A study reveals that an hour of vigorous exercise each day reduces breast cancer risk by 25 percent and walking for an hour each day reduces risk of breast cancer by 14 percent.

Women who engaged in at least an hour of vigorous physical activity every day had a 25 percent lower breast cancer risk, and those who walked for at least seven hours a week had a 14 percent lower breast cancer risk.

The researchers examined whether recreational physical activity, specifically walking, was associated with lower breast cancer risk. Given that more than 60 percent of women reported having walked daily, promoting walking as a healthy leisure-time activity could be an effective strategy for increasing physical activity among postmenopausal women. They found that without any other recreational activity, just walking an average of one hour per day was associated with lower risk of breast cancer in these women.

After making adjustments to the data, the researchers determined that the observed benefits of physical activity and walking were not influenced by body type (BMI and weight gain) or hormonal status (postmenopausal hormone use and estrogen receptor status).

Current guidelines recommend that adults should strive to get at least 2.5 hours per week of moderate-intensity activity, or 75 minutes per week of vigorous-intensity aerobic activity for overall health. Higher levels of activity may provide greater benefit for breast cancer prevention.

All participants provided information on the average number of hours they spent on various physical activities including walking, jogging, swimming, playing tennis, bicycling, and performing aerobic exercises every week, and the number of hours spent in leisure time sitting, including watching television and reading. The researchers calculated the total hours of metabolic equivalent (MET) per week for each participant, which is a ratio of the energy spent during a specific activity to the resting metabolic rate.

Among the study participants, 4,760 of them subsequently developed breast cancer.

The researchers found that about 9.2 per cent of the participants did not partake in any physical activity, and about 47 per cent of them reported walking as their only activity. The median MET expenditure among active women was 9.5 MET hours per week, which translates to 3.5 hours of moderately-paced walking.

They found that the most active women with 42 MET hours per week or more (at least one hour of vigorous activity every day) had a 25 per cent lower risk for breast cancer compared with women who were least active, with less than seven MET hours per week (e.g., moderately-paced walking for two hours a week). Among women who reported walking as their only activity, those who walked for seven hours or more per week had a 14 per cent lower risk for breast cancer, compared with those who walked for three hours or less. They did not find any risk for breast cancer associated with time spent sitting.