Women who are obese are at higher risk for breast cancer. But even just a little… (Ben Edwards )
For postmenopausal women who are obese, breast cancer is more likely. That's because fat tissue seems to behave essentially as an organ of the endocrine system, pumping out the hormone estrogen. And estrogen is a driver of many common breast cancers. But losing as little as 5% of one's body weight — 10 pounds for a 200-pound woman — drives down levels of estrogen and other hormones that raise breast cancer risk, a new study finds.
In combination with weight loss, exercise drove down hormone levels even more, an effect that is likely to reduce a woman's risk of developing breast cancer. But by itself, exercise did not have a significant impact on circulating hormones.
The more weight a woman lost, the greater the reduction in hormones linked to breast cancer risk.
While many studies have linked obesity to breast cancer, this is the first to show that losing weight will drive down that risk. Obesity is just one of many risk factors for breast cancer that a woman can do something about. Others include breastfeeding one's babies, reducing alcohol consumption and steering clear of hormone-replacement therapy. Women at high risk of breast cancer are sometimes urged to consider taking one of several medications, including two first approved to prevent osteoporosis, to reduce their risk.
"This shows it's never too late to make lifestyle changes to reduce your risk for breast cancer," said co-author Dr. Anne McTiernan from Fred Hutchison Cancer Center.
While more significant weight loss drove down estrogen and other helper hormones more deeply, even a weight loss far short of what would be needed to exit the obese category helps a lot. The study's authors estimate that a 5% loss of body weight would lower a woman's breast cancer risk by 22%.
The study took 439 postmenopausal Seattle-area women who were sedentary and overweight or obese, and divided them into four groups. One group got an intensive weight-loss program focused on reducing calories and fat and choosing fruits, vegetables and high fiber; another group got an intensive exercise program only — five 45-minute aerobic exercise classes a week. A third group got a program that had exercise (mostly walking) and diet help. Members of the fourth group were asked not to change their diet or exercise habits but were offered four weight-loss classes and eight weeks of physical exercise training at the end of the study.
After a year, the average weight loss among the dieters was 10.8% of body weight, and among those who added exercise to their regimen, 11.9% (while the control group's average weight inched up almost imperceptibly, the exercise-only group added 3.3% of body weight.)
The big changes were in levels of circulating hormones such as estradiol, estrone, testosterone and sex-hormone binding globulin, which takes estrogens out of circulation and is therefore protective against breast cancer. With diet, average levels of those hormones fell by 10% to 20%, and adding exercise generally subtracted as many as five percentage points from subjects' hormone levels. The hike in SHBG — the breast-cancer protective agent — was 22.4% in dieters and 25.8% in those who did both diet and exercise.
The diet-and-exercise group also saw a decline in fasting insulin levels and measures of inflammation. The exercise-only group saw much smaller changes in all these measures — on the order of 5%.
The authors use hormone levels as a direct measure of increased or decreased breast cancer risk: they did not follow women to see who went on to develop breast cancer and who didn't. Epidemiological studies have found that women who exercise regularly are less likely to get breast cancer. Acknowledging that, the authors suggested the exercise may help lower breast cancer risk by some means other than lowering estrogen levels.