Menopausal women who suffer hot flashes are nearly twice as likely to later suffer hip fractures, one of the most debilitating injuries of old age, according to a new study by a UCLA medical school professor.

The still elusive question is why.

Carolyn  J. Crandall of UCLA's David Geffen School of Medicine and a large team of researchers examined data for more than 23,000 women ages 50 to 79 enrolled in the Women's Health Initiative Clinical Trial at 40 medical centers around the United States. They also looked at more than 4,800 women who were part of a study of bone mineral density. The women they studied were not using hormone therapy as a treatment for menopause.

Though the number of hip fractures was small, the researchers discovered that women who experienced moderate to severe hot flashes during menopause -- known as "vasomotor symptoms" -- were 1.78 times as likely to eventually suffer a hip fracture, and had lower bone mineral density, a measure of bone health used to diagnose osteoporosis. Curiously, however, they did not discover a connection between hot flashes and fractures of the vertebrae.

The reason for the connection to hip fractures is still "entirely a mystery," Crandall said. "We’re the first ones to ever look at this."

The data were strong enough to rule out age, race, weight, estrogen supplements and smoking as possible causes. Crandall said one possibility could be swings in hormone levels that medical technology does not yet detect. She noted that other research has shown an association between hot flashes and "cardiovascular risk markers" but no causal relationship.

"There has to be something different about the reactivity that happens in women with hot flashes," she said. Perhaps "it's something we don't yet have the technological capability to measure."

At least 60 percent of women suffer hot flashes as they go through menopause, and 80 percent of them experience the symptom for five years, she said. Some women can have hot flashes for as long as 10 years, she said.

About 300,000 people 65 and older suffer hip fractures every year, and 20 percent to 30 percent of them die within 12 months, according to a 2009 study. Many others lose the ability to perform everyday functions such as climbing stairs and raising themselves from a chair.

Crandall said women who suffer hot flashes should follow recommendations for bone health, which include physical activity such as walking, avoiding smoking and excess alcohol consumption and making sure they get enough calcium and vitamin D.