Young women with a high body mass index (BMI) had a reduced risk for developing breast cancer before menopause, according to a large, prospective observational study published in JAMA Oncology.1 This study confirms what previous studies have suggested: that obesity protects against developing breast cancer prior to menopause. In contrast, obesity is believed to increase the risk of developing breast cancer among postmenopausal women.

“Breast cancer is a leading cancer diagnosis among young adults. To guide effective prevention strategies, we need to better understand the contributors to breast cancer development at young ages,” co-primary study author Hazel Nichols, PhD, from the University of North Carolina at Chapel Hill, told Cancer Therapy Advisor. “In this case, we’re not recommending that premenopausal women increase their BMI to lower breast cancer risk — because we know that’s not a good strategy for overall health. Discovering the biological mechanisms behind this pattern may help us to identify other strategies to prevent breast cancer and enhance overall health.”

The study was conducted by the Premenopausal Breast Cancer Collaborative Group, an international group of researchers that includes scientists from the National Institutes of Health. Researchers pooled data from 758,592 premenopausal women across 19 prospective cohorts in North America, Europe, Asia, and Australia. Participants did not have a breast cancer diagnosis at the start of the study.

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Participants provided weight, height, and other information via various questionnaires. The relationship between BMI and breast cancer was assessed across 4 age groups: 18 to 24, 25 to 34, 35 to 44, and 45 to 54 years. Participants were followed for a median of 9.3 years, until menopause, at age 55, or until they died. In total, 13,082 participants (1.7%) eventually developed in situ or invasive breast cancer.

The researchers found that as BMI increased, the relative risk of breast cancer decreased across all age groups. The largest reduction was seen for women aged 18 to 24, who had a 23% lower risk of breast cancer (HR, 0.77; 95% CI, 0.73-0.80). The smallest reduction in risk was seen for women aged 45 to 54, who had a 12% lower breast cancer risk (HR, 0.88; 95% CI, 0.86-0.91).