Women who get calcium from food have higher bone density than those who get calcium from supplements, a study has found. This is true even if the supplements contain more total calcium than the diet with which they are compared.

In the study published in the May 2009 edition of the American Journal of Clinical Nutrition, researchers at the Washington St. Louis had 168 post-menopausal women keep a weeklong dietary record. The researchers used the records to calculate how much calcium was ingested.

They grouped the women by major source of calcium: dietary, supplemental or both. And they looked at how the ingested amounts compared to bone mineral density (BMD).
The results: Women who obtained calcium from both diet and supplement had the highest intake of the mineral and also the highest BMD.

But even though the group that obtained most of its calcium from supplements consumed more total calcium (1033 mg/day compared with 830 mg/day), its BMDs were lower than either of the other groups.

Hormone levels could be the reason for the superiority of dietary calcium, the study suggests. The researchers examined estrogen metabolites (formed when the hormone estrogen is broken down) in the women’s urine. They found that those women who got most of their calcium from the diet had comparatively more “active” estrogen metabolites (which still act somewhat like estrogen) than the other groups. High levels of these active metabolites are known to be associated with stronger bones.

But whether or not this hormone mechanism is correct, there are several possible reasons for dietary calcium’s superior effect on bone.

The simplest is that women who get most of their calcium from food have more balanced diets. They may also have maintained them for longer, including during the key, bone-building years. Supplements, on the other hand, might be added only later in life, after a period of deficiency.

Dr. Reina Armamento-Villa-real, lead author of the study and assistant professor in Washington University’s Division of Bone and Mineral Diseases, says since women’s peak bone mineral density occurs around age 30, an ideal next step would be to study them in the growing period. That’s when calcium intake may have the greatest effect on estrogen metabolism, she says.

Another reason for the poorer performance of supplements is that they, unlike dietary calcium, are absorbed best only under certain conditions. The highly acidic digestive juices that automatically flow after a meal are not necessarily present after chewing a tablet, which means that the calcium salts may not completely dissolve.

In addition, supplements may contain one of several different kinds of calcium salts, and some are absorbed better than others. (For this study, the women were not told which supplements to use, or when.)

Connie Weaves, head director of food and nutrition at Purdue University in West Lafayette, Indiana, says that research provides an interesting entry point into how diet might be linked to hormonal processes. A previous study also suggested that the mineral can have an effect on female development, possibly via hormones, Weaver says.