Higher BMI Decreases Risk of Breast Cancer in Premenopausal Women

A large pooled analysis of 758,592 premenopausal women, conducted by the Premenopausal Breast Cancer Collaborative Group, identified an inverse association between body mass index (BMI) and the risk of breast cancer.

The most common type of cancer diagnosed in women, breast cancer accounts for approximately 25% of all female cancers. Although female gender remains the biggest risk factor for breast cancer, other factors such as obesity are also being explored. In postmenopausal women, obesity is positively correlated with breast cancer risk. However, in premenopausal women, previous findings suggested an inverse relationship where an increased BMI lowered breast cancer risk. In a study conducted by the Premenopausal Breast Cancer Collaborative Group, data from premenopausal women was retrieved to estimate the relative risk of breast cancer in association with BMI while also exploring other breast cancer characteristics.

A total of 758,592 premenopausal women were pooled together from 19 prospective cohort studies, 13,082 of whom developed in situ or invasive breast cancer during a median follow-up period of 9.3 years. Data from these studies were categorized according to BMI, age, and other relevant breast cancer characteristics.

After the data were categorized, investigators found that as BMI increased, the risk of breast cancer decreased, with findings particularly evident in younger women. For women aged 18 to 24 years old, there was a 23% risk reduction per 5 kg/m² difference (hazard ratio [HR], 0.77; 95% CI, 0.73-0.80). The difference resulted in a 4.2-fold risk gradient between the highest and lowest BMI categories (BMI ≥35.0 vs <17).

Women who were in the oldest age group (45 to 54 years old) also had risk reductions (12%), although not as large as the younger women (HR, 0.88; 95% CI, 0.86-0.91).

An interesting discovery was that there were differences in relative risk even among women who had normal BMI (18.5-25). Premenopausal women who were in the BMI range between 23.0 to 24.9 had a relative risk reduction of 20% compared with women who were in the BMI range between 18.5-22.9 (HR, 0.80; 95% CI, 0.75-0.86).

Obesity also impacted breast cancer characteristics. For women aged 25 to 34 years, associations of breast cancer risk for every 5 kg/m² difference in BMI were significantly larger for in situ (HR, 0.76; 95% CI, 0.69-0.85) than for invasive breast cancer (HR, 0.88; 95% CI, 0.84-0.92) (P = .02 for interaction). Similar observations were made for women between 35 to 44 years old.

For women at 18 to 24 years old, estrogen and progesterone receptor (ER/PR)—positive breast cancer showed stronger associations with BMI than ER/PR–negative breast cancer. Notably, there were no associations between BMI and triple-negative breast cancer in women 25 years or older or with ERBB2/HER2 in women 35 years or older. Use of oral contraceptives was associated with less risk reduction in relation to BMI difference in the age groups 18 to 24 (P = .02) and 34 to 44 years (P = .009). Women who gave birth also had less risk reduction in relation to BMI at ages 25 to 34 years than women who have not given birth (P = .03 for interaction).

Findings from this study revealed that obesity was inversely associated with the risk of breast cancer in premenopausal women, particularly in younger women. Future studies should be aimed at uncovering the reasoning behind these results to implement better breast cancer prevention strategies.

Reference

Schoemaker MJ, Nichols HB, Wright LB, et al; The Premenopausal Breast Cancer Collaborative Group. Association of body mass index and age with subsequent breast cancer risk in premenopausal women. JAMA Oncol. 2018;4(11):e181771. doi: 10.1001/jamaoncol.2018.1771.