Adding yet one more detail to the picture of the benefits and risks of hormone replacement therapy (HRT), a new study has found that HRT may help protect postmenopausal women against cardiovascular disease.

Hormone replacement therapy should be begun within about six years of the start of menopause to make the most of its heart-protecting effects, researchers at the University of Southern California in Los Angeles report.

“So if HRT is protective in its actions, you want to start it early, while cells are still healthy.”

In women who were within six years of entering menopause, HRT with estrogen (estradiol) prevented the progression of atherosclerosis, the thickening of the artery walls, compared with placebo. HRT did not prevent the development of atherosclerosis in women who had been in menopause for 10 years or more.

The study provides the first direct evidence that the timing of the start of HRT is important for its success in preventing cardiovascular disease and stroke, Howard Hodis, the principal investigator of the study, told TheDoctor. The importance of the timing of the start of treatment is probably also applicable to most preventive approaches to cardiovascular disease in women.

Nearly 650 women were divided into two groups for the study: those who were six years or less from menopause and those who were 10 years or more from menopause. Women in each group were randomly assigned to receive HRT or a placebo.

The researchers found that among women who were six years or less since menopause, those who received HRT had less thickening of their artery walls compared to women who received the placebo. HRT had little or no effect among women for whom it had been 10 years or more since menopause began — they had thickening of artery walls similar to that of women who had been given a placebo.

These results may be the result of what neuroscientists and cardiologists call the “healthy cell bias,” Hodis explained. Basically, if patients start HRT when they have healthy brain cells, HRT tends to maintain the health of the brain, Hodis said. If the brain cells are already diseased, as they are in patients with Alzheimer’s disease, HRT will not help, and may even accelerate their problems.

This same healthy cell bias applies to the cells in the vascular wall, said Hodis, the director of the Atherosclerosis Research Unit at the Keck School of Medicine at the University of Southern California. When the cells are healthy, HRT has a cardioprotective effect, but when they already have lesions, it does not help.

As people age, their vessels get diseased, just like their brains do, Hodis said. “So if HRT is protective in its actions, you want to start it early, while cells are still healthy.”

Now the scientists need to sort out the mechanism behind the cardioprotective effect of HRT, said Hodis. “This mechanism probably applies to everything we do in prevention, especially in women,” he said.

The study was published online recently in the New England Journal of Medicine.