How might physical activity be linked to reduced risks of cancer?
Exercise has a number of biological effects on the body, some of which have been proposed to explain associations with specific cancers, including:
- Lowering the levels of hormones, such as insulin and estrogen, and of certain growth factors that have been associated with cancer development and progression23 [breast, colon]
- Helping to prevent obesity and decreasing the harmful effects of obesity, particularly the development of insulin resistance (failure of the body’s cells to respond to insulin)
- Reducing inflammation
- Improving immune system function
- Altering the metabolism of bile acids, resulting in decreased exposure of the gastrointestinal tract to these suspected carcinogens24,25 [colon]
- Reducing the amount of time it takes for food to travel through the digestive system, which decreases gastrointestinal tract exposure to possible carcinogens [colon]
Is being sedentary linked to health risks?
Yes. Sedentary behavior, such as prolonged periods of television viewing, sitting, and lying down, is a risk factor for developing chronic conditions, including cancer, cardiovascular disease, and type 2 diabetes, and for premature death.26,27 In some studies, the association of sedentary behavior with these outcomes is independent of physical activity—that is, the increased risks of disease associated with being sedentary are not lessened by physical activity. However, in one large meta-analysis28 an increase in cancer mortality was seen only in those sedentary people with the least physical activity.
How much physical activity is recommended for general health benefits?
The U.S. Department of Health and Human Services 2008 Physical Activity Guidelines for Americans recommends that, for substantial health benefits, adults engage in at least 150 minutes (2 hours and 30 minutes) of moderate-intensity aerobic physical activity, 75 minutes (1 hour and 15 minutes) of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity activity, every week. Aerobic physical activity should be performed in episodes of at least 10 minutes, preferably spread throughout the week. Examples of moderate-intensity and vigorous-intensity physical activities can be found on CDC’s Physical Activity website.
For children and adolescents, the guidelines recommend at least 60 minutes (1 hour) of physical activity daily. Most of the 60 or more minutes a day should be either moderate- or vigorous-intensity aerobic physical activity, and should include vigorous-intensity physical activity at least 3 days a week. As part of their 60 or more minutes of daily physical activity, children and adolescents should include muscle-strengthening physical activity on at least 3 days of the week and bone-strengthening physical activity on at least 3 days of the week.
Is physical activity beneficial for cancer survivors?
Research indicates that physical activity may have beneficial effects for several aspects of cancer survivorship—specifically, weight gain, quality of life, cancer recurrence or progression, and prognosis (likelihood of survival).29,30 Most of the evidence for the potential benefits of physical activity in cancer survivors comes from people diagnosed with breast, prostate, or colorectal cancer.26
Weight gain. Both reduced physical activity and the side effects of cancer treatment can contribute to weight gain after a cancer diagnosis. In a cohort study (a type of epidemiologic study), weight gain after breast cancer diagnosis was linked to worse survival.31In a 2012 meta-analysis of randomized controlled clinical trials examining physical activity in cancer survivors, physical activity was found to reduce both body mass index and body weight.32
Quality of life. A 2012 Cochrane Collaboration systematic review of controlled clinical trials of exercise interventions in cancer survivors indicated that physical activity may have beneficial effects on overall health-related quality of life and on specific quality-of-life issues, including body image/self-esteem, emotional well-being, sexuality, sleep disturbance, social functioning, anxiety, fatigue, and pain.33 In a 2012 meta-analysis of randomized controlled trials examining physical activity in cancer survivors, physical activity was found to reduce fatigue and depression and to improve physical functioning, social functioning, and mental health.32
Recurrence, progression, and survival. Being physically active after a cancer diagnosis is linked to better cancer-specific outcomes for several cancer types.34