A new study of 80,000 women with early breast cancer in 70 clinical trials has found that obesity is associated with a 34 percent higher risk of breast cancer-related death only among the 20,000 pre-menopausal women with oestrogen receptor (ER)-positive disease.

Obesity had little effect in post-menopausal ER-positive disease or in ER-negative disease.

“Obesity substantially increases blood oestrogen levels only in post-menopausal women, so we were surprised to find that obesity adversely impacted outcomes only in pre-menopausal women,” said Hongchao Pan, PhD, a researcher at the University of Oxford in the United Kingdom.

Dr. Pan continued: “This means we don’t understand the main biological mechanisms by which obesity affects prognosis.”

This Early Breast Cancer Trialists’ Collaborative Group (EBCTCG) study compared records from women who received the same treatment in the same clinical trial.

Body-mass index (BMI) was used to define normal weight, overweight, and obesity (20-25, 25-30 and ≥30 kg/m2).

To assess the independent effects of BMI on prognosis, the researchers adjusted the findings for tumour characteristics such as size and nodal spread, and for any differences in treatment.

Among the 20,000 pre-menopausal patients with ER-positive disease, the breast cancer mortality rate was one-third higher in obese women than in women of normal weight.

This would, for example, change a 10-year breast cancer mortality risk of 15 percent into a 10-year risk of 20 percent.

In contrast, obesity had little effect on breast cancer outcome either among the 40,000 post-menopausal women with ER-positive disease or among the 20,000 with ER-negative disease.

The study was funded by Cancer Research UK, the MRC and the British Heart Foundation.

ASCO Perspective

"This study is part of the growing body of evidence showing that patients who are obese generally fare worse with cancer— in this case, younger women with breast cancer," explained Clifford A. Hudis, MD, FACP, ASCO President.

“With some two-thirds of our nation’s adult population now obese or overweight, there’s simply no avoiding obesity as a complicating factor in cancer care. ASCO is working to support physicians and patients in addressing this challenge, and we urge researchers to examine new strategies for reducing obesity’s cancer-related toll."

Source: ASCO