The study, which followed nearly 80 000 Japanese adults for up to a decade, found that regularly active men and women had lower risks of developing any type of cancer. When the researchers looked at specific types of cancer, they found that exercise was linked to lower risks of colon, liver, pancreatic and stomach cancers.
They also found that the protective effect was strongest among normal-weight men and women - supporting the theory that physical activity helps lower cancer risk at least partly through better weight control.
Dr Manami Inoue and colleagues at Japan's National Cancer Centre, in Tokyo, report the findings in the American Journal of Epidemiology.
Cancer risk drops with rise in activity
The researchers followed cancer incidence rates among 79 771 men and women who were between the ages of 45 and 74 at the outset. Between 1995 and 1999, study participants were surveyed about their physical activity levels, diet and other lifestyle habits; the researchers then followed them through 2004, documenting more than 4 300 new cancer diagnoses.
Overall, according to the researchers, the risk of developing any cancer dipped slightly as participants' activity levels climbed. On average, the most-active men were 13% less likely than the least active men to develop cancer; the most-active women had a 16% lower cancer risk than their sedentary counterparts.
The link held true when the researchers accounted for a range of other factors, including participants' age, weight, smoking habits, daily calorie intake.
Physical activity was defined not only as leisure-time exercise, but also the amount of time participants typically spent walking, doing physical labour and housework.
All activity beneficial
"Our results suggest that increased daily total physical activity - not only exercise - may be beneficial in preventing the development of cancer among Japanese men and women," Inoue told Reuters Health.
The researcher also pointed out that Japan's population is a relatively lean one and that the relationship between physical activity and lower cancer risk was weaker among overweight study participants.
It's thought that exercise may help prevent cancer, in part, by controlling body fat. But physical activity also has other effects that could theoretically stave off cancer, Inoue and colleagues point out.
Exercise can, for example, stimulate immune system activity, one of the body's natural defences against cancer. It may also alter levels of certain hormones, including sex hormones and insulin-like growth factors, which can feed the growth and spread of tumours. - (Amy Norton/Reuters Health)
SOURCE: American Journal of Epidemiology, August 15, 2008.