Canadians are miscalculating their weight and height, resulting in faulty national statistics.
Two different studies looked at the effects of data collected through Statistics Canada's Canadian Community Health Survey (CCHS) and National Population Health Survey (NPHS). The studies relied heavily on self-reported weight and height information by respondents in order to estimate body mass index (BMI). The information was then used by StatsCan determine national obesity rates.
"People tend to over-report their height and under-estimate their weight and that results in huge problems," Ian Janssen, Queen's University obesity expert told Canada AM.
As people get older, the reporting errors tend to increase as well.
StatsCan discovered that men who responded to the 2005 CCHS survey tended to over-estimate their height by roughly one centimetre and women by about half that much.
Weight miscalculations were more pronounced. Women tended to under-report their weight by about 2.5 kilograms on average, and men by 1.8 kilograms. The study found the heavier the person, the more they under-reported.
In total, the study found 33 per cent of overweight individuals and 48 per cent of obese individuals (class I) were placed too low on the BMI index. Consequently, prevalence of obesity in Canada and its rate of change over time are unknown.
Furthermore, the relationship between obesity and disease such as diabetes, high blood pressure and heart disease may be distorted.
For example, among those classified as obese based on self-reported data in the 2005 CCHS, 360,000 people aged 40 years or older had diabetes. But among those classified as obese based on actual measured values, 30,000 people -- nearly 50 per cent more -- had diabetes.
But Janssen told CTV he is most concerned by trends he sees in children. Obesity rates among this age group have tripled in the past 25 years.
"Statistics suggest the burden of obesity will get worse in the next 20 or 30 years," said Janssen, making accurate surveillance of the problem more crucial than ever.
BMI is calculated by a person's weight in kilograms divided by their height in square metres. The clinical definition "overweight" is a BMI reading of about 25. A BMI above 30 is considered "obese."
Statistics Canada has been using self-reported data since the 1990s and is currently working on methods of addressing the bias.
According to previous CCHS' 5.5 million adults were obese in 2004 and 8.6 million were overweight.
