A study of more than 95,000 nurses provides what may be the strongest evidence yet that women who put on a lot of weight during adulthood raise their risk of breast cancer substantially.
Researchers said the reason is that body fat increases the amount of estrogen in the bloodstream. And estrogen is thought to promote breast tumors.The study was reported in The Journal of the American Medical Association.
Women who were not taking estrogen after menopause increased their risk of breast cancer with every pound they gained after age 18. Among these women, those who put on at least 45 pounds were twice as likely to develop spreading breast cancer as women whose weight changed less than five pounds.
Previous studies have linked adult weight gain to breast cancer. But these were much smaller studies that looked back at participants' health habits instead of following the women over several years.
Also, the previous studies did not explore the separate contributions of weight gain and the use of estrogen, which has been linked to breast cancer, both in the new study and in some other research.
"After menopause, blood estrogen is derived largely from body fat, so the estrogen level is increased with obesity and weight gain," said the lead author of the new study, Dr. Zhiping Huang of the Harvard School of Public Health. "So you expect that obesity and weight gain increase postmenopausal breast cancer."
The study tracked 95,256 nurses, ages 30 to 55, for 16 years ending in 1992. During that period, 2,517 cases of spreading breast cancer were found, 1,000 in premenopausal women and 1,517 in postmenopausal women.
Overall, 33 percent of the postmenopausal cancer resulted from weight gain, estrogen use or interaction of the two, Huang said. Weight gain alone probably accounted for 16 percent, and estrogen use alone for 5 percent, she said.
The 33 percent estimate "is both sobering and encouraging: sobering because this is a large percentage and encouraging because both of these exposures are modifiable in many women," two experts, Jennifer L. Kelsey of Stanford University School of Medicine and Dr. John Baron of Dartmouth Medical School, said in an accompanying editorial.
Huang said the findings do not mean that women should forgo estrogen, which other studies indicate can have other important benefits.