CHICAGO — Women who lose weight by exercising and eating better may reduce their risk of breast cancer more than women who lose the same amount of weight through diet alone, according to a new study of postmenopausal women.
Both exercising and eating better are thought to reduce women's risk of breast cancer by decreasing body fat and levels of the sex hormones related to breast cancer, according to the researchers. But the researchers investigated whether there is any additional benefit to exercising, beyond the effect of weight loss in reducing cancer risk.
The results suggest exercising has a stronger effect on breast cancers fueled by hormones, compared with dieting, and also offers additional benefits such as preserving lean body mass, said study researcher Anne Maria May, of the University Medical Center Utrecht, in the Netherlands.
"Exercise is the preferred weight loss strategy to decrease breast cancer risk," May said. [7 Cancers You Can Ward Off with Exercise]
About 240 overweight women, ages 50 to 69, who didn't regularly exercise participated in the study, presented here this week at the annual meeting of the American Society of Clinical Oncology (ASCO). The women's goal was to lose 11 to 13 pounds (5 to 6 kilograms) over 16 weeks.
About one-third of the women dieted, whereas another third enrolled in an endurance and strength training program, working out for four hours weekly. They also followed a slightly healthier diet, with a small decrease in their calorie intake. The rest of the participants didn't change their habits, and served as controls for the study.
By the end of the study, women in both the exercising and dieting groups achieved their weight-loss goals. However, the exercising participants preserved their lean body mass (which includes muscles and bones), and reduced more of their body fat, compared with the dieting participants.
Moreover, blood tests showed the exercising participants reduced their levels of estrogen more than dieting participants did. (Many breast cancers need estrogen to grow.)
Compared with women in the control group, the exercising women showed decreases in all types of estrogen in the body, whereas women in the diet group showed a decrease in only one type of estrogen, according to the study.
The researchers also found the exercising group showed a benefit in levels of other breast cancer related hormones, such as testosterone.
It is likely that physical activity influences sex hormone levels mainly through reducing body fat, May said.
The findings demonstrate the importance of exercising for postmenopausal women, she said. Previous studies have shown that lack of physical activity is one of the risk factors for developing breast cancer.
Other than influencing the sex hormones, it is possible that exercising affects women's cancer risk by reducing inflammation in the body, or decreasing levels of the hormone insulin, studies have suggested.