If you’re postmenopausal, extra pounds boosts your risk of breast cancer. And it doesn’t have to be many pounds.
“It’s not just obesity,” notes Regina Ziegler, senior investigator at the National Cancer Institute. “Postmenopausal women who are overweight have a higher risk than those who are normal weight.”
(Premenopausal women who are heavy have a lower risk of breast cancer. But excess weight raises their risk of type 2 diabetes and other health problems.)
How does extra pounds promote postmenopausal breast cancer? After menopause, a woman’s ovaries stop producing estrogen. At that point, most of her estrogen is made by fat cells. So the more fat cells she has, the higher her blood levels of estrogen—which travels through the blood as estradiol.
“Women who are obese have about three times the circulating levels of estradiol compared to lean women,” says Walter Willet, chair of the nutrition department at the Harvard School of Public Health. “That’s a huge difference. And we see some increase in risk even in women who gain 5 to 10 pounds. It’s not just women who gain 50 or 60 pounds.”
Roughly 80 percent of breast tumors are hormone-receptor positive—that is, they are fueled by estrogen and/or progesterone. (Those tumors are usually easier to treat—with drugs that block estrogen production—than tumors that are not fueled by those hormones.)
But estrogen alone may not explain why heavier women have a higher breast cancer risk. They’re also more likely to have higher insulin levels, which may also fuel tumors.
When researchers looked at women who were not taking estrogen after menopause, those with the highest insulin levels had 2 ½ times the risk of breast cancer of those with the lowest insulin levels, after taking estradiol levels and weight into account.
“Extra weight works only partially by raising estradiol,” explains Ziegler.
What’s more, extra pounds may increase the risk of a recurrence in women who have had breast cancer, according to the American Cancer Society and a workshop at the Institute of Medicine (IOM), which is part of the National Academy of Sciences.
But regardless of how extra padding works, the message is the same.
“The main point for postmenopausal breast cancer is to weigh as close as you can to what you weighed at age 18,” says Willett.
“To anybody who’s afraid of estrogen-like chemicals in the environment, I would say, we have seen the enemy and it is us,” he adds. “We are the big source.”
Sources: BMJ 335: 1134, 2007; J. Natl. Cancer Inst. 106: dju0557, 2014; J.Natl Cancer Inst. 101: 483, 2009; CA Cancer J. Clin. 2014. Doi: 10.3322/caac.21124; iom.edu/Reports/2012/The-Role-of-Obesity-in-Cancer-Survival-and-Recurrence.aspx.