No bones about it. (Quick Studies).

Roughly half of all women aged 50 or older have either low bone mass or osteoporosis and don't know it, according to a new study that tracked more than 200,000 healthy women for one year.

Forty percent of the women had low bone mass, which roughly doubled their risk of fracture compared to women with normal bone mass. Another seven percent had osteoporosis (very low bone mass), which quadrupled their risk of fracture. (Women who had previously been diagnosed with osteoporosis were excluded from the study.)

DEXA, or dual-energy X-ray absorptiometry, is the gold standard test for bone density. But this study found that less expensive tests, which only measure bone density in the heel, are good enough to detect low bone mass.

"This report reaffirms the existence of a large population of women expected to live well into the 21st century who are at risk for future fracture," conclude the authors.

And it's not just a future risk that worries them. Eleven percent of the women in the study had already broken a bone after the age of 45. Yet none of them had been diagnosed with osteoporosis or treated for it.

What to do: Women who are over 65 should get their bone density tested (see Jan./Feb. 2002, cover story). So should younger women who are postmenopausal and have at least one other risk factor for fractures (for example, they weigh less than 127 pounds or have broken a bone as an adult).