To examine the impact of the COVID-19 pandemic on body mass index (BMI) and obesity status among Canadian residents and explore how this association varied by sociodemographic and health status.

Prospective cohort study.

Canada.

41 302 adults, aged 45–85 at baseline, participating in the Canadian Longitudinal Study on Aging.

BMI and BMI-defined obesity were measured at baseline, follow-up 1 and follow-up 2 (FUP2), with 33% of FUP2 data (n=13 444) gathered after 16 March 2020, when COVID-19 restrictions began. Correction factors were applied for self-reported BMI and weighted generalised estimating equations assessed BMI changes before and during the pandemic.

We found a significant interaction between follow-up time and timing of FUP2 data collection (before or during the pandemic). Participants measured during the pandemic had an excess BMI increase of 0.21 kg/m² (95% CI 0.15 to 0.28) and 1.06 times higher odds of obesity (95% CI 1.03 to 1.09) compared with prepandemic trends. Increases were more pronounced among females, middle-aged adults and those without diabetes.

The COVID-19 pandemic was associated with a modest increase in BMI and obesity among Canadian adults. Ongoing research is needed to assess long-term trends.