A new study found that women who were postmenopausal and obese were more likely to develop breast cancer than women of normal weight.
“Obesity is associated with an [increased] postmenopausal breast cancer risk, particularly for estrogen receptor- and progesterone receptor-positive disease, but risk does not vary by [hormone therapy] use or race/ethnicity," wrote lead study author Marian L. Neuhouser, PhD, of the Fred Hutchison Cancer Research Center in Seattle, and colleagues.
Graham A. Colditz, MD, DrPH, epidemiologist and associate director for prevention and control at the Alvin J. Siteman Cancer Center, Washington University School of Medicine and Barnes-Jewish Hospital in St. Louis, MO, told dailyRx News that past research has reached similar conclusions.
"The data are so strong that the International Agency for Research on Cancer (IARC) and also the World Cancer Research Fund classify obesity as a known cause of postmenopausal breast cancer," Dr. Colditz said. "It is estimated that about 20 percent of postmenopausal breast cancer could be prevented if women did not gain weight and become obese. In addition, we have shown that major weight gain in premenopausal years can also increase risk of premenopausal breast cancer."
Dr. Colditz added, "All could be avoided if we increased physical activity and avoided weight gain through the premenopausal and postmenopausal years."
Obesity is determined by body mass index (BMI). BMI is a measure of body fat based on height and weight. A BMI of 25 to 29.9 is considered overweight, according to the Centers for Disease Control and Prevention (CDC). A BMI of 30 or higher is considered obese.
The female sex hormones estrogen and progesterone can influence the development of breast cancers. Obesity can affect the levels of these hormones in the blood.
Past research found that obesity may be a risk factor for breast cancer. According to Dr. Neuhouser and team, this study was designed to look at that link more closely.
Dr. Neuhouser and team used data from the Women’s Health Initiative (WHI) clinical trials on 67,142 postmenopausal (over the age of 50 but under the age of 80) women between 1993 and 1998.
Dr. Neuhouser and team looked at data on height, weight, mammograms and breast cancer diagnoses. These women were followed for an average of 13 years. During that period, there were 3,388 breast cancer diagnoses.
These researchers found that obese women had an increased risk of invasive breast cancer. Women who were overweight also had an increased risk, but it was not as significant.
Dr. Neuhouser and team found that women with BMIs over 35 had a 58 percent higher risk of invasive breast cancer — compared to women of normal weight.
When these obese women developed breast cancer, they were also more likely to have large tumors and tumors that spread. Dr. Neuhouser and team found that obese women were also more likely to die from the disease.
The women who initially had a normal BMI but gained more than 5 percent of their initial body weight also were found to have a raised risk of breast cancer.
According to Dr. Neuhouser and team, hormone replacement therapy for menopause did not affect the risk of breast cancer in these women.
“First, we need to refine our understanding of why overweight and obesity raise the risks of some cancer," wrote Clifford Hudis, MD, of the Memorial Sloan Kettering Cancer Center in New York, and Andrew Dannenberg, MD, of the Weill Cornell Medical College in New York, in a related editorial. "Overweight and obesity are a growing global challenge and the increased burden of malignant disease, to which it contributes, is another one."
The study and editorial were published in the June issue of the journal JAMA Oncology.
The National Heart, Lung and Blood Institute, the National Institutes of Health and the US Department of Health and Human Services funded this research.
Study author Dr. Rowan Chlebowski received fees from Novartis, Novo Nordisk, Pfizer, Amgen and Genentech. These companies make drugs used in breast cancer treatment.