Researchers at Washington University School of Medicine in St Louis have revealed that women who get most of their daily calcium from food have healthier bones than women whose calcium comes mainly from supplemental tablets.

The researchers' conclusions about calcium intake has come from a study of 183 postmenopausal women. The researchers asked the women to meticulously detail their diet and their calcium supplement intake for a week.

"We assumed that this sample represented each woman's typical diet," said senior author Reina Armamento-Villareal, MD, assistant professor of medicine in the division of bone and mineral diseases and a bone specialist at Barnes-Jewish Hospital.

"In addition to analysing the volunteers' daily calcium intake, we tested bone mineral density and urinary concentrations of oestrogen metabolites," she added.

The researchers found that the women could be divided into three groups: one group, called the 'supplement group', got at least 70 percent of their daily calcium from tablets or pills; another, the 'diet group', got at least 70 percent of their calcium from dairy products and other foods; and a third, the 'diet plus supplement group', consisted of those whose calcium-source percentages fell somewhere in between these ranges.

The diet group took in the least calcium, an average of 830 milligrams per day. Yet this group had higher bone density in their spines and hipbones than women in the supplement group, who consumed about 1,030 milligrams per day. Women in the diet plus supplement group tended to have the highest bone mineral density as well as the highest calcium intake at 1,620 milligrams per day.

The hormone oestrogen is known to maintain bone mineral density. But the standard form of oestrogen is broken down or metabolized in the liver to other forms -- some active and some inactive. Urinalysis showed that women in the diet group and the diet plus supplement group had a higher ratio of active to inactive oestrogen metabolites than women in the supplement group.

"This suggests that dietary calcium is associated with a shift in oestrogen metabolism that favours production of active forms of oestrogen," said Armamento-Villareal.

"Although we're not yet certain what underlies this effect, it could be that nutrients other than calcium cause this shift. It's also known that dairy products, which are a major source of calcium, can contain active estrogenic compounds, and these can influence bone density and the amount of estrogenic metabolites in the urine," she added.

Calcium supplements differ in how well their calcium can be absorbed, and this also could play a role in the study's findings, according to its authors. For example, calcium carbonate tablets need to be taken with a meal so that stomach acid can facilitate absorption, but calcium citrate tablets don't have this limitation. If the study participants taking calcium carbonate weren't conscientious about the timing of their supplements, they might not have received the highest benefit from them.

"Only about 35 percent of the calcium in most supplements ends up being absorbed by the body," Armamento-Villareal said.

"Calcium from diet is generally better absorbed, and this could be another reason that women who got a high percentage of calcium in their food had higher bone densities," Armamento-Villareal added.

Although dairy foods are excellent sources of calcium, Armamento-Villareal suggests that individuals with dairy sensitivities could consume other calcium-rich food sources such as calcium-fortified orange juice. Dark green leafy vegetables also contain calcium, but it is not as readily absorbed as calcium from dairy sources.