Overweight and obese women are at increased risk of developing breast cancer after menopause, compared to normal-weight women, a large new analysis from Cancer Prevention Program at the Fred Hutchinson Cancer Research Center in Seattle 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 body weight.
For the new study, 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.
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 29.9 is considered overweight and 30 and greater 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.
A research trial looking specifically at whether weight loss decreases breast cancer risk is needed to determine if it's helpful for women, the researchers said.
While more studies need to be conducted, the new results are "a caution that once you're overweight the damage may be done."
Some research suggests inflammation generated by fat tissue, and possibly other effects on hormones, could fuel cancer growth.
Source: JAMA Oncology, online June 11, 2015.
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