Cancers are classified as a family of related diseases that result from uncontrolled growth and spread of abnormal cells (Dishman, Washburn & Heath, 2004) that usually become a tumor. The most common causes of cancer related deaths in the United Kingdom are seen in Figure 1 below.
The evidence linking low levels of physical activity and an increased potential for development of cancer is growing. More studies are focused on determining if physical activity can be used as a preventative measure in the incidence of cancer.
Physical Activity and Breast Cancer
Thune et al (1997) found in their study of over 25,000 participants that an increased amount of physical activity during leisure time and work is associated with a decreased risk of breast cancer. The study evaluated the influence of physical activity, both at work and leisure time, in a large sample (n =25, 624, age 20 to 54) of pre-menopausal and post-menopausal women. Further support to the notion of increased physical activity and lower incidence of breast cancer is supported in studies of former college athletes (Frisch et al., 1985) and general population (Friedenreich et.al.,1995). One hypothesis for the lower incidence of breast cancer in former athletes relates to the later age of menarche usually found in this population (Frisch et al., 1985). Both early menarche and late menopause would be expected because they (the athletic population) would generally carry less body fat.
However, there are a number of studies that have found no evidence to suggest physical activity can help to reduce the incidence of breast cancer in a population. One such study looked at a large sample (n = 10118) of female teachers in Finland (Pukkala, et.al.,1993) and compared the incidence of cancers in a group of physically active teachers (PE teachers) with a less active group (language teachers). Another study showed that the risk for male and female breast cancer became elevated after socio-economic status was taken into consideration (Dosemeci et al., 1993). A suggestion may be that lower socio-economic groups may not have sufficient physical activity and consume a “less healthy” diet (i.e., with higher fat intake), thus potentially increasing the risk of cancer. This article on the BBC website provides an interesting discussion on possible lifestyle links to breast cancer: http://news.bbc.co.uk/1/hi/health/1633831.stm.
A review by Van Gaal and Mertens, cited in Kopelman and Stock (1998), shows conflicting evidence in relation to breast cancer and fat distribution. Some studies have reported a correlation between breast cancer risk and fat distribution (Van Gaal et al., 1998). Other studies have not come to the same conclusions.
The connection between physical activity and body fat distribution is that physical activity can help control body weight.
Physical Activity and Colon Cancer
Much of the research conducted on cancer and physical activity combine the data for colon and rectal cancers. Friendreich and Orenstein (2002) state that the evidence for a link between physical inactivity and the appearance of colon cancer is strong and consistent. Giovanucci et al (1995) found that physical activity was inversely related with colon cancer, after adjustments for age and other factors.
Waist to hip ratio and waist circumference were also indicative of risk factors for colon cancer. Therefore, it could be hypothesized that increased abdominal adiposity is associated with an elevated risk. This can be viewed as a significant finding as the study examined a large group of participants (n = 51529). However, because the sample group consisted of health professionals, it is could be argued that this is not representative of the general population. Dosemeci et al. (1993) suggested that the lower the socio-economic status, the higher the incidence of colon cancer. Additionally cancers of the colon and prostate showed an increased risk with males participating in lower levels of activity. Slattery et al. (1997) lend support to the notion that low levels of physical activity and high energy intake (therefore, a potential increase in body mass) are associated with colon cancer. The same study found that a high level of energy intake that was balanced by a high level of physical revealed a non-significant risk of colon cancer.
Another study (Paffenbarger et al. 1987) highlighted varying populations, (i.e., dockworkers, college alumni etc.) and reviewed them for the incidence of cancer in relation to the levels of physical activity involved during both leisure time and occupational time. The conclusions suggested that the more active men have an increased likelihood to die from pancreatic and colorectal cancers. No differences by activity for the incident of prostate cancer were noted. However, Friedenreich (2001) suggests that the evidence for correlation between physical activity and colon/colorectal cancer is convincing. A significant number of studies have clearly demonstrated a large reduction in risk of cancer in the physically active. (Thun et al., 1992; Paffenbarger et al., 1987; Lee et al.,1994)
Batty and Thune (2000) suggest plausible mechanisms of protection of physical activity on insulin amongst others, which positively influence cells in the colon. Additionally, they suggest that physical activity improves the transit time of the bowel, thus reducing the contact time with carcinogens in faecal matter. The authors report the appearance of an increased risk of colon cancer associated with sedentary or light physical activity. This supports the statement that increased physical activity appears to aid in the reduction of risk for colon and rectal cancers.
Other Cancer Sites
An examination of data from the National Health and Nutrition Examination Survey (NHAMES 1) by Albanes et al. (1989) showed a nearly two-fold risk increase in the inactive group for all cancer sites. The study used 5,138 men and 7,407 females, age 25 to 74 years of age. The most common sites of cancer identified in the follow up period of the study for males were lung, prostate and colorectal. The common sites in females were identified as breast and colorectal.
Exercise has an important role in cancer prevention by helping to maintain a healthy body weight especially when combined with a healthy diet. It may also have a more direct effect on cancer risk. Exercise is thought to influence the levels of hormones such as estrogen and insulin in the body and may have a particular effect in hormone-related cancers.
There is also evidence that physical activity significantly reduces the risk of bowel cancer and may help lower the risk of breast, prostate, lung and endometrial cancer. Cancer Research UK suggests moderate exercise five days per week for around thirty minutes.
The protective mechanism suggested by Thune and Furberg (2001) is that physical activity improves ventilation and perfusion. Therefore, concentration of carcinogenic agents in the airways will be reduced as will the duration of agent/airway interaction. Albanes et al. (1989) supports a protective effect from leisure time activity and is supported by Lee et al. (1999) and Severson et al. (1989). Due to the relatively small number of supportive studies on lung cancer and physical activity, the current results must be treated with caution, particularly when considering the participants of the studies. The majority of participants in this area have been male, so perhaps more females need to be recruited to establish a male/female risk factor. There is little mention of the ethnicity of the participants.
Practical Considerations
Exercise prescription as part of the disease management for cancer patients must be highly individualized because of the extreme variability of the effects of cancer and treatment regimens on functional capacity. Furthermore, other concurrent or prior health problems should be anticipated and taken into account in developing any exercise prescription. Although it is acknowledged that the recommendations for exercise should be modified for specific patients, it is generally accepted to recommend 30 minutes of continuous exercise such as brisk walking or swimming three to four times per week. Attention should be given to safety measures and general health maintenance. These guidelines are the general exercise recommendations outlined by American College of Sports Medicine (2006) and more recently updated (Haskell, et al., 2007). Furthermore, Cancer UK published a good healthy living leaflet for general health (http://info.cancerresearchuk.org/healthyliving/tentoptips/).
Walking and cycling are recommended as safe and generally well tolerated exercise modes involving large muscle groups. De-conditioned patients should begin with daily sessions of shorter duration and lower intensity. In general, moderate intensity exercise (50-75% HR reserve, RPE 11-14) sessions of between 20 and 30 minutes duration are recommended, with modifications as needed, including an interval approach to cardiovascular exercise, consisting of short exercise bouts (three to five minutes) followed by rest periods.
Table 2 outlines the practical considerations of exercise programming for cancer patients.
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Table 2. Summary of Practical Considerations for cancer patients
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ACSM (2003) Exercise Management for Persons with Chronic Diseases and Disabilities (Second Edition)
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Conclusion
While there is a link between physical activity and certain cancers, there still seems to be a gap in the knowledge of why physical activity seems to have a preventative effect on certain cancers. Some studies offer suggestions, but there is no clear reason of the “why” increased physical activity may help and “how” it may help.
There is not sufficient conclusive evidence to say that an increased physical activity level has a preventative measure against the development of all site cancers. However, as discussed above, there is strong evidence for the decreased risk of certain cancers.
Practically speaking it is difficult to make general recommendations regarding frequency, intensity, and duration. No one individual or demographic will be the same. The key is to remember this and use gradual progressive exercise programming.
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