Canadian adults who spend hours a day watching television or sitting in front of a computer are more likely to be obese, according to a new Statistics Canada study.

Sedentary Behaviour and Obesity, released Wednesday, is based on data from the 2007 Canadian Community Health Survey of more than 42,600 Canadians aged 20 to 64, interviewed throughout 2007.

It found that television viewing or computer use was strongly associated with obesity. A third sedentary activity — reading — was not associated with obesity.

The odds of obesity among men and women who reported watching television 21 or more hours a week were almost twice those for study subjects watching an average of five or fewer hours a week.

The link between obesity and "screen time" persisted even when demographic factors such as age, education, household income and urban-rural residence were considered, the authors found.

Physical activity and diet were linked to the sedentary habits, but Margot Shields, author of the study along with Mark Tremblay, said that even when these factors were controlled, the relationship between "screen time" and obesity remained.

This suggests "screen time" should be considered a distinct factor when developing interventions to reduce obesity, she said.

"Physical activity and diets haven't gone down and have actually improved somewhat, but obesity is still prevalent," said Shields, who works with Statistics Canada's health information and research division. "So we have to start looking at other factors."

The authors suggest it may be time to revisit what "physically active" means.

The metabolic energy demand when watching television is much lower, for example, than vacuuming, putting away groceries or even playing card and board games, they said.

"This underscores the importance of accurately measuring physical activity in all domains of life [including both structured and unstructured activities during leisure and non-leisure time] to understand the potential mediating role of other activities in the association between television viewing and obesity," they wrote.

Statistics Canada has done studies looking at the screen time habits of young Canadians and the connection with obesity, but this is the first to look at adults, said Shields.

3 in 10 adults frequent TV viewers: study

A separate but linked study, by the same authors, of "screen-time" habits found over a third of Canadian adults are spending hours of their daily leisure time sitting in front of either a computer or television screen.

Almost three in 10 Canadian adults (29 per cent) were frequent television viewers: that is, they watched more than 15 hours of television a week, or more than two hours a day. Nineteen per cent of those surveyed, or one in five, reported watching 21 or more hours a week.

About 15 per cent of adults were considered frequent leisure-time computer users, logging more than 11 hours a week on a computer.

Only five per cent of respondents were both frequent computer users and frequent television viewers. About 61 per cent Canadians were not frequent leisure time users of either a television or a computer.

The study said that among younger-age groups, computer use is replacing television viewing as the screen time activity of choice. Close to half (45 per cent) of the screen hours reported by 20- to 24-year-olds were spent on a computer.

In general, younger ages and higher levels education were associated with increased computer use and decreased television use. Recent immigrants were also more likely to be frequent computer users and less likely to be frequent television viewers.