Women can help prevent brittle bones in old age by regularly consuming dairy products throughout their lives, but calcium supplements taken after menopause probably won't be effective, researchers say.

In a study in Friday's Journal of the American Medical Association, researchers said the key factors in bone density among post-menopausal women were lifelong calcium intake and the naturally occurring hormone estrone.Moderate exercise and calcium supplements taken later in life do not appear to prevent osteoporosis, a frequently debilitating disease most common among women in which the bones become thinner and more brittle, they found.

"Swallowing calcium pills at 50 years of age probably is not going to make your bones thicker," said Dr. Lewis Kuller, of the Epidemiology Department at the University of Pittsburgh's School of Public Health.

"The most important message . . . is that an adequate calcium intake during childhood, adolescence, post-adolescence and the reproductive years is probably a very important part of good health and will probably help prevent bone fracture and osteoporosis," he said.