The fracture risk study yielded some promising indications. Women who consistently took the full supplement dose experienced a significant 29 percent decrease in hip fracture. Women older than 60 had a significant 21 percent reduction in hip fracture.

The studies -- one assessing calcium and vitamin D supplementation on the risk of fractures and the other evaluating their effects on the risk of colorectal cancer -- are part of the Women's Health Initiative (WHI), sponsored by the National Institutes of Health (NIH) and the National Heart, Lung and Blood Institute (NHLBI).

The WHI Calcium with Vitamin D (CaD) trial of 36,282 postmenopausal women ages 50 to 79 found no detectable effect on the incidence of colorectal cancer.

During the trial, 374 women had hip fractures with a fracture rate of 14 per 10,000 cases per year in the supplemented group compared to 16 per 10,000 per year in the placebo group. This 12 percent reduction in hip fracture in those taking the calcium plus vitamin D supplement was not statistically significant. The supplements had no significant effect on spine or total fractures.

Not Conclusive

Although disappointing, the results of the studies are not considered conclusive.

The 400 IU of vitamin D per day used in the study may not have been enough to demonstrate the positive results on hip fracture risk previously shown to be effective, according to the authors. The majority of other studies reporting a benefit from calcium and vitamin D supplementation involved a vitamin D dose of 600 IU or higher.

In addition, the study notes that subjects may have been too young to detect a beneficial effect on hip fracture.

The latency period of 10 to 20 years for colorectal cancer could have played a role in the null results in the study on colon cancer, the authors suggested, given that the study duration averaged only seven years. They plan to assess the longer-term effect of calcium and vitamin D supplementation in an ongoing follow-up.

Promising Indications

The fracture risk study yielded some promising indications. Researchers found a small but significant 1 percent higher hip bone density for those taking calcium combined with vitamin D compared to those taking placebo. Women who consistently took the full supplement dose experienced a significant 29 percent decrease in hip fracture. Women older than 60 had a significant 21 percent reduction in hip fracture.

"The study showed that calcium with vitamin D supplementation resulted in a modest but significant improvement in hip bone density in postmenopausal women," said John Hathcock, PhD, vice president, scientific and international affairs, Council for Responsible Nutrition (CRN), a trade association representing dietary supplement industry interests. "It's very possible that if the study continued for a longer duration there would have been a significant reduction in fracture risk."

Noting the consistency of that point with other research, Dr. Hathcock continued, "Giving women an edge by building bone density earlier on helps bolster bones against the possible onset of osteoporosis later. Calcium and vitamin D supplementation can effectively increase bone density to help preserve women's bone health for the long term."