Exercise can ward off many diseases and a recent long-term study has found that it may help decrease breast cancer risk, too. Breast cancer is one of the most common cancer types in women and is especially common in women who have gone through menopause. Researchers have known for some time that increased exercise is likely linked to lower breast cancer risk, but they didn’t have a good sense of how much exercise needed to be done and for how long the effects of exercise lasted.

To answer those questions, the authors studied almost 60,000 women who had gone through menopause over 12 years. They asked them for a variety of health information that included their amount of exercise. They then looked at the diseases these women got and tried to see whether their level of activity at different points over the 12 years was related to how often they got certain diseases.

They found that exercise did indeed reduce breast cancer risk and that duration and amount of exercise played an important role. To reduce risk, women needed to do 12 metabolic hours (MET-h) per week of exercise, which is a scientific measure that comes out to about four hours of walking or two hours or more intense exercise per week. Doing more than this amount didn’t decrease risk any further. This is in line with general recommendations of at least two and a half hours of exercise per week from the World Health Organization and Health and Human Services in the U.S.

Importantly, the exercise had to be relatively recent to reduce risk. When women who used to exercise enough stopped exercising, their risk of breast cancer increased to pre-exercise levels over five to seven years. In other words, after several years of no exercise, it was as if they’d never been active.

For women at 12 MET-h, how much exercise they were doing in the five years prior to the study made no difference. Neither did BMI, waist size or weight gain during that time. Regardless of these factors, women saw a decrease in the risk of breast cancer if they were exercising enough. That means that no matter what your body size or exercise history, starting an exercise program now can decrease your breast cancer risk. It also means that if you’re currently exercising, you should continue to do so to maintain the benefits.

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