Overweight and obese women are at increased risk of developing breast cancer after menopause, compared to normal-weight women, a large new analysis finds.
The cancer risk rises with greater weight, researchers found, and women with the most severe obesity were 86 percent more likely to develop the most common form of breast cancer, and to be diagnosed with more advanced cancers.
While other research has pointed to a link between excess weight and breast cancer risk, it's important to confirm that link, especially for something changeable like weight, "because that suggests women can do something about it," said Marian Neuhouser, of the Cancer Prevention Program at the Fred Hutchinson Cancer Research Center in Seattle, who led the study.
For the new study, published in JAMA Oncology, the researchers analyzed data from the large, long-term Women's Health Initiative study.
They looked at data on 67,142 post-menopausal women ages 50 to 79 years old from across the U.S., and followed them for an average of 13 years. Overall, there were 3,388 breast cancers detected by 2010.
The study team grouped women by their body mass index (BMI), a measure of weight in relation to height. A BMI of less than 25 is considered normal, BMI between 25 and 30 is considered overweight and over 30 is obese. A BMI of 35 - the equivalent of a five-foot six-inch person weighing 216 pounds - or above is considered severely obese.
About 5 percent of women in each weight group were diagnosed with an invasive breast cancer during the study period, but the risk of breast cancer increased with weight.
Women with BMIs of 35 and up were about 56 percent more likely to be diagnosed with any type of invasive breast cancer during the study, compared to normal-weight women.
When the researchers looked at specific breast cancer subtypes, they found the most-obese women were 86 percent more likely than normal-weight women to be diagnosed with breast tumors that are fueled by the hormones estrogen and progesterone.
These so-called estrogen receptor-positive and progesterone receptor-positive breast cancers are the most common forms of the disease. There was no link between body weight and breast cancers that are hormone receptor-negative.
Using hormone replacement therapy after menopause did not change the relationship between breast cancer and weight, the researchers found.
The analysis did reveal that normal-weight women who gained more than 5 percent of their starting weight over the study period had a 35 percent increased risk of breast cancer.
But for women who were already overweight or obese, losing weight did not lower their increased breast cancer risk.
"I think it’s important to note that this was not a weight loss trial," Neuhouser said.
A research trial looking specifically at whether weight loss decreases breast cancer risk is needed to determine if it's helpful for women, she said.
While more studies need to be conducted, Dr. Clifford Hudis told Reuters Health the new results are "a caution that once you're overweight the damage may be done."
Hudis, who co-wrote an editorial accompanying the new study, said research also needs to find out why increased weight may increase breast cancer risk so that solutions can be found.
His own group's research has focused on the potential role of inflammation generated by fat tissue, and possibly other effects on the endocrine system that could fuel cancer growth, he writes in the editorial.
"Unfortunately (overweight and obesity) is just a growing problem in Western countries," said Hudis, who is chief of the Breast Medicine Service at Memorial Sloan Kettering Cancer Center in New York. "It’s a public health challenge."