Researchers looked at how kids’ weights changed before and during the COVID-19 pandemic using data from thousands of American children ages 2 to 19. They found that about 1 in 5 kids had obesity both before and during the pandemic, with only a small increase during COVID. Interestingly, kids who ate fewer ultra-processed foods and got more exercise had better results, especially when it came to preventing more severe obesity. The study suggests that staying active remains one of the most important things kids can do for their health, even during challenging times like a pandemic.

The Quick Take

  • What they studied: Whether kids’ obesity rates changed during the COVID-19 pandemic and how eating ultra-processed foods and physical activity affected their weight
  • Who participated: About 7,257 American children and teenagers ages 2 to 19 total: 4,756 before the pandemic (2017-March 2020) and 2,501 during the pandemic (August 2021-August 2023)
  • Key finding: Obesity rates were 21.2% before COVID and 22.6% during COVID—a small increase that wasn’t statistically significant. Kids who exercised more days per week had lower chances of obesity, especially the more severe types. Surprisingly, eating fewer ultra-processed foods didn’t show a clear connection to obesity severity during the pandemic period.
  • What it means for you: If you’re a parent or young person, this research suggests that physical activity is one of the most reliable ways to help prevent obesity, even during stressful times. The good news is that obesity rates didn’t skyrocket during the pandemic as some feared, though staying active remains crucial for health.

The Research Details

This study compared two groups of children at different time periods rather than following the same kids over time. Researchers used information from the National Health and Nutrition Examination Survey, which is like a giant health checkup program for Americans. They looked at kids before the pandemic (January 2017 through March 2020) and then again during the pandemic (August 2021 through August 2023). For each child, they measured weight and height to calculate BMI (body mass index), which is a number that shows whether someone’s weight is healthy for their height. They also asked kids and parents about what they ate over 24 hours and how many days per week kids did exercise that made them breathe hard or sweat.

The researchers divided obesity into three levels based on how far above a healthy weight each child was: Class I (mildly obese), Class II (moderately obese), and Class III (severely obese). They then used statistical methods to figure out which factors—like exercise, ultra-processed food intake, age, sex, race, ethnicity, and family income—were connected to obesity.

This approach is important because it captures real-world data from a representative sample of American kids rather than just studying one specific group. By comparing the same time periods before and after COVID, researchers could see whether the pandemic actually changed obesity rates. Understanding which behaviors (exercise and diet) protect kids from obesity helps doctors and parents know what to focus on.

This study has good strengths: it includes a large number of children from different backgrounds across the entire United States, and it uses standardized measurements rather than relying on people’s memories. However, because it’s a snapshot in time rather than following kids over years, we can’t be completely sure that exercise causes lower obesity—only that they’re connected. The study also relies on people accurately reporting what they eat and how much they exercise, which can sometimes be inaccurate.

What the Results Show

The main finding was that obesity rates barely changed from before to during the pandemic: 21.2% of kids had obesity before COVID compared to 22.6% during COVID. This 1.4 percentage point increase was small enough that it could have happened by chance. Before the pandemic, kids who exercised more days per week had noticeably lower odds of having Class II obesity (the moderate type) and overall obesity. For every additional day per week of exercise, the odds of Class II obesity dropped by about 14%, and overall obesity odds dropped by about 9%.

During the pandemic, the pattern was similar but weaker. Kids who met exercise guidelines (which typically means 60 minutes of moderate-to-vigorous activity most days) had lower odds of overall obesity, though the effect wasn’t as clear-cut as before. Interestingly, the percentage of calories kids got from ultra-processed foods actually decreased slightly from 66% before the pandemic to 62.7% during the pandemic, which was a statistically significant drop.

However, this decrease in ultra-processed food intake didn’t translate into a clear protective effect against obesity severity during the pandemic period. This was surprising and suggests that other factors beyond just what kids eat may have been at play during COVID.

The study found that the protective effect of physical activity was most consistent for Class II obesity (moderate obesity) before the pandemic, but less clear for the most severe obesity (Class III). During the pandemic, while exercise still showed protective effects, they were weaker and didn’t reach statistical significance for specific obesity classes. The decrease in ultra-processed food consumption during the pandemic was unexpected and might reflect changes in how families ate at home during lockdowns, though the study didn’t find this translated to better obesity outcomes.

Previous research has consistently shown that physical activity helps prevent obesity in children, and this study confirms that pattern holds true even during a pandemic. The finding that obesity rates didn’t increase dramatically during COVID is somewhat reassuring compared to early pandemic predictions that suggested childhood obesity would spike due to school closures and reduced activity. However, the lack of a clear connection between ultra-processed food intake and obesity severity during the pandemic is unusual and differs from most previous studies, suggesting the pandemic may have changed how diet affects weight in ways researchers don’t yet fully understand.

This study has several important limitations. First, it’s a snapshot comparison rather than following the same kids over time, so we can’t prove that exercise causes lower obesity—only that they’re associated. Second, the pandemic period data was collected starting in August 2021, which was after vaccines became available and restrictions eased, so it may not fully capture the most severe pandemic period. Third, people self-reported their exercise and diet, which can be inaccurate—people often overestimate exercise and underestimate unhealthy eating. Fourth, the sample size during the pandemic was smaller than before, which could affect the results. Finally, the study couldn’t explain why ultra-processed food intake decreased but didn’t show a protective effect, leaving some questions unanswered.

The Bottom Line

Based on this research, the strongest recommendation is that children and teens should aim to get regular physical activity—ideally 60 minutes of moderate-to-vigorous exercise most days of the week. This appears to be one of the most reliable ways to help prevent obesity. While reducing ultra-processed foods is still important for overall health, this particular study didn’t show it was the key factor in preventing obesity during the pandemic. Parents and young people should focus on making movement a regular part of daily life. Confidence level: Moderate to High for exercise recommendations; Lower for specific dietary conclusions from this study.

This research is most relevant for parents of children ages 2-19, pediatricians, school administrators, and policymakers making decisions about kids’ health programs. It’s especially important for families concerned about obesity or for those who experienced reduced activity during the pandemic. However, this study shouldn’t replace personalized advice from a doctor, especially for children who already have obesity or other health conditions. Kids with severe obesity may need additional support beyond just exercise.

Realistic expectations: Kids who increase their physical activity may see improvements in weight and health markers within 8-12 weeks, though significant weight loss typically takes several months of consistent activity. The benefits of regular exercise for overall health (like stronger heart and bones) can start appearing within weeks, even if weight changes more slowly.

Want to Apply This Research?

  • Track the number of days per week your child gets 60 minutes of moderate-to-vigorous physical activity (activities where they breathe hard or sweat, like running, sports, or fast cycling). Set a goal of 5-7 days per week and log actual days achieved.
  • Use the app to set weekly exercise goals and celebrate when kids hit them. Create a family challenge where everyone tracks their active days together. Include a variety of activities kids enjoy—sports, dancing, biking, or playing—to make it sustainable rather than feeling like a chore.
  • Check in monthly to see if activity levels are consistent and adjust goals based on what’s working. Track not just exercise days but also how kids feel (energy levels, mood, sleep quality) to show the broader benefits of activity beyond just weight. Consider pairing this with periodic check-ins with a pediatrician to monitor weight and health markers over time.

This research provides general information about obesity trends and the relationship between exercise and weight in children. It should not replace personalized medical advice from your child’s doctor. If you’re concerned about your child’s weight or health, please consult with a pediatrician or healthcare provider who can assess your individual child’s situation, medical history, and needs. This study shows associations between behaviors and obesity but cannot prove cause-and-effect relationships. Always talk to a healthcare professional before making significant changes to your child’s diet or exercise routine, especially if your child has existing health conditions.