While going through menopause, many women face an unfortunate fact of life: They gain weight, in part because of a slowing metabolism. Excess body fat, especially belly fat, increases a postmenopausal woman’s risk of breast cancer. But exercise can help women—especially postmenopausal women—build muscle, reduce fat and lower their breast cancer risk, says Christine Friedenreich, an epidemiologist at Alberta Health Services and the University of Calgary in Alberta, Canada.

It’s unknown how much exercise is needed to reduce breast cancer risk. General physical activity guidelines call for 150 minutes of moderate-intensity aerobic exercise or 75 minutes of vigorous-intensity aerobic activity per week, along with two or more days of strength training. However, in a study published in the September 2015 issue of JAMA Oncology, Friedenreich and her colleagues found that previously inactive, postmenopausal women who had 300 minutes of moderate-to-vigorous activity per week lost more body fat than those who had 150 minutes of moderate-to-vigorous activity per week for one year. Participants did not change their diets.

Researchers divided 400 women who ranged from normal weight to obese into two groups. The women wore heart rate monitors and spent at least 50 percent of each exercise session at 65 percent to 70 percent of their maximum heart rate reserve, which is a person’s maximum heart rate minus her resting heart rate.

Fitting in Fitness

Here are tips for getting started with exercise.

Struggling to find the time or energy to exercise? Kathryn Schmitz, an epidemiologist at the University of Pennsylvania in Philadelphia who studies exercise, cancer risk and prevention, offers these tips:

Build up safely and slowly. Track the amount of time you spend exercising the first week. Then add 10 percent a week. If you exercise 60 minutes week one, exercise for 66 minutes week two, and build from there.

What gets measured gets managed. Keep a log or use an activity tracker. Seeing your progress might motivate you to keep going.

Plan for it. Schedule exercise as you do with other activities. Walk on your lunch hour or bike to work. Plan for three hours of activity on weekends, when you might have more free time.

Turn off the TV. “The question I always like to ask people when they say ‘I don’t have time’ is ‘OK, how long did you watch television yesterday?’ ” Schmitz says. Cutting out just 30 minutes of TV a day can create more time for fitness.​

Women in the 300-minute group lost an average of 2.2 percent of body fat compared with women in the 150-minute group, who lost 1.2 percent. Women who were assigned to exercise for 300 minutes a week also lost more belly fat—a breast cancer risk factor—and decreased their body mass index, waist circumference and waist-to-hip ratio compared with women who were assigned to exercise for 150 minutes. Obese women who exercised more lost more fat and weight compared with normal and overweight women, likely because they had more fat and weight to lose.

Exercising for 150 minutes a week is beneficial, says Friedenreich, but exercising for 300 minutes per week is even better. “The take-home message is for people to try to be physically active in their lives and to try to achieve as much as they can. And if they can achieve higher levels, then they are going to achieve higher benefits.”