In a new review of past research, people who spent lots of time watching TV or otherwise sitting were more likely to be diagnosed with colon or endometrial cancer than those who were less sedentary.
"Cancer is a complex disease and has numerous possible causes, including genetic factors, environmental factors and lifestyle factors," said Dr. Daniela Schmid, from the department of epidemiology and preventive medicine at the University of Regensburg in Germany, who worked on the review. Too much sitting could be one of the factors involved, she said.
"Prolonged sedentary time has been linked to other chronic conditions like diabetes and heart disease as well as cancer," she said.
In total, the 43 studies analyzed by Schmid and her co-author followed close to 69,000 cancer patients. For every two-hour increase in sedentary time per day, on average, colon-cancer risk increased by 8 per cent and endometrial cancer risk increased by 10 per cent. Overall sitting time was related to lung-cancer risk as well.
Risk increased more with TV-viewing time than work-related or total sitting time. "TV viewing is often accompanied by unhealthy food consumption and smoking, which pose risks of cancer," Schmid said.
Colon- and endometrial-cancer risk seemed to be most strongly related to sedentary time, but overall sitting time was related to lung-cancer risk as well.
Sitting time was not related to other cancers, including breast, rectal, ovarian, prostate, esophageal and testicular cancers.
Too much sitting may lead to health problems through several mechanisms, Schmid said. Obesity may partly explain the connection, but there might also be disease-specific reasons that sitting would be linked to cancer, she said.
Observational studies like the ones used in this review can't prove that sitting causes cancer, and it is possible that people who sit a lot tend to do other things that increase their cancer risk, but studies account for that, said Dr. Graham A. Colditz, a chronic-disease epidemiologist at Washington University School of Medicine in St. Louis, Mo.
Colditz coauthored an editorial accompanying the review in the Journal of the National Cancer Institute.
Lifestyle factors can influence cancer risk almost as much as they influence diabetes and heart disease, Colditz said.
"We evolved over thousands of years with a good amount of activity in daily life – we have removed activity at work, in commuting, and largely at home – all in the relatively short time of 150 years or less," he said. "The physiologic effect of refined carbohydrates and so forth add to the adverse effect of not exerting much effort in activity throughout the day."
In older studies, people with the most sedentary jobs had the highest death rates from colon cancer, and postal workers, who walked quite a bit for their jobs, had the lowest, Colditz noted.
"Endometrial cancer is the cancer most strongly linked to obesity – and since sitting is directly related to weight gain, diabetes and so forth, this fits too," he said.
There is currently no guidance for how much sitting is too much, but keeping active and breaking up long periods of sitting would be good steps to take, he said.
"Given occupational settings, breaking up the day with some walking at lunch is more likely possible than reconfiguring the workplace," he said.
People at risk for colon or endometrial cancer are not the only ones who should sit up and take notice of these results, Schmid said.
"This recommendation is not limited to people at risk but also applies to the general population, who will benefit from being more active and sitting less," she said.