Postmenopausal Women Who Lose Weight May Have Reduced Breast Cancer Risk

Losing even some extra weight significantly reduces the risk of breast cancer for postmenopausal women, a researcher reported during the 2017 San Antonio Breast Cancer Symposium, held Dec. 5-9 in Atlanta.

The results came out of a large, prospective, observational study that took note of participants’ heights and weights at baseline, and again after three years. After that, investigators followed up with each woman yearly, over an average 11.4 years, to monitor for any development of breast cancer. Postmenopausal women with an anticipated survival of at least three years were enrolled at 40 U.S. centers between 1993 and 1998, and data collection closed on Sept. 30, 2015.

“In the three-year window of the study, relatively modest weight loss was associated with significant lowering of breast cancer incidence,” said Rowan Chlebowski, M.D., Ph.D., a research professor in the Department of Medical Oncology and Therapeutics Research at City of Hope in Duarte, California. “From this study, we have evidence that a weight loss strategy can be effective in lowering breast cancer risk in postmenopausal women.”

Chlebowski said that he and fellow investigators embarked on the study because breast cancer is a common disease and cause of death in American women, obesity rates have been increasing, and several previous studies have associated a high weight-to-height ratio, or body mass index (BMI), with increased breast cancer risk in postmenopausal women. He noted that 65 percent of American women are overweight or obese.

“We wanted to determine if weight loss was associated with lower breast cancer incidence, as studies have not been able to consistently show that losing weight reduces the risk of breast cancer,” Chlebowski said. Past studies used measurements that were self-reported by participants, rather than the type of clinical data collected in the City of Hope project, he said.

For this project, Chlebowski and colleagues analyzed data from the Women’s Health Initiative (WHI) Observational Study, which tracks the health of postmenopausal women between the ages of 50 and 79. Any participant in the WHI with normal mammogram results and no prior breast cancer, and who was not underweight (defined by a BMI of 18.5 k/mg² or less), was eligible for enrollment. At baseline, in addition to height and weight data, the investigators asked participants to fill out questionnaires detailing their medical histories, breast cancer risk factors, medication use and mammogram frequency.

The study enrolled 61,335 women. At baseline, 41 percent of them were normal weight, 34 percent were overweight and 25 percent were obese. They were studied in four groups categorized by BMI, age, race/ethnicity and history of using hormonal drugs, but no differences in outcomes were found based on these distinctions.

Of the women enrolled, 3,061 developed invasive breast cancer during their follow-up period. Compared to women whose weight remained stable, those who lost 5 percent or more of their body weight were 12 percent less likely to develop breast cancer. Weight loss of 15 percent or more was associated with a 37 percent reduction in breast cancer risk.

Conversely, weight gain of 5 percent or more was not tied to increased overall breast cancer risk — but it was associated with more than a 50 percent increased risk of triple-negative breast cancer.

Protection against breast cancer was not significantly statistically different based on whether or not the weight loss was intentional, Chlebowski said.

“These are very optimistic findings that provide a lesson to postmenopausal women that even a moderate degree of weight loss may be associated with health benefits,” Chlebowski said. “It’s a relatively new finding, and it’s going to have public health implications.”

To make the most of the information, he said, stakeholders in the health system will need to determine which types of experts should be responsible for helping women form and implement weight loss plans, and working to make sure that those efforts are covered by health insurance.

The study was supported by the National Institutes of Health.