When mothers around the nation insist that their daughters drink more milk, they have a lot of research to stand on. More than 30 years ago, studies found that calcium absorption decreases with age, especially after menopause. But since those first studies were done, not many more have followed, and scientists still don't know whether the change in calcium absorption is caused by menopause, aging, or something else. A group of Australian scientists took a fresh look at this topic.

What the researchers wanted to know: Is there a decrease in calcium absorption in elderly women, and what might cause that?

What they did: The researchers recruited 262 post-menopausal women, ages 40 to 87, for the study. None of the women included had osteoporosis, and all had a bone density test done at the beginning of the study. The women visited the lab at 9 a.m. after fasting for the night, were weighed, measured, and had a blood sample drawn. The women then took a calcium supplement and had blood drawn again an hour later. The researchers measured the calcium level in the blood before and after the women took the calcium supplements.

What they found: Once women got to be about 75 years old, their ability to absorb calcium declined by an average of a third. The scientists are not entirely sure what causes this decline because it cannot be attributed to a decrease in the hormone estrogen, which fuels a similar decline in calcium absorption during menopause. The decrease was also not due to a decrease in the level of vitamin D in the blood, which helps absorb calcium, so the researchers speculated that it had something to do with a degrading of the small intestine, where calcium seeps into the body.

What it means to you: This study is a good reminder of why it's important to drink your milk. Calcium builds up your bones, preventing osteoporosis and fractures. As these scientists found, it's harder to beef up your calcium intake in old age.

Caveats: This study looked at immediate calcium absorption—within an hour of taking a supplement. So, there's a possibility that some of the calcium had not yet been absorbed into the blood, though that would probably not account for the variation in absorption by age.

Find out more: The George Washington University Hospital in Washington, D.C., has a Web page about bone loss in women as they age and what you can do about it.

In Columbia University's student question-and-answer health forum, a nurse answers a college woman's question about how much calcium she needs with some good research and information on the amount of calcium in common foods.

Read the article: Nordin, B.E.C. et al. "Effect of Age on Calcium Absorption in Postmenopausal Women." American Journal of Clinical Nutrition. Oct. 1, 2004, Vol. 80, No. 4, pp. 998–1002.

Abstract online: www.ajcn.org