Stepping on a scale doesn't give the whole picture about your risk of complications from excess belly fat, according to a Statistics Canada study released today.

The federal agency looked at the heart, stroke and other health risks of waist circumference within different categories of the body mass index (BMI) for Canadians aged 20 to 69.

"This study reveals that abdominal obesity within BMI categories is increasing," today's issue of Health Reports concluded. "As a result, Canadians with a given BMI are, on average, at higher risk of obesity-related health conditions than they were in 1981."

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If waist circumference climbs to more than 40 inches for most men or more than 35 inches for women, the risk of Type 2 diabetes and coronary heart disease rises, doctors say. (Howie McCormick/AP)

The percentage of overweight men at increased or high risk based on waist circumference rose from 49 per cent in 1981 to 62 per cent from 2007 to 2009, according to the report.

Among overweight women, the percentage at increased or high risk rose from 64 per cent to 93 per cent.

About 21 per cent of women who wouldn't be considered at normal weight based on BMI are at risk because of their abdominal obesity, said study author Margot Shields of Statistics Canada's health analysis division in Ottawa.

BMI is a person's weight in kilograms divided by the square of the height in metres. It doesn't differentiate between lean muscle and fat, or properly distinguish between different body frames.

Scale nudge versus pant belt tightening 

Visceral (intra-abdominal) fat surrounding the internal organs has been found to be more metabolically active, producing more changes in cholesterol, blood pressure and blood sugar, than fat in the legs and buttocks.

"People need to recognize that there are some treatments, particularly exercise, where the scale might not change or change very subtlely and yet the pants start to feel a lot looser, and that's a very good thing," said study co-author Ian Janssen, Canada Research Chair in physical activity and obesity at Queen's University in Kingston, Ont.

The report did not explain why Canadians' waistlines are expanding faster than their BMIs.

Janssen speculated it could be because people are now on certain types of medication, are less physically active or have different diets compared to 30 years ago.

Under Canadian guidelines released in 2007, a waistline of more than 94 centimetres (37 inches) for a man and 80 centimetres (32 inches) for a woman should be considered a risk factor for cardiovascular disease and Type 2 diabetes.

In 2007, measuring waist circumference became part of a preventive-care list for family doctors in Canada.

"There's guidelines in Canada that we should be using to classify a person's weight and their risk and using that to dictate treatment," Janssen said. "But the uptake is not great at the clinical level."

study of Ontario teenagers that was published last month concluded waist measurements seem important when assessing blood pressure measurements and lipids such as cholesterol in adolescents with high BMI.

With files from CBC's Amina Zafar