Can your level of physical activity lead to cancer?

The world is experiencing two pandemics: one overt and one insidious. The overt pandemic, of course, is COVID-19, where fear of infection has made public-health proposals—such as social distancing and shelter-in-place—become realities. The insidious pandemic is the growing trend of sedentary behavior, according to the authors of an article in Progress in Cardiovascular Diseases. But, these pandemics are not entirely separate. The alarming concern is that the COVID-19 pandemic will exacerbate worldwide deficiencies in physical activity, which may have dangerous health effects.

“The world will recover from the COVID-19 pandemic and so-called normal activities will resume. However, the [physical inactivity/sedentary behavior] pandemic will continue and, more troublingly, we may be at risk for this pandemic to worsen as a result of COVID-19. As a global society, we simply cannot let this happen,” wrote the authors of the aforementioned article.

Obvious repercussions of sedentary behavior include obesity and cardiovascular disease, but more insidious threats also exist. Although research on the association of sedentary behavior and increased cancer risk is still emerging, there is some evidence to suggest a correlation. 

Sedentary lifestyle and cancer risk

Sedentary behavior refers to activities of low-energy expenditure and involves prolonged sitting or lying (ie, devoid of whole-body movement). Examples include sitting at a computer or sitting to watch television for long periods of time.

In a systematic review published in Cancer, Epidemiology, Biomarkers & Prevention, researchers investigated the relationship between cancer outcomes and sedentary behavior. They evaluated 18 studies, 10 of which showed a significant association between cancer outcomes and a sedentary lifestyle. 

Per the authors, a number of high-powered, population-based observational studies (adjusted for confounding variables) have shown a correlation between sedentary behavior and the development of colorectal, endometrial, ovarian, and prostate cancers.

As for underlying mechanisms, the authors suggested the following: “Of the possible pathways that may mediate an association between sedentary behavior and cancer, the most consistent evidence has accumulated for adiposity. Sedentary behavior and adiposity are consistently associated in cross-sectional studies; results from prospective studies, however, suggest that the relationship may be bidirectional.”

“Modest evidence has also accumulated linking sedentary behavior with biomarkers of metabolic function, with stronger associations again emerging from cross-sectional studies. Although biological plausibility exists, there is insufficient epidemiologic evidence to draw any conclusions about the associations of sedentary behavior with sex hormones, inflammation, and vitamin D,” they added.

In a separate meta-analysis assessing 43 prospective studies and published in JNCI: Journal of the National Cancer Institute, authors mined quantitative evidence on the correlation between cancer risk and sedentary behaviors. 

They found that with higher television viewing times, there was a higher risk of colon cancer, with a relative risk (RR) of highest vs lowest levels of sedentary of 1.54. With respect to both occupational sitting time and total sitting time, the RRs were each 1.24. Furthermore, with regards to endometrial cancer, the RR from more television viewing time was 1.66, and that for total sitting time was 1.32. Lastly, with lung cancer, the RR was 1.21. Of note, no association was observed between sedentary lifestyle and cancers of the esophagus, ovaries, prostate, rectum, stomach, and testes, as well as non-Hodgkin lymphoma and renal cell cancers. Associations with colon cancer were particularly strong, with support from high-quality research. 

Implications

In an editorial accompanying the meta-analysis, the authors reflected on the implications of the study: “Given the strength of the data, the dose-response relation, and the lack of heterogeneity among studies, these data support a causal relation between sedentary behavior and both colon and endometrial cancers.”

They also presented specific calls to action with rationale. “Cancer prevention requires a sufficient evidence base, political will to fund programs to address the prevention potential, and a social strategy or plan by which we apply our knowledge to initiate or improve programs,” they wrote.

They added: “Currently, only a few guidelines, American Cancer Society Nutrition and Physical Activity and the UK Department of Health, specifically address the need to reduce sedentary behavior, but they offer no quantitative recommendations, nor do they outline strategies to achieve population-level improvements in reduced sitting time. Therefore, research to advance context-specific objective sedentary behavior assessment is needed to inform quantitative recommendations on daily sedentary time for cancer prevention and wellness.”

The authors proposed potential settings for interventions, including the worksite via policy changes and adjustments to the work environment. Changes to office layout to curb sedentary behavior could involve collaboration from multiple stakeholders, including public health experts, architects, built environment researchers, occupational researchers, and behavioral-science researchers. Future studies may want to address interventions for people working from home, though, which may become a new reality for a majority of people for the foreseeable future.

Another potential area of intervention is transportation. By replacing sedentary time in transport with active recruiting, as well as modifying the environment with additional infrastructure and promoting an active-transport culture, a shift in sedentary behavior could occur.

Notably, the authors stressed that any initiatives to reduce the risk of cancer death from a sedentary lifestyle be focused on cancer specifically and not obesity or prevention.

Physical activity and cancer risk

In contrast to sedentary behaviors, physical activity can decrease the risk of some cancer types. Physical activity refers to skeletal muscle movement that expends more energy than resting. Examples include exercising, biking, running, dancing, and household chores.

According to the National Cancer Institute (NCI): “Evidence linking higher physical activity to lower cancer risk comes mainly from observational studies, in which individuals report on their physical activity and are followed for years for diagnoses of cancer. Although observational studies cannot prove a causal relationship, when studies in different populations have similar results and when a possible mechanism for a causal relationship exists, this provides evidence of a causal connection.”

The NCI highlighted several high-quality studies in which physical activity was shown to decrease the risks for the following cancers:

• 12% to 21% lower risk of breast cancer 

• 19% lower risk of colon cancer

• 20% lower risk of endometrial cancer

• 21% lower risk of esophageal cancer

• 19% lower risk of stomach cancer

Of note, these risk reductions represent comparisons between the most physically active and least physically active individuals. The NCI also cited a 13% reduction in bladder cancer and a 23% lower risk of kidney cancers based on a pooled analysis of over 1 million individuals who participated in leisure-time physical activity.

The NCI also notes that physical activity provides the following protective measures that have been shown to decrease the risk of cancer development: obesity prevention; lower inflammation; enhanced immunity; altered metabolism, decreasing gastrointestinal exposure to carcinogens (ie, colon); decreased gastrointestinal transit time, decreasing exposure to carcinogens (ie, colon); lower levels of sex hormones such as estrogen and growth factors linked to cancer development (ie, breast, colon); and the prevention of increased insulin levels linked to cancer development (ie, breast, colon).