Research published in JAMA Oncology highlighted a dose-response relation where a higher BMI (body mass index) was associated with greater relative risk (Neuhouser et al., 2015).
The study found no notable modification of risk with the use of hormone replacement therapy, race or ethnicity.
The WHI clinical trials included more than 67,000 women over 50 who enrolled from 1993 to 1998 and followed for a median of 13 years. A total of 3388 invasive breast cancers were observed during follow-up.
Women with a BMI greater than 35 had a 58% higher risk of invasive breast cancer than women with a BMI lower than 25 (hazard ratio 1.58 (95% confidence interval 1.40 to 1.79)). A BMI of 35 or higher was strongly associated with a raised risk of oestrogen and progesterone receptor positive cancer (1.86 (1.60 to 2.17)) but not oestrogen receptor negative cancers.
Women who had a BMI lower than 25 at baseline but who gained more than 5% in body weight during the follow-up period also had a raised risk of breast cancer (1.36 (1.1 to 1.65)).
Obesity was also associated with markers of poor prognosis. Women with a BMI greater than 35 were more likely to have large tumours, evidence of lymph node involvement and poorly differentiated tumours.
Neuhouser, M.L., Aragaki, A.K., Prentice, R.L., Manson, J.E., Chlebowski, R., Carty, C.L., Ochs-Balcom, H.M., Thomson, C.A., Caan, B.J., Tinker, L.F., Urrutia, R.P., Knudtson, J. & Anderson, G.L. (2015) Overweight, Obesity, and Postmenopausal Invasive Breast Cancer Risk: A Secondary Analysis of the Women’s Health Initiative Randomized Clinical Trials. JAMA Oncology. Published online June 11, 2015. doi:10.1001/jamaoncol.2015.1546.