Adverse health effects differ with various levels of obesity, but limited national data existed previously for the Canadian population.  The authors examined the associations of sociodemographic and behavioural factors with obesity levels in Canada, and measured the impact of each level on major chronic diseases.

Data were extracted from the 2003 Canadian Community Health Survey.  The authors grouped overweight/obese participants aged 18 years and over into four levels based on body mass index (BMI, kg/m2): overweight (25.0-29.9), class I obesity (30.0-34.9), class II obesity (35.0-39.9), and class III obesity (extreme/clinical obesity, BMI ≥ 40.0).  They used logistic regression models to identify potential risk factors for the obesity levels and to estimate adjusted odds ratios (ORs) for major chronic diseases related to each level.

The investigators calculated population attributable risks (PARs) to help understand the impact of obesity levels on these chronic diseases.  The overall prevalence of obesity was 16.2% in men and 14.6% in women, and the prevalence of obesity III was 1.0% in men and 1.4% in women.  All levels of obesity increased with age, but then decreased in elderly participants.  The prevalence of diabetes, hypertension, heart disease, arthritis, and asthma increased with increasing BMI level, and the highest values appeared in participants at the obesity III level.  PAR was highest in the obesity III group for hypertension, followed by diabetes, and lowest for heart disease.  When correlated with risk factors, fewer statistically significant ORs, compared to the normal weight category, appeared for obesity II and III levels than for overweight and obesity I.  ORs for the combination of low education level, infrequent exercise and low household income rose significantly with BMI levels until the obesity II level, and in obesity III level, the OR remained at the same level as for obesity II, most significantly in women.

These results suggest that the impact of obesity on Canadians' health should be studied and dealt with by obesity level.  The greatest impact of clinical obesity was on hypertension and diabetes control in Canada.