Regular exercise or physical activity reduces risk of cancer, according to a new study led by James McClain of the National Cancer Institute and colleagues and reported at a meeting of the American Association for Cancer Research.

The study of 5,968 women in Maryland confirmed previous studies that have shown people who did physical exercise regularly were at lower risk of developing cancer.

The study also found among women who were in the upper half with regard to the amount of physical activity each week, those who slept less than seven hours per night were 47 percent more likely to develop cancer than those who slept longer.

Physical activity has also been associated with reduced risk of breast cancer risk in many previous studies.

One study led by Michael F Leitzmann and colleagues at the National Cancer Institute and published in the Oct. 2008 issue of Breast Cancer Research found that postmenopausal women with body mass index lower than 25 kg/m2 who engaged in vigorous physical activity were 23 percent less likely to develop breast cancer.     But no such association was found in overweight and obese women.

The researchers followed up 32,000 women who enrolled in the Breast Cancer Detection Demonstration Project Follow-up Study to examine if there was an association between risk of breast cancer and physical exercise.

Another study led by Freedman DM and colleagues from National Cancer Institute and published in Oct 21, 2008 issue of Cancer Causes and Control found physical activity like walking and hiking for 10 or more hours per week rendered the greatest protection against breast cancer in women, a 43 percent reduction in the risk.

Coyle YM from University of Texas Southwestern Medical Center says in a report published in the Dec 2008 issue of Cancer Causes and Control that physical activity is a protective factor for breast cancer.

The author says "Animal studies suggest that physical activity decreases breast tumor growth by promoting changes in cellular proliferation and apoptosis. Human studies provide some support for exercise producing favorable changes in estrogen metabolism that may lead to reduced breast epithelial cell proliferation."