For two decades, Danish children's BMI has remained relatively stable, but this new study has now examined BMI before, during, and after the COVID-19 pandemic. The study shows that the pandemic significantly altered children's BMI across all BMI categories.

The study, titled "Body Mass Index in Children, Before, During, and After COVID-19-Pandemic", is a cross-sectional study including data from 426,935 Danish schoolchildren in 1st and 6th grade, before, during, and after the COVID-19 pandemic. Data is sourced from the Child Database (Børnedatabasen), which contains data on all children in Danish public schools from mandatory health examinations. The study is newly published in the U.S. journal JAMA Network Open.

“The fluctuations in children’s BMI are very significant. In the study, we analyzed data going back to 2011, and we see that during COVID-19, all BMI categories change—but in different directions,” says Jens Meldgaard Bruun, the lead supervisor of the study.

He is a professor and senior physician at Steno Diabetes Center Aarhus, Aarhus University Hospital, and Aarhus University, and the medical director of the National Center for Obesity.

“The study shows that the COVID-19 lockdown affected all children. Changes in BMI can be observed across all categories, from underweight to severe obesity. That is quite unusual,” he adds.

All children were affected

The analysis of the children's data was carried out by Frederik Kirkemann Jensen, as part of his Master’s thesis in Public Health at Health, Aarhus University, where he examined health data from schools in the three periods before, during, and after the pandemic.

Jens Meldgaard Bruun got the idea to examine Danish registries based on data he saw from other countries, which showed a particular increase in the proportion of children with overweight and severe obesity during the COVID-19 lockdown. With data from the Child Database, he and Frederik Kirkemann Jensen were able to investigate how the COVID-19 lockdown affected Danish schoolchildren—even after reopening.

“The fact that weight changed so dramatically during COVID-19 is very interesting. Shifts are seen in all BMI categories, from underweight to severe obesity. The clear takeaway from this study is that when children are removed from their everyday routines and structures, it affects all children. From the data in this study, we unfortunately cannot determine the cause of these changes, but it does inspire further research,” says Jens Meldgaard Bruun.

He emphasizes that one of the study’s great strengths is access to such a large dataset through the Danish registers, making it possible to analyze data from different age groups.

“The analysis includes data from children in both early school years and middle school, which makes the results unique—even internationally. The study shows that the lockdown significantly affected both groups. But we also see that middle school children were affected differently compared to younger children, which adds to the study’s value,” he explains.

Unique data source

In Denmark, health examinations have been carried out on all schoolchildren for many years to ensure their well-being, and these include BMI measurements. Data from these examinations is stored in open registers, providing a uniquely rich dataset.

The study shows that from 2011 up to the COVID-19 lockdown, children’s BMI was relatively stable across all weight classes and age groups. Fortunately, most children in the categories of normal weight, underweight, and overweight returned to pre-COVID-19 levels after society reopened. However, two BMI categories deserve special attention.

During the lockdown, the proportion of children with severe obesity nearly doubled, especially among younger schoolchildren, and even one year after reopening, there were still 12% more children with severe obesity compared to before the lockdown.

For 6th-grade children, there were still more children with underweight after the pandemic than before.

Jens Meldgaard Bruun plans to conduct further studies, not only on BMI categories, but also by linking weight data to other important factors such as well-being.

“It would be interesting to re-analyze the data to see if some of the observed BMI shifts normalize over time or if they, for instance, can be linked to well-being or other psychosocial factors,” he says.

Key findings from the study

• All BMI categories changed during the COVID-19 pandemic, but most changes normalized after reopening.

• There were still more children in early school years with severe obesity one year after the COVID-19 lockdown.

• In middle school, there were more children with underweight one year after the COVID-19 lockdown.

About the study

The study includes data from 426,935 Danish schoolchildren in 1st and 6th grade, collected in three periods: before, during, and after the COVID-19 pandemic.

Data breakdown:

• 268,761 children in 1st grade

• 158,174 children in 6th grade

Time periods compared:

• Before COVID-19: March 11, 2019 – March 10, 2020

• During COVID-19: September 1, 2020 – June 30, 2021

• After COVID-19: August 1, 2022 – January 31, 2024

Data sources:

• Child Database (Børnedatabasen) – includes height and weight measurements from mandatory school health exams.

• Statistics Denmark – provided data on parental income and education.

BMI categories

BMI was age- and sex-adjusted (iso-BMI) and categorized according to WHO standards:

• Underweight: ≤18.5

• Normal weight: 18.5–25

• Overweight: 25–30

• Severe obesity: ≥30

Background on the research

• Study type: Cross-sectional study

• Collaborators: Steno Diabetes Center Aarhus and Aarhus University

• External funding: None

• Potential conflict of interest: Jens Meldgaard Bruun has lectured for Boehringer Ingelheim, Eli Lilly, and Novo Nordisk in the past three years.

• Read more about the study at: stenoaarhus.dk