The vast majority of Canadians believe they eat a healthy diet and would like to do even better, according to a Leger Marketing/CBC poll.

And while Canadian adults only log about 2.2 hours of vigorous activity per week — about half what they should be doing — they do about 7.5 hours per week of light activity, close to the 8.5 hours that are recommended

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Statistics Canada figures show 4.2 million Canadians are obese.

Those findings appear to run contrary to Statistics Canada figures that show 4.2 million Canadians, or 17.2 per cent of the population, are obese. That's included in the 58.6 per cent of men, and 43.5 per cent of women who are overweight.

And while sedentary lifestyles and poor food choices do play a factor, one of Canada's leading obesity experts believes much of the problem stems from basic metabolism and the yoyo dieting that so many obese and overweight people experience.

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"I think one of the biggest misconceptions when we talk about obesity in general is that obese people are obese largely because of their lifestyles and because of the way that they live," Dr. Arya Sharma of the University of Alberta, told CBC News.

Sharma points to studies where people's eating and activity are carefully monitored. They show that some people can eat an additional 1,000 calories per day and not gain a gram, while others would gain five to six kilograms over a six-week period.

'There's a huge variability in how people can cope with extra calories.' —Dr. Arya Sharma, scientific director, Canadian Obesity Network

"There's a huge variability in how people can cope with extra calories," he said in an interview with CBC News.

He says people who tend to pack on the pounds simply have bodies that burn calories very efficiently and store the excess as fat.

"They just take their extra calories, they don't even burn them because they're very fuel efficient, they'll just store those calories and they'll put them away."

Yoyo dieting blamed

Sharma, who is the chair of obesity research and management at the University of Alberta and medical director of the Weight Wise program at Edmonton's Royal Alexandra Hospital, is no fan of most diets.

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Dr. Arya Sharma wants Canadians to take a more sympathetic view of people who are overweight and obese. ((CBC))

"The minute you go on a diet, the minute you start losing weight the first thing that happens is your body says, 'OK we're going to shut down the furnace, there's not enough fuel coming in, we're not going to be shifting to fourth gear anymore,' and it happens automatically," he said.

Once the body gets used to a diet it adjusts and weight loss stops, leading to frustration on the part of the dieter. Inevitably most people will go back to their old eating habits, but with the body's metabolism slowed down, they gain even more weight than before.

"And that's your yoyo effect," he said.

Exercise not effective for weight loss

Sharma is equally reticent on exercise as a means of weight loss.

"There's a million benefits of exercise. It does everything including improving depression, preventing cancer; it'll do a million things for you. The one thing it does not do is help you lose weight."

'So much for exercising to prevent weight gain.' —Dr. Arya Sharma

He points to a recent study, reported in the Journal of the American Medical Association, which followed 3,554 men and women over 20 years. Half the group exercised vigorously, the others led more sedentary lifestyles. Both groups gained weight, but the average difference was only 2.6 kilograms for men and 6.1 kilograms for women — after two decades.

"So much for exercising to prevent weight gain," said Sharma.

"Some people are just naturally lean. They can have crappy lifestyles and it doesn't seem to affect them."

Sharma believes the key to maintaining a healthy weight, is maintaining a healthy lifestyle. One of the most important points he drives home is the need for a healthy breakfast. He says eating properly throughout the day helps prevent snacking and binging.

"Why is eating when you're hungry a bad idea? First of all, you make stupid choices. The second one is, you eat too fast."

He says sitting down and taking time to eat a meal is the healthiest approach because it takes about 20 to 40 minutes for the body to realize it's full.

Sharma says new research into obesity should cause a shift in how the condition is viewed.

"We keep hammering home the stereotype of the fat, lazy slobs who are eating fast food all the time who are not moving, not exercising or not taking care of themselves, making poor choices, when there's very little science that actually backs this up."

The science around obesity does support the contention of so many people that in spite of a healthy diet and getting exercise, they continue to gain weight.

The online poll of 1,514 adults and 506 youth aged 12 to 17 was conducted by Leger Marketing from Nov. 10 to Nov. 17, 2010. The margin of error for the adult sample is plus or minus 2.5 percentage points, 19 times out of 20, and plus or minus 4.4 percentage points, 19 times out of 20 for youth.