Carrying extra fat is bad for us, we're told: Among other things, it raises the risk of certain cancers, including ones of the breast. Thus, it was confusing to read last week of a study reporting a reduced risk of breast cancer in overweight and obese women.
What's going on? Does obesity raise the risk of breast cancer or lower it? Could this be a health benefit to being fat?
The study by epidemiologist Dr. Karin Michels of Harvard University and colleagues was not entirely a surprise. In fact, it adds to a long line of confusing studies: Depending on a woman's age, being overweight either protects her from breast cancer or puts her at heightened risk.
A high body mass index (BMI) before puberty or after menopause increases the risk of this cancer. But a high BMI during childbearing years appears to do the opposite.
The newest thing about Michels' study, published in the Archives of Internal Medicine, is that the earlier the researchers looked in a woman's life, the stronger the relationship they found between fat and breast cancer protection. "Even being overweight in adolescence protected for breast cancer," Michels says. Researchers say the results square with the current understanding of how breast cancer develops. The female hormone estrogen is key to the process. Estrogen encourages breast cells to grow, and the more they grow, the more likely one is to become cancerous.
Many studies show that women who have a lower lifetime exposure to estrogen -- ovulating fewer times during their life because of multiple pregnancies, for example -- have a lower risk of breast cancer.
So how does being overweight play into the picture -- and how does it help premenopausal women yet harm them when they're postmenopausal? Fatty tissue, like ovaries, also produces estrogen, but at very low levels. In ovulating women, this estrogen actually feeds back on the ovaries and dampens their cyclical release of estrogen. Net result: Their bodies are exposed to less estrogen overall. "It's like taking a little birth control pill," says Dr. Anne McTiernan, director of the Prevention Center at the Fred Hutchinson Cancer Research Center in Seattle. "Obese and overweight women are not ovulating normally." (They don't generally notice this abnormal ovulation until they try to become pregnant, Michels adds.)
It's different for postmenopausal women because they have far lower levels of estrogen in their bodies and their monthly cycling has ceased -- and thus, any extra estrogen from fat is contributing to their total load, not subtracting.
The protection conferred by fat in premenopausal women may extend beyond estrogen. Overweight and obese women are also not producing another hormone, progesterone, in the same way as women of regular weight, McTiernan notes. Some researchers think progesterone exposure is even more of a risk factor for breast cancer than estrogen. For example, in a large clinical trial known as the Women's Health Initiative, women on therapy that included estrogen and progesterone had a higher risk of developing breast cancer, but the women taking only estrogen did not.
The lowered risk of getting breast cancer associated with obesity does not mean women of childbearing age should put on pounds to keep their breasts healthy, experts stress. For one thing, "obese premenopausal women will become obese post-menopausal women," says breast cancer researcher Dr. Amtul Carmichael of the Russells Hall Hospital in Dudley, England. For another, there are other health problems associated with obesity -- such as an increased risk for heart disease and diabetes.
And obese women who do get breast cancer are worse off than their lean counterparts. "Obese women are two to three times more likely to die from breast cancer, and that's common to both pre- and postmenopausal women," Carmichael says.
It isn't clear why. Perhaps the cancers are more aggressive in such women: Fat encourages cancer through high insulin levels and inflammation.
Alternatively, the treatments might be less effective: Chemotherapy doses are based on weight, yet doctors might give obese women slightly lower doses to avoid toxicity, Carmichael says.