Women who have hot flashes as they enter menopause may have more fragile bones, new research shows.

Dr. Carolyn J. Crandall of the University of California, Los Angeles and her colleagues found that women who reported having hot flashes and night sweats-known collectively as vasomotor symptoms-had lower bone mineral density (BMD). And the more hot flashes they had, the thinner their bones.

Most women have hot flashes at some point during the transition into menopause, Crandall and her team point out, with about 60% of women experiencing them just before menstruation actually ceases.

Because hot flashes peak as bone density declines, and both menopausal symptoms and loss of bone density have been linked to low estrogen levels, Crandall and her team investigated whether there might be any relationship between the two.

They looked at 2,213 women participating in the Study of Women’s Health Across the Nation, in which researchers are following a multi-ethnic group of US women through menopause. At the beginning of the study, participants were 42 to 52 years old, and had either not yet undergone menopause or were in early perimenopause, meaning they had menstruated at least once in the three months before they entered the study.

Women having vasomotor symptoms had significantly lower BMD than women who didn’t have these symptoms, no matter what their menopausal stage, the researchers found. The link remained after the researchers adjusted for ethnicity, weight, age and other relevant factors.

There were differences in where bones were thin based on menopausal stage, however. Among the women who were not yet undergoing menopause and those in early perimenopause, those with vasomotor symptoms had lower bone density in the femoral neck (the upper part of the thigh bone linking the shaft of the bone to the “ball” that rests in the hip socket). For postmenopausal women, the difference was seen in the lower back and the hip.

And the women who had more frequent vasomotor symptoms had lower average BMD than those who didn’t have the symptoms frequently.

The fact that vasomotor symptoms signaled lower BMD even in early perimenopause suggests that estrogen levels alone can’t explain the relationship, Crandall and her colleagues say, because estrogen levels don’t drop significantly until just before menstruation ceases, and, in some cases, actually spike in early perimenopause.

They note that the sympathetic nervous system is activated during hot flashes, and that neurotransmitters and hormones released during this activation have been linked to bone loss. Future studies should investigate the “neurobiological pathways” that may connect vasomotor symptoms and BMD, they conclude.

SOURCE: Menopause, March/April 2009.