Postmenopausal women who lose weight may have a significantly reduced chance of developing breast cancer, according to data presented at the 2017 San Antonio Breast Cancer Symposium, held Dec. 5–9.

“Breast cancer is among the leading types of cancer and causes of death in American women,” said Rowan Chlebowski, MD, PhD, research professor in the Department of Medical Oncology and Therapeutics Research at City of Hope in Duarte, California. “Obesity rates have been increasing in the United States. We wanted to determine if there was a link between obesity and breast cancer risk.”

Several studies have associated high BMI with increased breast cancer risk, noted Chlebowski. While this risk factor is largely preventable, it is estimated that more than 65% 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,” said Chlebowski. Past studies have been limited to analyses with self-reported measurements, while the prospective study conducted by Chlebowski and colleagues utilizes a short-term, 3-year period of measured body weight and height followed by a long period of follow-up.

Chlebowski and colleagues analyzed data from the Women’s Health Initiative (WHI) Observational Study. This program tracks the health of postmenopausal women between the ages of 50 and 79. Participants who had a normal mammogram, no prior breast cancer, and were not underweight (BMI ≥18.5) were eligible for enrollment in Chlebowski’s study. Measurements for height and weight were obtained upon enrollment, and measurements were reassessed 3 years following.

At baseline, 41% of women were normal weight, 34% were overweight, and 25% were obese.

Of the 61,335 patients enrolled in Chlebowski’s study, 3,061 developed invasive breast cancer during an average of 11.4 years of follow-up. Compared to women with stable weight, those who lost weight (≥5% weight change) were 12% less likely to develop breast cancer following multivariable analysis. Weight loss of ≥15% was associated with a 37% reduction in breast cancer risk.

“In the 3-year window of the study, relatively modest weight loss was associated with significant lowering of breast cancer incidence,” said Chlebowski. “From this study, we have evidence that a weight loss strategy can be effective in lowering breast cancer risk in postmenopausal women.”

While weight gain (≥5% weight change) was not associated with increased overall breast cancer risk, it was associated with more than 50% increased risk of triple-negative breast cancer.