Being active lowers risk of several cancers. How inactivity links to cancer risk is a growing area of research, with several studies linking it to increased risk of diabetes, obesity and cancer. Looking at all the research together, a recent study finds that being too sedentary – whether at work or home – increases the risk of three cancers: colon, endometrial, and lung.
Each two-hour per day increase in sitting time linked to a modest but increased risk of cancers of the colon and endometrium.
The study, published this month in the Journal of the National Cancer Institute, adds to the evidence of a growing body of research that inactivity, independent of activity, may play a role in cancer risk.
“Our study suggests that sedentary behavior is associated with detrimental risk for developing certain types of cancer,” says lead study author Daniela Schmid, PhD, of the University of Regensburg in Germany. “This implies that reducing TV viewing and sedentary time is important.”
The analysis investigated sedentary behavior and cancer incidence by looking at several indicators of sitting, including TV viewing time, recreational sitting time and occupational sitting. In total, the analysis included 43 studies with over 4 million people. Data in the studies was through questionnaires or interviews, in which participants reported how long they spent watching TV or sitting about. For sitting time at work, the authors included studies that estimated sitting time based on job title.
An analysis with fewer studies suggests the effect was independent of how active participants were, suggesting that sedentary behavior represents a potential cancer risk factor distinct from physical inactivity.
When the highest levels of sedentary behavior were compared to the lowest, the study found a higher risk for colon, endometrial, and lung. For every two hours of sitting, the risk increased 8 percent for colon cancer and 10 percent for endometrial cancer. Long periods of watching TV showed the strongest relationship with increased risk of colon and endometrial cancers. This might be due to the link between watching TV and poor eating habits, notes Schmid.
The Obesity Role
This correlation seen between high amounts of sitting and cancer risk may be mitigated by other factors, such as being overweight or obese. A sedentary lifestyle links to higher risk of being overweight. In this analysis, the authors did not adjust for obesity, a risk factor for both endometrial and colon cancers, along with six other cancer types.
Understanding the mechanisms by which sedentary behaviors may link to cancer risk is an area of research that has received little focus and therefore, remains largely unknown, says Brigid Lynch, a cancer epidemiologist now at Baker IDI Heart and Diabetes Institute whose research focuses on how physical activity and sedentary behaviour link with cancer risk. “ From the broader literature looking at sedentary behavior and mechanisms, it seems that adiposity, inflammation and insulin resistance are key pathways that are likely to be important.”
In the JNCI analysis, some studies did not adjust for BMI while others adjusted for BMI, as well as waist circumference, she notes. “There is evidence to support the hypothesis that there is a bidirectional association between sedentary behavior and obesity. This complex behavior-biology connection is something that we are still trying to understand. Most of my studies have found that obesity is certainly a mediating factor in the association between sedentary behavior and cancer risk, but that sedentary behaviour is associated with cancer risk over and above this obesity link,” said Lynch.
In Lynch’s latest study on prostate cancer and sedentary behaviors, although the study found no significant association between either television viewing time or total sitting and prostate cancer risk, there was some evidence of effect modification by BMI category, she says.
Making Time to Move
AICR research currently shows that being moderately active at least 30 minutes a day reduces the risk of endometrial, postmenopausal breast and colorectal cancers. And the research also found that sedentary living increases risk of overweight and obesity.
Back in 2011, when the field of inactivity and cancer risk was just emerging, AICR highlighted new findings at our Annual Research Conference. Scientists presented data on the protective link between high levels of physical activity and reduced risk of certain cancers, along with data suggesting inactivity may be an independent risk factor for metabolic risk factors linked with cancer risk, along with type 2 diabetes and cardiovascular disease.
We wrote about the research in this issue of CRU.
At that time and more so today, the evidence suggests that moving more throughout the day may make a difference for cancer risk, along with improving overall health. (See our Make Time for Break Time infographic.) That can be challenging in an environment of sitting, note experts.
An accompanying editorial to the JNCI study by Washington University School of Medicine researchers note that: “interventions targeting sedentary behaviors in adults are scarce, despite the association of sitting with mortality being independent of physical activity.” Where people work and transportation offer key targets for interventions to help people replace sedentary time with active moving and commuting, they write.
- D. Schmid, M. F. Leitzmann. Television Viewing and Time Spent Sedentary in Relation to Cancer Risk: A Meta-analysis. JNCI Journal of the National Cancer Institute, 2014; 106 (7).
- Lin Yang and Graham A. Colditz. An Active Lifestyle for Cancer Prevention. JNCI J Natl Cancer Inst (2014) 106 (7).
- Lynch BM et al. Sedentary behavior and prostate cancer risk in the NIH-AARP Diet and Health Study. Cancer Epidemiol Biomarkers Prev. 2014 May;23(5):882-9. doi: 10.1158/1055-9965.EPI-13-0808. Epub 2014 Feb 13.