It is increasingly clear that maintaining a healthy weight throughout adulthood is one of the best things women can do to protect themselves against breast cancer.

Findings from a study involving more than 87,000 female nurses show that weight gain during adulthood is a strong risk factor for breast cancer. Researchers also found that weight loss after menopause helped lower breast cancer risk.

Gaining 55 pounds or more after age 18 was associated with a 45 percent increase in breast cancer risk after menopause over women who maintained a healthy weight throughout their 20s, 30s, and 40s.

The Brigham and Women’s Hospital and Harvard Medical School study is not the first to find weight gain during adulthood to be an important risk factor for breast cancer. But it is the first to show that losing weight after menopause can lower breast cancer risk.

The research is published in the July 12 issue of The Journal of the American Medical Association.

“It seems from our findings that it is never too late to reduce breast cancer risk by losing weight,” researcher A. Heather Eliassen, ScD, tells WebMD. “But we all know how hard it is to lose weight. So the best advice that we can give to women is to avoid the weight gain in the first place.”

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The Estrogen Connection

Estrogen fuels the growth of most breast cancers. All women produce estrogen during their reproductive years primarily by the ovaries. But after menopause, circulating estrogen levels drop dramatically and the main estrogen source comes from the body’s fatty tissue. The more body fat a woman has after menopause the more estrogen she will have circulating.

Eliassen and colleagues followed two groups of women in their effort to better understand the impact of weight change during adulthood on breast cancer risk after menopause.

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Doubled Breast Cancer Risk

Data from the ongoing Nurses Health Study were analyzed, and weight changes throughout adulthood were assessed for 87,143 postmenopausal women followed for 26 years. Researchers also examined the impact of weight change following menopause among more than 49,000 women, followed for up to 24 years.

Researchers reported that weight gain was most strongly associated with breast cancer risk in women who had never taken estrogen hormone therapy.

Gaining 55 pounds or more after age 18 was associated with a doubling of breast cancer risk among women who had never used hormone therapy. Gaining 22 pounds or more after menopause was associated with an 18 percent increase in breast cancer risk for all postmenopausal women.

Among the other major findings from the study:

The researchers estimate that for all the women included in their study, 15 percent of breast cancer cases may be linked to a gain in weight of 4.4 pounds or more since age 18, and 4.4 percent of cases for similar weight gain since menopause.

For women who didn’t use postmenopausal hormones, the percentages were 24 percent for weight gained since age 18 and 7.6 percent for weight gained since menopause.

Women who lost 22 pounds or more after menopause and were able to keep the weight off were almost 60 percent less likely to develop breast cancer than overweight, postmenopausal women who did not lose weight.

The researchers note that including past use of oral contraceptives did not change the results.

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‘A Lot of Evidence’

“There is now a lot of evidence that obesity increases the risk of breast cancer in postmenopausal women,” epidemiologist Michael Thun, MD, of the American Cancer Society, tells WebMD.

Back in May, ACS researchers published their own similar study. Like the Harvard researchers, they found a strong link between weight gain during adulthood and breast cancer after menopause. But they also found that the biggest gainers often had the most aggressive cancers.

Women who gained more than 60 pounds after age 18 were three times as likely to be diagnosed with breast cancers that had spread to other areas of the body as women who gained 20 pounds or less.

Thun agrees that the major public health messages from all the research is that women should do all they can to maintain a healthy weight.

“That is our focus, because it is clear that it is much easier to avoid weight gain in the first place than to lose weight,” he says.

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By Salynn Boyles, reviewed by Louise Chang, MD

SOURCES: Eliassen, A.H. The Journal of the American Medical Association, July 12, 2006; vol 296: pp 193-201. A. Heather Eliassen, ScD, instructor in medicine, Harvard Medical School, Boston. Michael Thun, MD, vice president of epidemiology and surveillance research, American Cancer Society. WebMD Medical News: “Obesity Boosts Risk of Breast Cancer.”