Physical Activity and Cancer
What is physical activity?
How is physical activity related to health?
Researchers have established that regular physical activity can improve health by:
- Helping to control weight.
- Maintaining healthy bones, muscles, and joints.
- Reducing the risk of developing high blood pressure and diabetes.
- Promoting psychological well-being.
- Reducing the risk of death from heart disease.
- Reducing the risk of premature death (1).
In addition to these health benefits, researchers are learning that physical activity can also affect the risk of cancer. 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 percent of Americans do not engage in enough regular physical activity (2).
How much physical activity do adults need?
The Centers for Disease Control and Prevention (CDC) recommend that adults “engage in moderate-intensity physical activity for at least 30 minutes on five or more days of the week,” or “engage in vigorous-intensity physical activity for at least 20 minutes on three or more days of the week” (1). Examples of moderate-intensity and vigorous-intensity physical activities can be found on the CDC Physical Activity Web site at http://www.cdc.gov/nccdphp/dnpa/physical/pdf/PA_Intensity_table_2_1.pdf on the Internet.
What is the relationship between physical activity and colon cancer risk?
Colorectal cancer has been one of the most extensively studied cancers in relation to physical activity, with more than 50 studies examining this association. 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 percent relative to those who are sedentary regardless of body mass index (BMI), with the greatest risk reduction seen among those who are most active (3–7). The magnitude of the protective effect appears greatest with high-intensity activity, although the optimal levels and duration of exercise are still difficult to determine due to differences between studies, making comparisons difficult. It is estimated that 30 to 60 minutes of moderate to vigorous physical activity per day is needed to protect against colon cancer (6, 7). It is not yet clear at this time whether physical activity has a protective effect for rectal cancer, adenomas, or polyp recurrence (3).
Physical activity most likely influences the development of colon cancer in multiple ways. Physical activity may protect against colon cancer and tumor development through its role in energy balance, hormone metabolism, insulin regulation, and by decreasing the time the colon is exposed to potential carcinogens. Physical activity has also been found to alter a number of inflammatory and immune factors, some of which may influence colon cancer risk.
What is the relationship between physical activity and breast cancer risk?
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; however, the amount of risk reduction achieved through physical activity varies widely (between 20 to 80 percent) (6, 7). Although most evidence suggests that physical activity reduces breast cancer risk in both premenopausal and postmenopausal women (6), high levels of moderate and vigorous physical activity during adolescence may be especially protective. Although a lifetime of regular, vigorous activity is thought to be of greatest benefit, women who increase their physical activity after menopause may also experience a reduced risk compared with inactive women. A number of studies also suggest that the effect of physical activity may be different across levels of BMI, with the greatest benefit seen in women in the normal weight range (generally a BMI under 25 kg/m-squared) in some studies. Existing evidence shows a decreasing risk of breast cancer as the frequency and duration of physical activity increase. Most studies suggest that 30 to 60 minutes per day of moderate- to high-intensity physical activity is associated with a reduction in breast cancer risk (4, 6).
Researchers have proposed several biological mechanisms to explain the relationship between physical activity and breast cancer development. Physical activity may prevent tumor development by lowering hormone levels, particularly in premenopausal women; lowering levels of insulin and insulin-like growth factor I (IGF-I), improving the immune response; and assisting with weight maintenance to avoid a high body mass and excess body fat (7).
What is the relationship between physical activity and risk of endometrial cancer?
About 20 studies have examined the role of physical activity on endometrial cancer risk. The results suggest an inverse relationship between physical activity and endometrial cancer incidence. These studies suggest that women who are physically active have a 20 percent to 40 percent reduced risk of endometrial cancer (6), with the greatest reduction in risk among those with the highest levels of physical activity. Risk does not appear to vary by age (4).
Changes in body mass and changes in the levels and metabolism of sex hormones, such as estrogen, are the major biological mechanisms thought to explain the association between physical activity and endometrial cancer. However, fewer than half of the studies in this area have also adjusted for the potential effect of postmenopausal hormone use, which may increase the risk of endometrial cancer. A few studies have examined whether the effect of physical activity varies according to the weight of the woman, but the results have been inconsistent.
What is the relationship between physical activity and lung cancer risk?
At least 21 studies have examined the impact of physical activity on the risk of lung cancer. Overall, these studies suggest an inverse association between physical activity and lung cancer risk, with the most physically active individuals experiencing about a 20 percent reduction in risk (4, 6). An analysis of many existing studies found evidence that higher levels of physical activity protect against lung cancer, but was unable to fully control for the effects of smoking or respiratory disease in estimating the magnitude of the potential benefit (6, 8). The relationship between physical activity and lung cancer risk is less clear for women than it is for men.
What is the relationship between physical activity and risk of prostate 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 (4, 7). Although it is possible that men who are physically active experience a reduction in risk of prostate cancer, the potential biological mechanisms that may explain this association are unknown, but may be related to changes in hormones, energy balance, insulin-like growth factors, immunity, and antioxidant defense mechanisms (7). One recent study suggested that regular vigorous activity could slow the progression of prostate cancer in men age 65 or older (9).
How might physical activity affect cancer survivorship?
Research indicates that physical activity after a diagnosis of breast cancer may be beneficial in improving quality of life, reducing fatigue (7), and assisting with energy balance. Both reduced physical activity and the side effects of treatment have been linked to weight gain after a breast cancer diagnosis. One study found that women who exercised moderately (the equivalent of walking 3 to 5 hours per week at an average pace) after a diagnosis of breast cancer had improved survival rates compared with more sedentary women. The benefit was particularly pronounced in women with hormone responsive tumors (10). Another study found that a home-based physical activity program had a beneficial effect on the fitness and psychological well-being of previously sedentary women who had completed treatment for early-stage through stage II breast cancer (11). Increasing physical activity may influence insulin and leptin levels and influence breast cancer prognosis. Although there are several promising studies, it is too early to draw any strong conclusions regarding physical activity and breast cancer survival.
Two additional studies have suggested a protective association of physical activity after colon cancer diagnosis and survival. Researchers examined the relationship between levels of physical activity both before and after a diagnosis of colon cancer in two different observational studies. Whereas levels of pre-diagnosis physical activity were not related to survival, participants with higher levels of physical activity post-diagnosis were less likely to have a cancer recurrence and had increased survival (12). Although these studies suggest protective effects of physical activity, more research is needed to understand what levels of physical activity provide these benefits.
Is the National Cancer Institute (NCI) exploring the role of physical activity in the prognosis and quality of life of cancer patients?
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.
Is NCI studying the role of physical activity in cancer risk?
A number of NCI-funded studies are answering questions about the relationship between physical activity and the risk of developing cancer. NCI has established the Transdisciplinary Research on Energetics and Cancer (TREC) initiative, which links four research centers investigating how energy balance and physical activity modify the risk of cancer and influence the process of carcinogenesis. The TREC initiative also incorporates a broad range of scientists, ranging from experts in basic biological science to those with expertise in community behavioral interventions to increase physical activity. This combination of scientists and expertise will allow exploration of the role of physical activity across the full spectrum of cancer prevention. More information about TREC can be found at http://cancercontrol.cancer.gov/trec/ on the Internet.
Do any of these studies focus on special populations who are at increased risk of cancer?
NCI funds a number of research projects and interventions aimed at helping vulnerable populations reduce their risk of cancer by becoming more active, changing their nutritional behavior, and/or maintaining an optimal weight. Populations included in these projects include multiethnic working poor populations, African American women, rural communities, overweight or obese individuals, and cancer survivors. Several NCI-funded studies have started examining the factors related to long-term behavior change and increases in physical activity.
NCI is supporting national and regional surveys, as well as research methodology development, to gain more accurate information about physical activity across all age groups and diverse populations, as defined by race, ethnicity, income, and other factors known to influence levels of physical activity. This information will help identify groups who may benefit from programs to increase physical activity.