The finding comes from a detailed, seven-year study of more than 36,000 U.S. women aged 50 to 79 enrolled in the Women's Health Initiative. Half the women took 1,000 milligrams of calcium and 400 international units (IU) of vitamin D every day; the other half took identical-looking, inactive placebo pills.
The study proved what many have learned from experience: After menopause, women tend to gain weight until their mid-60s.
"This study clearly demonstrates what happens to women over time. The most weight gain was in the youngest postmenopausal women," Kaiser Permanente researcher Bette Caan, DrPH, tells WebMD.
But the study also showed that calcium/vitamin D supplements slowed this weight gain -- particularly among women who weren't getting enough calcium to begin with.
"Women not taking enough calcium were getting the greatest benefit. They were 11% less likely to gain weight and more likely to remain weight-stable or lose weight," Caan says. "The effect was not cumulative: Women got the benefit after three years, and then were able to maintain that benefit."
This is good news for women, says Leslie Bonci, MPH, RD, director of sports nutrition at the University of Pittsburgh Medical Center.
"The neat thing about this study is there may be a very easy little thing women can do to prevent some of that weight gain after menopause: Keep their calcium up," Bonci tells WebMD.
Caan and colleagues report the finding in the May 14 issue of Archives of Internal Medicine.
Calcium/Vitamin D: More at Stake Than Weight
The average weight benefit was not very large -- just over a quarter of a pound overall, and less than half a pound in women with low calcium intake.
But in this latter group, women who took the calcium and vitamin D supplements were 11% less likely to gain 2.2 to 6.6 pounds and also 11% less likely to gain over 6.6 pounds.
And weight isn't the main reason to take calcium and vitamin D. Caan and colleagues previously reported that the supplements slow bone loss and cut a woman's risk of bone fracture after menopause.
"This is not just about preventing that muffin top above the belt -- there is a whole-body benefit from calcium and vitamin D," Bonci says. "Calcium and vitamin D may make you a slightly smaller muffin, but it is vitally important for bone health."
Does it matter whether you get your calcium from a pill or from dairy foods? Bonci notes that dairy foods contain other valuable nutrients besides calcium. But they are not calorie-free.
"If you are getting your calcium from low-fat dairy foods, you have to swap those calories out for something else," she says. "It is not just a matter of adding the low-fat cottage cheese; it means having those dairy foods instead of the tuna or the turkey. And one is not going to get one's vitamin D needs met from the small amounts added to dairy foods."
Caan warns that calcium and vitamin D supplements are not magic weight loss pills.
"Postmenopausal women this age should be taking calcium supplements anyway," she says. "But to prevent weight gain, they should still consider calorie restriction and exercise."