Age- and weight group-specific weight gain patterns in children and adolescents during the 15 years before and during the COVID-19 pandemic.

Background/Objectives: There is concern that measures aiming to limit a further spread of COVID-19, e.g., school closures and social distancing, cause an aggravation of the childhood obesity epidemic. Therefore, we aimed to compare BMI trends during the 15 years before and during the COVID-19 pandemic.Subjects/Methods: To assess the change in weight dynamics during the first months of COVID-19, we compared the trends of BMI change (ΔBMI-SDS) and of the proportions of children gaining or losing weight between 2005 and 2019 and the respective changes from 2019 (pre-pandemic) to 2020 (after the onset of anti-pandemic measures) in more than 150 000 children.Results: During the COVID-19 pandemic, we found a substantial weight gain across all weight and age groups, reflected by an increase in mean BMI-SDS (β = 0.05, 95% CI 0.036 to 0.055; p < 0.001), an increase in the proportion of children gaining weight (OR = 1.37, 95% CI 1.28 to 1.46; p < 0.001), and a decrease in the proportion of children losing weight (OR = 0.72, 95% CI 0.67 to 0.77; p < 0.001). Besides, we found the same trends since 2005 on a low but stable level with a mean ΔBMI-SDS increase of β = 0.007 (95% CI 0.006 to 0.009; p < 0.001) every five years, the odds of gaining weight increased by OR = 1.05 (95% CI 1.03 to 1.06; p < 0.001) per five years, and the odds of losing weight decreased by OR = 0.95 (95% CI 0.94 to 0.96; p < 0.001) per five years. Alarmingly, both the long-term and the short-term effects were most pronounced in the obese subgroup.Conclusions: There are positive dynamics in different measures of weight change, indicating a positive trend in weight gain patterns, especially within the group of obese children. These dynamics are likely to be escalated by the COVID-19-related measures and may lead to a significant further aggravation of the childhood obesity pandemic.