What’s the role of exercise in preventing cancer?

Heavy weight and large size have been associated with increased risk of breast cancer. There is evidence to suggest that regular exercise is associated with a reduced incidence of breast cancer.

The relationship between physical activity and breast cancer incidence has been extensively studied, with over 60 studies published in North America, Europe, Asia, and Australia. Most studies indicate that physically active women have a lower risk of developing breast cancer than inactive women.

In the Women’s Health Initiative Observational Study published in 2001, it was reported that current exercise and exercise after menopause are both associated with a decreased risk of breast cancer. Even a small amount of exercise (1.25 to 2.5 hours per week of brisk walking) is beneficial. Women who had engaged in regular strenuous exercise at age 35 had a 14 per cent decreased risk of breast cancer compared with less active women (CMAJ 2004).

Evidence from population based studies suggests that occupational, leisure, and household activities are associated with about 30 per cent reduction in breast cancer rates – more you exercise, better the results (BMJ editorial 2000).

There is evidence to support potentially important protective effect of physical activity against colon cancer but not against rectal cancer. And there is no evidence that exercise increases the risk of any cancer. Colorectal cancer has been one of the most extensively studied cancers in relation to physical activity, with more than 50 studies examining this association (www.cancer.gov).

Many studies in the United States and around the world have consistently found that adults who increase their physical activity, either in intensity, duration, or frequency, can reduce their risk of developing colon cancer by 30 to 40 per cent relative to those who are sedentary regardless of body mass index (BMI), with the greatest risk reduction seen among those who are most active.

The risk of colorectal cancer begins to increase after age 40 and continues to increase as you get older. Obesity is linked to an increased risk of colorectal cancer. A lifestyle that does not include regular exercise may also be linked to an increased risk of colorectal cancer.

Research findings are less consistent about the effect of physical activity on prostate cancer, with at least 36 studies in North America, Europe, and Asia. Overall, the epidemiologic research does not indicate that there is an inverse relationship between physical activity and prostate cancer.

About 20 studies have examined the role of physical activity on endometrial cancer risk. These studies suggest that women who are physically active have a 20 per cent to 40 per cent reduced risk of endometrial cancer, with the greatest reduction in risk among those with the highest levels of physical activity. Risk does not appear to vary by age.

At least 21 studies have examined the impact of physical activity on the risk of lung cancer. Overall, these studies suggest that the most physically active individuals experience about a 20 per cent reduction in risk.

In conclusion, there is convincing evidence that physical activity is associated with a reduced risk of cancers of the colon and breast. Several studies also have reported links between physical activity and a reduced risk of cancers of the prostate, lung, and lining of the uterus (endometrial cancer).

Despite these health benefits, recent studies have shown that more than 50 per cent of the population do not engage in enough regular physical activity.

National Cancer Institute (NCI) funded studies are exploring the ways in which physical activity may improve the prognosis and quality of life of cancer patients and survivors. For more information about current research in this area, please visit NCI’s Cancer Survivorship Research Web site at http://cancercontrol.cancer.gov/ocs on the Internet.

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