Watching TV and other sedentary activities are associated with an increased risk for certain types of cancer, according to a new study.

A new meta-analysis of 43 different observational studies found that sedentary activities, which include prolonged periods of sitting, were associated with a higher risk of colon, endometrial and lung cancer.

The analysis, published Monday in the Journal of the National Cancer Institute, looked at the results of 43 different studies that included more than 4 million people and 68,936 cases of cancer. The studies assessed how much time the participants spent doing sedentary activities, like watching TV or sitting down, either for work or recreationally.

The researchers, based out of the University of Regensburg in Germany, found that the highest levels of sedentary behaviour had a statistically significant higher risk for colon, endometrial and lung cancer compared to the lowest levels of sedentary behaviour.

The risk went up with each two-hour increase in sitting time:

  • Eight per cent for colon cancer
  • 10 per cent for endometrial cancer
  • Six per cent for lung cancer (borderline statistical significance)

Dr. Daniela Schmidt, co-author of the study, said adults in developed countries spend approximately 50 to 60 per cent of their day engaged in sedentary behaviour.

And while previous studies have reported that prolonged television viewing is associated with increased risks of diabetes and heart disease, the “relation between TV viewing and cancer had not been addressed using meta-analysis.”

“Our study will now add literature that sedentary behaviour is also bad for cancer,” Schmidt told CTV News.

Sedentary behaviour was also unrelated to other types of cancer including breast, rectum, ovarian, prostate, stomach and esophageal cancer, according to the study.

The authors said there may be several biological mechanisms that are behind the association between sedentary behaviour and cancer.

They say that sitting can increase body weight, inflammation and hormonal changes that may promote certain types of cancers.

The authors also hypothesize that TV viewing time showed the strongest relationship with colon and endometrial cancer, possibly because people tend to eat junk food and drink sweetened beverages while watching TV.

Meanwhile, the relationship between sedentary behaviour and cancer risk appeared to be unaffected by levels of physical activity, suggesting that long periods of time spent sitting can negatively impact your health no matter how much you exercise, the authors said.

In a separate editorial, Drs. Lin Yang and Graham Colditz argue that promoting an active lifestyle is an important part of cancer prevention. However, besides advocating general weight control, strategies to achieve this goal are poorly defined.

"Priority should be placed on refining interventions, independent of physical activity and obesity prevention, to reduce sedentary time and lower cancer risk and overall mortality," they write.

They add that the workplace may be one ideal place to intervene, given the number of hours most people spend sitting at work.

"Given that working adults can spend eight or more hours a day at work, the worksite is an ideal and key setting to reduce sedentary time through worksite policies or changes to the physical environment," they write.

That’s why Mark Tremblay, a health and fitness researcher at the Children’s Hospital of Eastern Ontario Research Institute, holds his staff meetings while walking.

“There’s something about sitting too much -- even if you have some physical activity in your lifestyle – there’s something about it that is bad for your health. And we now know it’s bad for our metabolism, for our heart and for prevention of different types of cancer.”

Tremblay adds that people should start introducing “little bits of inconvenience” in order to break up sedentary time.

“We can do that through walking meetings, through standing meetings -- different things that’ll just get us out of our seat.”

With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip