“Prolonged TV viewing and time spent in other sedentary pursuits is associated with increased risks of certain types of cancer,” concluded a meta-analysis of data from 43 observational studies including more than 4 million people and 68,936 cancer cases. A positive association with overall sedentary behavior was found for colon, endometrial, and lung cancer.
“The primary finding from our meta-analysis is that prolonged TV viewing and time spent in other sedentary pursuits is associated with increased risks of colon and endometrial cancer. Each 2-hour per day increase in sedentary time was related to a statistically significant 8% increase in colon cancer risk and 10% increase in endometrial cancer risk,” Daniela Schmid, PhD, MSc, and Michael F. Leitzmann, MD, DrPH, Department of Epidemiology and Preventive Medicine, University of Regensburg, Germany, wrote in the Journal of the National Cancer Institute. “Sedentary behavior was unrelated to cancers of the breast, rectum, ovaries, prostate, stomach, esophagus, testes, renal cell, and non-Hodgkin lymphoma.”
The authors noted that sedentary behavior is emerging as an independent risk factor for chronic disease and mortality. Television viewing was singled out because it “is accompanied by increased consumption of unhealthy foods, such as sugar-sweetened beverages, sweets, and fast food and it is related to enhanced smoking initiation,” the investigators stated.
“We performed a comprehensive electronic literature search in Cochrane, EMBASE, Medline, and SciSearch databases through February 2014 for published articles investigating sedentary behavior in relation to cancer incidence. Because randomized controlled trials are difficult to perform on this topic, we focused on observational studies that met uniform inclusion criteria,” the researchers explained. Data in the studies had been obtained with self-administered questionnaires and interviews.
“Comparing the highest vs lowest levels of sedentary time, the relative risks (RRs) for colon cancer were 1.54 (95% confidence interval [CI] = 1.19 to 1.98) for TV viewing time, 1.24 (95% CI = 1.09 to 1.41) for occupational sitting time, and 1.24 (95% CI = 1.03 to 1.50) for total sitting time. For endometrial cancer, the relative risks were 1.66 (95% CI = 1.21 to 2.28) for TV viewing time and 1.32 (95% CI = 1.08 to 1.61) for total sitting time. A positive association with overall sedentary behavior was also noted for lung cancer (RR = 1.21; 95% CI = 1.03 to 1.43),” the authors wrote.
Among the biologic mechanisms that “may mediate the observed positive association between sedentary behavior and cancer,” the investigators listed decreased energy expenditure accompanied by weight gain and obesity, and vitamin D deficiency. The authors pointed out that while sedentary behavior was strongly associated with colon cancer and endometrial cancer, “tumors that are considered obesity-related,” sedentary behavior was not associated with breast cancer and renal cell cancer, “even though obesity is positively associated with those malignancies. This suggests that sedentary behavior and obesity mediate risk for certain cancers (eg, colorectal cancer and endometrial cancer) through shared mechanisms, whereas other cancers (eg, breast cancer and renal cell cancer) show distinct obesity-specific pathways.”
The authors called for stronger individual and public health efforts to reduce time spent on sedentary behavior.
“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. For other cancers that are related to obesity (breast, kidney), the association for sitting may operate through obesity-specific pathways,” noted an accompanying editorial by Lin Yang and Graham A. Colditz, MD, DrPH, of the Siteman Cancer Center and Department of Surgery, Washington University School of Medicine, St. Louis.
“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,” the editorialists continued. They mentioned several possible interventions including worksite modifications to reduce sedentary time and “replacing sedentary time in transport with active commuting.” ■
Schmid D, Leitzmann MF: J Natl Cancer Inst 106(7):dju098, 2014.
Yang L, Colditz GA: J Natl Cancer Inst 106(7):dju135, 2014.