Restrictions implemented by governments during the coronavirus disease 2019 (COVID-19) pandemic affected people’s eating habits and physical activity. We investigated the effect of COVID-19 lockdowns and restrictions on body mass index (BMI) and weight in a UK population, according to BMI class, sex, age and ethnicity.

This retrospective observational cohort study used the Clinical Practice Research Datalink AURUM database. Baseline spanned from 22 March 2017–22 March 2020, and the follow-up lockdown period was from 23 March 2020 (start of the lockdown in the UK) to 13 March 2021. The descriptive analysis included individuals with ≥ 1 valid BMI/weight measurements during both the baseline and follow-up periods, while the model-based analysis comprised individuals with ≥ 1 valid measurement(s) during baseline. Results were stratified by baseline BMI category, sex, age and ethnicity.

In the descriptive analysis (n = 273,529), most individuals did not change BMI category post-lockdown (66.4–83.3%). A greater proportion of women (12.6%) than men (9.5%) moved up BMI categories post-lockdown. Compared with older groups, a higher proportion of individuals < 45 years old increased post-lockdown BMI category. The model-based analysis (n = 938,150) revealed consistent trends, where changes in body weight and BMI trajectories pre- and post-lockdown were observed for women and for individuals < 45 years.

During COVID-19 restrictions, women and young individuals were more likely than other groups to increase BMI category and weight post-lockdown.