TORONTO - Canadians who are spending lots of their leisure time in front of screens -- especially TV screens -- are more likely to be obese, a new study suggests.

It's been known for awhile that rising screen time, as it's called, is contributing to childhood obesity. But this study, drawing from data in Statistic Canada's Canadian Community Health Survey, is one of the first looking to see if there is a link between screen time and obesity among Canadian adults.

And there appears to be.

Adults who watch more than 21 hours of TV a week were 80 per cent more likely to be obese than people who watched five hours or less television. Men who spent a lot of their leisure time in front of a computer screen were 20 per cent more likely to be obese and women were 30 per cent more likely than people who didn't send much time in front of a computer.

"From this, we would suggest that decreasing television viewing time in particular may be to the advantage of people at risk of overweight or obesity,'' said Mark Tremblay, one of the authors and the director of the Healthy Active Living and Obesity Research Institute at the Children's Hospital of Eastern Ontario in Ottawa.

The study, based on a survey of 42,600 men and women aged 20 to 64, found that in 2007, nearly three out of every 10 Canadian adults reported they watched an average of 15 or more hours of television a week and nearly two in 10 said they watched 21 or more hours a week.

Frequent leisure time computer use was less common, with about 15 per cent of adults reporting they averaged 15 or more hours a week at their computers. Only six per cent reported 21 hours or more a week of leisure computer screen time and nearly a third said they spent none of their leisure time in front of a computer screen.

The survey also asked about time spent reading, to see if sedentary behaviour in general raises the risk of obesity, or if there is something particular about TV and computer screen time that increases the likelihood a person will be obese.

Interestingly, rates of obesity were not higher among people who spent a lot of their leisure time reading than among infrequent or non-readers.

The researchers can't say why they saw the pattern they did. Because of the way the study was designed, it can't answer questions about cause and effect. It can only point out that there are associations between increased screen time and weight.

But obesity experts have a number of theories, including the overt prompting one gets to eat in commercials and the subliminal prompting of watching characters eating or drinking.

Another factor is that when people do eat in front of a screen, it is generally mindless repetitive eating, where a person goes through a bowl of chips without thinking or eats a handful of cookies instead of just one.

And people eating while watching TV generally don't scarf down steamed broccoli or celery sticks.

"Watching television is associated with snacking and often with foods that aren't of the best nutritional value,'' said Tremblay's co-author, Statistics Canada senior analyst Margot Shields.

"Chips and Coke and things that aren't of the best nutritional value.''

Another possibility is the low metabolic rate associated with TV viewing, the study said.

People watching TV expend only marginally more energy than people who are sleeping. Even playing a board game or typing on a computer keyboard raise the metabolic rate more, the authors noted.