The scale of the dataset is what sets the study apart. Using random cluster sampling, the researchers enrolled 120,459 students from 15 junior and senior high schools spread across four districts and prefectures in Xinjiang, selected according to geographical distribution characteristics. Height and weight were measured in strict accordance with the Chinese National Student Physical Health Standard, and each participant was classified as underweight, normal weight, overweight, or obese based on body mass index, with a combined overweight-plus-obesity category also analyzed. Because the same surveillance framework ran every year from 2017 to 2021, the team could compare each pandemic-era year against a stable pre-pandemic baseline rather than relying on snapshots from unrelated surveys.

The headline finding is a sharp, statistically significant bump in body mass index in 2020. Mean BMI among the adolescents rose by 0.2 kilograms per square meter compared with previous years, an increase the authors describe as significant, before the trend reversed and declined in 2021. The statistical evidence for these shifts was robust: analysis of variance yielded an F statistic of 104.862 with a P value below 0.001, meaning the probability that such differences arose by chance is vanishingly small. A rise of 0.2 kg/m² may sound modest at the population level, but across hundreds of thousands of adolescents even small shifts in the BMI distribution can translate into large numbers of young people crossing clinical thresholds for overweight and obesity.

Those thresholds moved in exactly the worrying direction during the acute pandemic phase. Compared with the 2017 to 2019 baseline, the detection rate of underweight fell to 10.6 percent in 2020, while the overweight rate climbed to 9.5 percent and the combined overweight-and-obesity rate rose to 12.0 percent, with both increases statistically significant. In other words, the pandemic year did not simply make some adolescents heavier; it shifted the entire weight distribution, pulling teenagers away from the low end of the scale and pushing them toward the high end. The researchers used chi-squared tests to confirm that these changes in category prevalence were unlikely to be artifacts of sampling variation.

To quantify risk more precisely, the team turned to binary logistic regression, using 2017 as the reference year and accounting for the clustered sampling design. The results were striking. In 2020, the odds of being underweight dropped to 0.668 times the baseline odds, a 95 percent confidence interval of 0.633 to 0.706, while the odds of overweight rose to 1.280 times baseline, with a confidence interval of 1.199 to 1.366, and the odds of combined overweight and obesity rose to 1.364 times baseline, with a confidence interval of 1.286 to 1.448. Because those confidence intervals sit entirely above 1.0 for the excess-weight outcomes and entirely below 1.0 for underweight, the associations are statistically secure at the conventional 5 percent significance level the study adopted.

Then came the reversal. As China’s pandemic response entered its normalized prevention and control phase, with schools operating under routine rather than emergency conditions, the adolescent weight distribution began drifting back toward its pre-pandemic shape. In 2021, the underweight rate rebounded to 13.9 percent, while overweight and combined overweight-and-obesity rates fell to 8.0 percent and 10.7 percent respectively, all changes statistically significant compared with 2020. The regression models echoed this pattern: the odds of underweight in 2021 were 0.917 times the 2017 baseline, with a confidence interval of 0.872 to 0.965, and the odds of combined overweight and obesity fell to 1.201 times baseline, with a confidence interval of 1.131 to 1.275. The elevated risk had not vanished, but it was clearly receding.

One of the most consequential findings concerns who was affected most. The BMI changes were not uniform across the population: girls showed greater fluctuation than boys, with the analysis of variance producing F statistics of 114.950 and 30.063 for the two groups, both with P values below 0.001, and statistically significant differences appeared across all age groups as well. The recovery in 2021 was also more pronounced among girls and among the 13-to-15-year-old group, the younger half of the cohort. This gender and age patterning matters for intervention design, because it suggests that the pandemic’s metabolic impact was concentrated in specific subgroups rather than spread evenly, and that younger adolescents may be more resilient, or more responsive to restored routines, than older ones approaching the end of puberty.

Why would a year of pandemic restrictions push adolescents toward higher body weight? The study’s design does not test mechanisms directly, but the plausible pathways are well understood in the broader literature on childhood obesity. School closures curtail structured physical activity, including physical education classes, organized sports, and the incidental walking that a school day imposes. Lockdowns alter eating patterns, increasing snacking and reliance on energy-dense convenience foods while removing the regularity of school meal schedules. Screen time rises, sleep rhythms drift, and the psychological stress of isolation can itself promote weight gain. The simultaneous decline in underweight detection during 2020 is consistent with this picture: adolescents who might otherwise have remained lean gained weight, thinning out the low end of the distribution even as the high end expanded.

The partial reversal in 2021 is equally informative. It suggests that the pandemic’s effect on adolescent weight was not a permanent scar but a perturbation tied to the disruption of daily routines. When normalized prevention and control restored a measure of regularity to school life, the forces that had driven BMI upward weakened, and the distribution began to revert. The authors emphasize, however, that the return was incomplete: the 2021 odds of combined overweight and obesity remained 1.201 times the 2017 baseline, indicating a lingering elevation even as the acute impact faded. Whether the distribution would have fully normalized with more time is a question the study’s five-year window cannot answer definitively.

For public health planners, the study carries a clear message about vigilance and precision. The researchers conclude that the initial stage of the pandemic was associated with a significant negative impact on the weight status of Chinese adolescents, and they call for strengthened long-term dynamic monitoring of adolescent weight changes alongside precisely targeted health interventions to prevent undesirable physical status and promote healthy development. The subgroup findings point the way: interventions aimed at girls and at younger adolescents during periods of major routine disruption could yield outsized benefits. With adolescent obesity rising across much of the world, and with future pandemics or other large-scale disruptions a certainty rather than a hypothetical, the Xinjiang dataset offers something precious, a quantified, longitudinal record of how quickly a population of teenagers can shift toward excess weight when normal life stops, and how quickly the trend can bend back when it resumes.

Subject of Research: The impact of the COVID-19 pandemic and its normalized prevention and control phase on BMI and overweight and obesity prevalence among Chinese adolescents aged 13 to 18.

Article Title: A longitudinal study on differential impacts of the COVID-19 pandemic and normalized prevention and control on overweight and obese adolescents in China

Article References: Maitituersun, Y., Su, R., Yang, G., & Taerken, A. Y. (2026). A longitudinal study on differential impacts of the COVID-19 pandemic and normalized prevention and control on overweight and obese adolescents in China. BMC Pediatrics, 26(1), Article 882. https://doi.org/10.1186/s12887-026-07558-z

Image Credits: AI Generated

DOI: 10.1186/s12887-026-07558-z

Keywords: COVID-19, adolescent health, obesity, overweight, body mass index, longitudinal study, China, Xinjiang, public health, school closures, pediatrics, weight status