Trends in obesity defined by body mass index among adults before and during the COVID-19 pandemic: a repeated cross-sectional study of the 2009–2023 Canadian Community Heath Surveys

BACKGROUND: The COVID-19 pandemic affected health behaviours and the social determinants of health. We sought to describe trends in the prevalence in body mass index (BMI) categories before and during the COVID-19 pandemic among adults in Canada. METHODS: We conducted a repeated cross-sectional study of adults in the 2009-2023 Canadian Community Health Surveys. We compared changes after the onset of the COVID-19 pandemic (April 2020 to December 2023) to an 11-year prepandemic period (January 2009 to March 2020). We calculated odds ratios (ORs) and absolute percentages from, respectively, weighted logistic and linear regression models. RESULTS: Our unweighted analytic sample included 746 250 adults from the 2009-2023 surveys. The prevalence of BMI-defined obesity increased from 24.95% in 2009 to 32.69% in 2023 (absolute increase 7.74%). The COVID-19 pandemic period was associated with an adjusted annual increase in the relative odds of obesity that was 1.02 (95% confidence interval [CI] 1.01-1.04) times greater than the prepandemic period. The absolute rate of increase of BMI-defined obesity nearly doubled during the pandemic, with an annual average excess rate of 0.44 (95% CI 0.14-0.74) percentage points. Class II and III obesity increased at a greater absolute rate than class I, indicating a shift toward more severe obesity. The relative increase in class III obesity was greater among young adults and females. INTERPRETATION: Since the COVID-19 pandemic, the prevalence of BMI-defined obesity, and especially class III obesity, increased at a faster rate than before the pandemic. Some groups that historically had lower levels of obesity were disproportionately affected during the pandemic.