Diet, exercise, genes strengthen bones.

Study after study these days touts the benefits of eating right and

exercising. But the value of that advice may depend on your genetic

inheritance.

A study of a gene associated with the development of strong bones indicates that the importance of exercising and drinking milk varies with the type of vitamin D receptor (VDR) gene your cells harbor.

"We found that the combination of dietary calcium intake and exercise can overcome a poor genetic predisposition," says epidemiologist Loran Salamone of the University of Pittsburgh team that conducted the study, "while for those with a beneficial genetic makeup, little activity and low calcium intake didn't adversely affect [bone strength.]"

The finding may help identify women who need to work at developing strong bones.

Through early adulthood, people continually add calcium and strength to their bones, which reach a peak density around age 35. Women rapidly lose bone density after menopause, and their ability to avoid osteoporosis--severe bone loss that can lead to life-threatening fractures--depends upon their bone density before menopause.

In 1994, researchers discovered that the vitamin D receptor is vital to achieving high bone density. But not all vitamin D receptors are the same. The VDR gene, which carries the blueprint for making the receptors, comes in two varieties. Both produce functional vitamin D receptors, but one stores calcium in bone a little more efficiently.

People who inherit two copies of the more efficient form of VDR--known as b--develop high bone densities. Those who inherit two copies of the less efficient gene--known as B--have somewhat less strong bones.

Salamone knew that a person's VDR gene complement would help determine bone strength, but she figured that factors such as dietary calcium and exercise would also play a role. She and her colleagues studied 470 premenopausal women age 44 to 50. They interviewed the women about their diets, exercise habits, and hormone usage, then analyzed the women's bone density and genetic makeup.

As Salamone reported at a meeting of the Society for Epidemiologic Research in Snowbird, Utah, in June, women with two B forms of VDR who exercised the most developed 7 percent higher bone density than women with the same genes who exercised the least. Women harboring B who had high calcium intake and who exercised enjoyed 10 percent more bone density, but a calcium-rich diet without exercise had no beneficial effect.

Women with the b genes developed strong bones regardless of exercise and calcium intake.

"This is a wonderful study," says epidemiologist Marian T. Hannan of the Boston University Arthritis Center. Salamone "has shown that it is possible to make modifiable changes that can have an impact on osteoporosis."

Salamone points out that her study population is white, so she isn't sure how the genes affect black women, who ordinarily have much higher bone density.