How does a woman’s BMI impact her risk of premenopausal breast cancer?

A high body mass index (BMI) may be associated with a reduced risk of premenopausal breast cancer, according to new research published in JAMA Oncology.

The authors, members of the Premenopausal Breast Cancer Collaborative Group, studied more than 758,000 premenopausal women taken from 19 patient cohorts from all over the world. They noted an inverse association of breast cancer risk with the BMI for women between the ages of 18 and 54; as BMI increased, the breast cancer risk decreased. This inverse association was the most apparent in women between the ages of 18 and 24.

“The stronger inverse associations of risk with BMI at younger than older ages suggest that adiposity in young adulthood or earlier, if adiposity at approximately 20 years of age is a proxy marker for adiposity in childhood, is the critical factor,” wrote author Minouk J. Schoemaker, PhD, division of genetics and epidemiology at the Institute of Cancer Research in London, England, and colleagues. “No cohorts in our collaboration had information on BMI at younger than 18 years, but published analyses of subjective body size compared with peers at these ages have found strong inverse associations with premenopausal and postmenopausal breast cancer risk.”

Schoemaker and colleagues also made it clear they are not, in any way, suggesting that patients gain weight to combat breast cancer.

“Obesity has many adverse effects on general health, and we do not advocate weight gain as a preventative measure against premenopausal breast cancer,” the authors wrote. “However, understanding the mechanistic action underlying the inverse association of premenopausal adiposity with breast cancer risk could potentially identify modifiable pathways.”