Obesity increases Breast Cancer Risk in postmenopausal women, finds study

A new study published in The Breast journal found that obesity increases the risk of estrogen receptor–positive (ER-positive) breast cancer in postmenopausal women, especially among those not using hormone replacement therapy. Overweight was also linked to a higher risk of HER2-positive breast cancer in postmenopausal women. Also, before menopause, being overweight was associated with a lower risk of ER-positive breast cancer but a higher risk of ER-negative and triple-negative breast cancer.

This comprehensive systematic review and meta-analysis was set to examine how body mass index (BMI) relates to different breast cancer subtypes and menopausal status. The study analyzed data from 33 scientific studies, including 9 cohort studies and 24 case-control studies, encompassing more than 2.1 million women globally. This research searched major medical databases to identify relevant studies published up to September 2024.

Women classified as overweight had a BMI between 25 and 29.9 kg/m², while those with obesity had a BMI of 30 kg/m² or higher. Their breast cancer risk was compared with women considered underweight or normal weight (BMI below 25 kg/m²). The analysis found that obesity was linked to a modestly higher risk of developing hormone-sensitive breast cancer, specifically ER-positive Breast Cancer. Across all women studied, obesity increased the risk of this subtype by about 13%.

In postmenopausal women, obesity was linked to a 29% increased risk of ER–positive breast cancer. Being overweight also raised risk slightly for both ER–positive disease and HER2-Positive Breast Cancer. In premenopausal women, being overweight was actually associated with a reduced risk (20%) of ER–positive breast cancer. Also, higher BMI appeared to increase the risk of more aggressive forms of the disease.

Premenopausal women who were overweight had a higher risk of ER–Negative Breast Cancer and Triple-Negative Breast Cancer. After menopause, body fat becomes a major source of estrogen production, by increasing the risk of hormone-sensitive cancers. Before menopause, ovarian hormone production dominates, which may alter how body fat influences cancer risk.

The findings caution that BMI alone does not determine cancer risk, and genetics, lifestyle, reproductive history, and metabolic factors also play important roles. Overall, the results highlight the importance of considering both menopausal status and cancer subtype when studying breast cancer risk.

Reference:

Dauccia, C., Bruzzone, M., Arecco, L., Blondeaux, E., Sirico, M., Gerosa, R., Franzoi, M. A., Brandão, M., Perrone, L., Pedrazzoli, P., Desmedt, C., Di Cosimo, S., Vernieri, C., Collet, L., Lambertini, M., de Azambuja, E., & Agostinetto, E. (2026). Association between body mass index and risk of breast cancer according to breast cancer subtypes: A systematic review and meta-analysis. Breast (Edinburgh, Scotland), 86(104710), 104710. https://doi.org/10.1016/j.breast.2026.104710