she said. "It's not linked to total calcium consumption."
The rate of hip fractures in the study population was about half of what was expected when the study was designed. This decreased the study's statistical power to show a significant finding, according to LaCroix. "The low rates could be due to a number of factors. These women were already taking an average of 1,100 milligrams of calcium a day, and they weighed more on average than expected [heavier people have stronger bones]. Many were taking hormone therapy, which lowers risk of hip fracture, and they were a little younger on average than we expected."
Nonetheless, LaCroix said the findings provided valuable information. "There haven't been large fracture trials in healthy postmenopausal women, so this study gives more evidence than we've had in the past," she said. "Calcium supplements definitely won't take the place of osteoporosis medications for the women who need them, but as an overall preventive measure, it does have a positive role."
The ongoing five-year WHI extension study will continue to track occurrences of colorectal cancer — as well as other diseases — and may provide answers on later effects of calcium/vitamin D supplementation.
The National Heart, Lung and Blood Institute of the National Institutes of Health funded the study, which involved numerous researchers and institutions nationwide. Other Center co-authors of these papers include Drs. Ross Prentice, Garnet Anderson, Shirley Beresford and Barbara Cochrane.