A study in the Journal of Nutrition suggests that the widely cited “fat rebound” may reflect an increase in lean muscle mass rather than an increase in body fat.
A new study published Food magazinethe peer-reviewed journal of the American Society for Nutrition, offers new insights into the decades-old puzzle of childhood obesity. The study found that while body mass index (BMI) rises during early childhood, known as obesity recovery, when BMI rises after an initial decline, waist-to-height ratio, a measure that better reflects body fat, declines. This suggests that the increase in BMI at this age mainly reflects the increase in lean tissue such as muscle and bone rather than excess body fat.
The findings come from an analysis of 2,410 children and adolescents ages 2 to 19 who participated in the 2021-2023 National Institutes of Health and Nutrition Examination Survey (NHANES). The study was conducted by Andrew Agbaje, MD, MPH, PhD, associate professor of clinical epidemiology and child health at the University of Eastern Finland and head of urFIT-child (Unorstanding FITto be and ccardiometabolic chproperty in la little darlings) research group. The researchers found that BMI follows a classic pattern of decline in early childhood, increasing again around age 6, while waist-to-height ratio continues to decline. This suggests that the increase in BMI during this period mainly reflects an increase in lean tissue rather than excess body fat, disproving the long-held view that early onset of obesity increases the risk of obesity.
“The recent global consensus statement on the redefinition and diagnosis of obesity recommended that obesity should not be diagnosed by BMI alone, but confirmed by non-invasive measures such as waist-to-height ratio,” Agbaje said. “This new study reinforces the inappropriate use of BMI in children whose body composition changes rapidly during growth and the possibility of attributing physiological functions to pathology that may lead to unnecessary interventions. Waist-to-height ratio should be used as a low-cost first measure in the diagnosis of pediatric obesity with BMI as a confirmatory tool.”
For more than 40 years, researchers have described fat recovery as the point in childhood, usually around age 6, when BMI begins to increase again after an early decline. BMI is calculated using a person’s height and weight and is commonly used as a tool to diagnose obesity. Previous relapse was often considered a warning sign for future obesity.
However, BMI cannot distinguish between fat mass, such as body fat, and fat mass, including muscle and bone. For this reason, some scientists have questioned whether the apparent increase in BMI reflects an increase in body fat or just normal changes in body composition in children as they grow.
In a new study, researchers revisited the issue by using the waist-to-height ratio, which compares a child’s waist to their height, to estimate belly fat. Belly fat is linked to a higher risk of health problems, including heart disease, type 2 diabetes, high blood pressure, liver disease and broken bones. Because the waist-to-height ratio reflects fat distribution and is less affected by normal muscle growth, it can provide a clearer picture of body fat in children.
The researchers describe the discrepancy between BMI and waist-to-height ratio as a “reset of body composition,” a shift toward the development of lean tissue that can support healthy growth.
This study also adds to the growing evidence that BMI alone may not reflect body composition in children whose bodies change rapidly during growth. Incorporating a waist-to-height ratio can provide a clearer picture of body fat and help clinicians avoid misclassifying normal developmental changes as obesity risk.
Agbaje’s work in pediatric cardiometabolic health has received international recognition. He is the recipient of the inaugural American Society for Nutrition Novo Nordisk Foundation Flemming Quaade Award for Innovative Approaches to Childhood Obesity. This award recognizes early career physicians who have made exceptional contributions to the prevention or management of obesity. Now in its second year, the Flemming Quadda Award will be presented at the ASN Annual Scientific Meeting, NUTRITION 2026, July 25-28, 2026, in Harbor, Maryland, near Washington, DC.
Quote: Reversal of obesity or fat-free mass anabolism in children – Solving the 42-year BMI puzzle with waist-to-height ratio: The inaugural 2025 American Society for Nutrition Award for Innovation in Childhood Obesity Lec. Andrew O. broken J Nutr 2026; 101437.
