Unique Swedish data on young children's growth help to fill the gap identified by the World Health Organization

A well-known anecdote shows a man looking for his lost keys under a lamppost at night. A passer-by stops to help and, after searching in vain, asks him ‘Are you sure this is where you lost the keys?’ The man replies ‘No. I lost them over there. But it is pitch dark there’. In research terms, only searching in the circle of light created by a streetlight would be the equivalent of selection bias and raise questions about how representative the findings would be. Instead, we love data that shed light on every corner. High coverage and large sample sizes make us trust information. We all dislike messing around in the dark. Valid data can be used to change things. In this issue of Acta Paediatrica, Miregård et al. present cross-sectional data on weight and height for 100 001 four-year-old children who were measured by Swedish Child Health Services in 18 of the 21 regions in 2020. The study covers 85% of all the children who were born in Sweden in 2016. Because 2020 was the first year of the COVID-19 pandemic, the authors chose to compare that data with 104 455 children who were 4 years of age in 2018, before the pandemic.1 At the beginning of the COVID-19 pandemic, the Swedish strategy for limiting transmission. stood out.2 Sweden chose to keep preschools and schools open and daily life was not as limited for young children as it was in many other countries. Despite this, food habits, social interactions and physical activity were still affected in Sweden. The data from Miregård et al. show a remarkable change in the prevalence of overweight and obesity among four-year-old children. The girls had a higher prevalence of overweight and obesity, which is often the case for the youngest ages. The increase from 2018 to 2020 was more notable among the boys than the girls, but significant for both sexes in 13 of the 18 regions that reported data for both years. Most importantly, the increase was more remarkable for obesity than for overweight with obesity rising by an alarming 31.8%, from 2.2% to 2.9%. It seems that COVID-19 played against a backdrop of social or genetic vulnerability, where some children were more prone to developing obesity when their support structures vanished. A similar pattern was observed in a partly longitudinal study on 25 049 children aged 3–5 years from three regions in Sweden.3 In that study, overweight and obesity increased more among children attending Child Health Services in less socially privileged areas.3 The Miregård et al. paper discusses how a regional social care need index only partly correlated with their data on overweight and obesity in four-year-old children. They state that there are likely to be many explanations. In my eyes, the central message of the Miregård et al. paper is not the likely effect of the COVID-19 pandemic on children's weight, but the uniqueness of the data that the study produced. As far as I know, these are the only comprehensive national data on four-year old children's heights and weights in the world. The maps that illustrate the differences between regions will be of particular interest. What factors make children's weight increase too fast? What successful strategies have been used? As Miregård et al. point out, parents and health professionals both prefer, and ask for, weight issues to be targeted early in a child's life.4, 5 Good data mean that the effects of interventions can be analysed. Finland is a forerunner in gathering national data on children's growth. The Finnish Institute for Health and Welfare automatically collects data from primary health care and school health services for children aged 2–16 years. The coverage is 50% for children aged 2–6 years.6 We can look on with envy at Finland's data for school-age children, whereas the Swedish data have been sparse.7 Hopefully, we may not need to depend on researchers´ stoic ambition to gather data much longer, but rather on routine data collection in an emerging patient quality register for Child Health Services, Barnhälsovårdsregistret, which is including more and more regional reports.8 At a paediatric obesity congress in 2022, Kremlin Wickramasinghe, the acting head of the World Health Organization's European Office for the Prevention and Control of Non-Communicable Disease, was particularly concerned about the lack of high-quality data on preschool children. Thanks to the hard work of the authors, Acta Paediatrica can now proudly present high-quality data on over 200 000 children from Sweden.