OTTAWA — The expanding waistlines of Canadians are nothing new - obesity has been one of the country's biggest health problems for years now - but a new Statistics Canada survey indicates the population is actually fatter than previously thought.

The reason? It seems that when a survey simply asks a respondent for their height and weight, the results are not the same as when people are actually weighed and measured.

Detailed data released Tuesday as part of the Canadian Community Health Survey of more than 130,000 people indicates the national obesity rate in 2005 was 15.5 per cent. But that rate is based on people volunteering their own weight and height.

The survey also did a smaller sample of 4,000 people who were put on a scale and had their height measured by a Statistics Canada interviewer.

The results are not only interesting to health-care officials, but may be of interest to sociologists as well. Because men tended to over-estimate their height and women generally under-estimate their weight, Statistics Canada says an extrapolation of the people who were measured works out to a national obesity rate of 24.3 per cent.

Dr. Arya Sharma, scientific director of the newly established Canadian Obesity Network, said some people offering their own heights and weights "might be making true mistakes, not knowing."

"A lot of people don't weigh themselves regularly. They may just recall the last time they stepped on a scale, and it might have been a while ago. Some of it might just be wishful thinking. Some of it might be just embarrassment. There could be all kinds of reasons."

People lose height over time, and can gain weight without realizing it, he added.

"Measured is a better reflection of reality," said Vincent Dale, survey manager of the CCHS. But because this sample size was small, an obesity rate based on measured data wasn't available for the provinces or at the community health region level.

Easier-to-gather self-reported numbers continue to be the norm when most large surveys are conducted, but now experts can factor in what they know about the discrepancy between the two methods.

No matter how you measure it, obesity among Canadians is a problem that's only getting worse.

In terms of regional variation, Canadians get slimmer as you move from east to west. Among provinces, the highest obesity rates were in Newfoundland and Labrador, where 23.8 per cent of the population was obese (self-reported data). By comparison, the rate was 13.2 per cent in British Columbia.

Nunavut had the highest obesity rate in the country - at 25.9 per cent. And nearly four in 10 women were obese in the northern Saskatchewan health region of Mamawetan/Keewatin/Athabasca.

Again, these numbers are all self-reported, and Mark Tremblay, a professor of kinesiology at the University of Saskatchewan, notes that self-reporting systematically underpredicts the extent of the obesity problem.

He said that "anyone in the know" recognizes that self-reported data is clearly an underestimate, "the degree to which it underestimated we're not sure - well, we're becoming increasingly sure now."

The latest measured numbers appear to confirm the findings of a 2004 nutrition survey based on actual weighing and measuring where the obesity rate landed at 23.1 per cent.

Sharma, who holds a Canada Research Chair in cardiovascular obesity research at McMaster University in Hamilton, said it's great to have more "precise" data now.

"I think that level of obesity was always out there," he said. "It was just not being recorded."

Overweight and obesity rates are based on the Body Mass Index, or BMI, which is calculated using a person's height and weight to gauge total body fat.

A BMI of 25 or more in adults indicates someone is overweight. A BMI of 30 or more indicates obesity and a high to extremely high risk of developing health problems.

For example, an adult male who is five-foot-10 and weighs 210 pounds would have a BMI of 30 and would be considered obese. A woman who is five-foot-four and 175 pounds would also have a BMI of 30.

There are various issues surrounding the use of BMI because it doesn't tell you anything about body composition, noted Tremblay, who is also a senior scientific adviser on health measurement for Statistics Canada.

"There is the sort of Arnold Schwarzenegger argument, that if you measured all of the NFL halfbacks in the National Football League or the equivalent in some other sport, you'll find that the majority of them are obese by BMI indexes... they're heavy for their height because they're very muscular," he said.

"However, I think it's reasonable to assume that most Canadians have not become increasingly like NFL halfbacks over time, at least that seems like a reasonable assumption to me," he noted.

"The more viable explanation for the BMI creep that we're seeing in the country is that Canadians, and Americans and almost every other country in the world, are becoming increasingly fat."

Waist girth is a very good index to measure health risks. Even better is taking skin-fold measurements, and better than that is an MRI, Tremblay said.

Sharma said he doesn't think people have a good feel for the importance of obesity as a contributor to disease, and most who try losing weight do so for cosmetic reasons.

"People are a lot more scared of high blood pressure and cancer and a lot of other things, and not realizing that the excess body weight is killing them. It's killing them slowly but it's killing them steadily."

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Top five health regions in Canada with the highest percentage of adult obesity among its population, according to the Canadian Community Health Survey released Tuesday by Statistics Canada:

• Mamawetan-Keewatin-Athabasca (northern Saskatchewan): 31.6
• Chatham (New Brunswick): 27.0
• Prince Albert Parkland (Saskatchewan): 26.7
• Central Regional (Newfoundland and Labrador): 26.6
• Kelsey Trail (Saskatchewan): 26.4
• National average: 15.5

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Percentage of the adult population considered obese, as reported in the Canadian Community Health Survey released Tuesday by Statistics Canada:

• Canada: 15.5
• Newfoundland and Labrador: 23.8
• P.E.I: 22.5
• Nova Scotia: 20.7
• New Brunswick: 22.5
• Quebec: 14.2
• Ontario: 15.1
• Manitoba: 18.1
• Saskatchewan: 20.6
• Alberta: 15.8
• British Columbia: 13.2
• Yukon: 17.8
• N.W.T: 24.5
• Nunavut: 25.9

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Top five health regions in Canada with the lowest percentage of adult obesity among its population, according to the Canadian Community Health Survey released Tuesday by Statistics Canada:

• Vancouver: 8.0
• North Shore-Coast Garibaldi (B.C.): 8.5
• City of Toronto: 10.1
• Richmond (B.C.): 11.1
• Fraser North (B.C.): 11.5
• National average: 15.5