Cancer is increasingly recognised as a public health threat, especially in developing countries which are going through the effects of rapid urbanisation. The most effective means of tackling this emerging menace is through efforts in preventing cancers. An important prevention tool is promoting healthy life style – such as healthy diet and habits (avoiding tobacco use). Can we add exercise to this list?
Several studies have noted a beneficial effect for exercise in reducing cancer risk. Recently, the largest study to have investigated the association of leisure-time physical activity on the risk of 26 types of cancer was published in the Journal of American Medical Association Internal Medicine.
The study pooled data from 12 US and European cohorts comprising a total of 1.44 million adults. Within each cohort of population, individuals with the highest level of self-reported physical activity (90th percentile) were compared to the lowest level of activity (10th percentile). Some 186,932 cancers were identified in these 1.44 million individuals. The investigators concluded that a high level of leisure-time physical activity lowered the risk for 13 cancers including esophageal, liver, lung, stomach, uterine, leukaemia, myeloma, kidney, breast, colon, head and neck, rectal and bladder cancers. The protective effect of exercise was seen despite adjusting for body size (body mass index) or tobacco exposure. Cumulatively, higher levels of exercise resulted in a 7% lower risk for cancer.
The conclusions drawn from this study are quite convincing, especially due to the fact that these cohorts were drawn in a prospective manner. Unlike several prior studies on the association of exercise and cancer, there was no bias by recall. The large sample size provided adequate power to the study to assess for several cancer types individually, rather than as a group. At the moment, it is unclear as to what kind of physical activity will be most effective at cancer risk reduction. Based on information from several cohorts, it appears that at least 150 minutes per week (30 minutes a day for five days a week) of moderate exertion such as walking is needed to obtain benefit.
Despite such convincing studies, scientists have always been perplexed at how exercise mediates in reducing risk of cancer. The answer may well lie in a person’s immune system. An elegant study in mice demonstrated that exercise induced a higher level of immune milieu, particularly increased level of natural killer cells and T lymphocytes. Such an altered immune system in exercising mice resulted in tumour growth suppression, thereby suggesting the role of immune cells in fighting cancer.
In fact, the journal Science selected cancer immunotherapy as the breakthrough of the year in 2013. Already, several immunotherapeutic drugs such as Ipilimumab, Nivolumab and Pembrolizumab are used in fighting cancers such as melanoma, lung, gastrointestinal and liver cancer. These drugs work by blocking the immune checkpoint induced by tumour cells on the immune system of the individual. Through this blockade, the body is able to unleash its own immune cells in fighting aggressive tumour cells. The remarkable nature of these drugs is the much lower degree of side effects compared to traditional chemotherapeutic agents. Recently, former US president Jimmy Carter announced that such an immunotherapy has made his skin cancer disappear from the liver and the brain, and that he is no longer on treatment. A couple years ago, such a declaration would have been found only in the pages of science fiction.
What is the take home message from this study? In addition to tobacco control, we can add one more effective tool to our armamentarium aimed at cancer prevention: exercise.
Aju Mathew is an assistant professor of medicine at the University of Kentucky Markey Cancer Center, Lexington.