From the Journals

Higher BMI tied to lower breast cancer risk in women before menopause



Although obesity increases the risk of breast cancer in postmenopausal women, a large multicenter analysis has confirmed the opposite effect in premenopausal women.

The association had “a greater magnitude than previously shown and across the entire distribution of body mass index,” wrote Minouk J. Schoemaker, PhD, of the Institute of Cancer Research in London, with his associates, on behalf of the Premenopausal Breast Cancer Collaborative Group. The protective effect of adiposity was strongest during young adulthood (ages 18-24 years), when it spanned breast cancer subtypes. “Understanding the biological mechanisms underlying these associations could have important preventive potential,” they wrote in JAMA Oncology.

Prior studies have linked greater body fat with reduced risk of breast cancer in younger women, but the effect has not been well characterized. For this analysis, the investigators pooled data from 19 cohort studies that included a total of 758,592 premenopausal women; median age was 40.6 years (interquartile range, 35.2-45.5 years).

For each 5-unit increase in BMI, the estimated reduction in risk of breast cancer was 23% among women aged 18-24 years (hazard ratio, 0.77; 95% confidence interval, 0.73-0.80), 15% in women aged 25-34 years, 13% in women aged 35-44 years, and 12% in women aged 45-54 years. There was no BMI threshold for risk reduction: the inverse correlation existed even when women were not overweight. Risk also did vary significantly among subgroups stratified by other risk factors for breast cancer. Adiposity was more protective against estrogen receptor-positive and progesterone-receptor positive breast cancers and less protective against hormone receptor–negative breast cancers, which “implies a hormonal mechanism,” the investigators said. “Body mass index at ages 25-54 years was not consistently associated with triple-negative or hormone receptor–negative breast cancer overall.”

Funders included Breast Cancer Now, the Institute of Cancer Research, the National Institutes of Health, and many others. The researchers reported having no relevant conflicts of interest.

SOURCE: Schoemaker MJ et al. JAMA Oncol. 2018; Jun 21. doi: 10.1001/jamaoncol.2018.1771.

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