CHICAGO, IL—Obesity is associated with a 34% increase in mortality rates among premenopausal women with estrogen receptor (ER)-positive breast cancer, according to a study of 80,000 participants in 70 clinical trials, which will be presented at the 2014 American Society of Clinical Oncology Annual Meeting on Saturday, May 31st.

Obesity was not associated with breast cancer deaths among women who had ER-negative disease or who were postmenopausal, noted lead author Hongchao Pan, PhD, of the University of Oxford in England.

“Obesity substantially increases blood estrogen levels only in postmenopausal women, so we were surprised to find that obesity adversely impacted outcomes only in premenopausal women,” said Dr. Pan during a pre-conference press conference. “This is the opposite of what we expected.”

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“In women with early breast cancer, obesity appears strongly independently related to breast cancer mortality only in pre/peri-menopausal ER-positive disease,” Dr. Pan concluded.

The study is part of a “growing body of evidence showing that patients who are obese generally fare worse with cancer—in this case, younger women with breast cancer,” commented ASCO President Clifford A Hudis, MD, FACP. “Obesity is slated to overtake tobacco as a cause of cancer.”

“With 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,” Dr. Hudis said.

The Early Breast Cancer Trialists’ Collaborative Group (EBCTCG) analysis included data from 20,000 premenopausal women with ER-positive breast cancer. Body mass index (BMI) values of 30 kg/m2 or higher were defined as obese. The researchers statistically controlled for tumor size, lymph node involvement, and treatment regimens to isolate the effects of BMI on mortality.

BMI was positively associated with mortality in both pre- and postmenopausal women (P < 0.00001), Dr. Pan reported—but after statistical adjustment for tumor size and lymph node involvement, the association remained significant only among 20,000 pre- and perimenopausal women with ER-positive breast cancer (cancer-specific mortality rate ratio for obese vs. normal-weight women [RR] = 1.34; 95% CI: 1.22-1.47; P < 0.00001).

In addition to tobacco use, ASCO “notes with concern the rising toll of overweight and obesity” on cancer outcomes, Dr. Hudis said, highlighting ASCO’s recent roll-out of an obesity information resources “tool kit”.

“There are really two risk factors with obesity: one is the risk of developing cancer, and the other is the negative impact of being obese on cancer outcome,” Dr. Hudis noted. “This study did not address the first of these.”

While most Americans understand the association between tobacco and cancer, only 10% understand that obesity is also a cancer risk, Dr. Hudis said.


  1. Pan H, Gray RG. Abstract 503. Presented at: American Society of Clinical Oncology (ASCO) Annual Meeting 2014; May 30-June 3, 2014; Chicago, IL.