Although as much as 60 percent of our chance for developing strong bones comes from how we live, heredity is still a major player in bone health. In 1994, researchers discovered a gene that contains instructions for a crucial vitamin D receptor. We need the receptor so our bones can use vitamin D to absorb calcium. If a version of this gene turns out to be a reliable marker for osteoporosis, women still in their teens will be able to take a blood test to assess their risk and begin preventive ac­tion decades before bone loss actually begins.

Osteoporosis causes about 250,000 hip fractures each year and they occur three times more often in women than in men. Lean women who lose as little as 5 percent of their body weight after age fifty double their chance of hip fracture, as do women of average body mass who lose 10 percent of their body weight after age fifty. Researchers speculate that this link may be due to some combina­tion of decreased muscle mass, less available estrogen from fat cells, less gravitational force on bone, and loss of natural hip padding. Most of these hip fractures cause some kind of permanent disabil­ity and 20 percent actually result in death.

Spinal fractures are also common in women over age fifty. Some­times these fractures are painless, but they also can result in severe pain and disfigurement as the spine literally collapses on itself.

You are at even greater risk for osteoporosis if there is a history of the disease in your family. Heredity plays a role in your potential peak bone mass, determining just how strong your bones can be­come. Genes also determine the shape of your bones. Asian women, for example, have only about half the hip fractures of Cau­casian women, even though both groups lose bone at about the same rate as they age. Researchers attribute the lower incidence of fracture to Asians' shorter hip axis, which is much stronger.