Staying highly active may protect against breast cancer whether it's walking, running, or anything in between, researchers found.

Women who got around 2 hours of exercise a day most days of the week were about 30% less likely to develop breast cancer whether pre- or post-menopausal in a population-based study by Lauren E. McCullough, MSPH, of the University of North Carolina in Chapel Hill, and colleagues.

High-intensity exercise didn't appear any better than low-intensity activity, the group reported online in Cancer.

"Given that three-quarters of the U.S. population participates in some physical activity, it is conceivably one of the most important lifestyle risk factors associated with the incidence of breast cancer," they wrote.

The study included 1,504 women with cancer and 1,555 without it in the population-based Long Island Breast Cancer Study Project.

Simply getting some physical activity outside of work didn't appear to have much impact on breast cancer risk, with an odds ratio of 0.94 (95% CI 0.79 to 1.12).

Reproductive status did seem to matter. Exercise during the reproductive years -- between the birth of a first child until menopause -- or after menopause had the biggest influence.

Even then, less than average activity levels (9 or 10 hours) were not significantly protective in either group.

Only the third quartile in both showed statistically significant benefits in terms of reduced breast cancer risk.

Women who got 10 to 19 hours of exercise a week during their reproductive years showed an odds ratio of 0.67 for breast cancer compared with inactive women (95% CI 0.48 to 0.94).

Women in their childbearing years who got more than 19 hours of activity a week showed a reduced odds ratio of 0.86 as well but with a wide, nonsignificant CI.

With regard to activity levels after menopause, women in the quartile that got 9 to 17 hours a week of physical activity had an odds ratio of 0.70 for breast cancer (95% CI 0.52 to 0.95). Those who got more than 17 hours again tended to have reduced risk but not significantly so (OR 0.84, 95% CI 0.63 to 1.13).

The lack of a linear dose-response "could be interpreted as weak evidence of an association" but a J- or U-shaped curve is also possible, McCullough's group noted.

Sustained activity can generate cellular and DNA damage and depress immune function, they pointed out.

Results were similar for in situ and invasive cancer and across hormone receptor status.

Every weight category showed lower breast cancer risk with more activity versus none.

Substantial weight gain after menopause appeared to eliminate the benefits of exercise for breast cancer risk, though still apparently better than a big gain without staying active (OR 1.02 versus 1.28).

"Collectively, these results suggest that women can still reduce their breast cancer risk later in life by maintaining their weight and engaging in moderate amounts of activity," the researchers wrote.

The group noted that exercise's benefits likely come from cutting down on insulin resistance and inflammation by keeping energy balance and obesity under control.

They cautioned that the study cohort was richer and better educated than typical in the U.S. and included few women who didn't have children, which may have an impact on generalizability.