A new study published in The Journal of Nutrition, a peer-reviewed journal of the American Society for Nutrition, offers new insights into a decades-old puzzle in childhood obesity. The study found that while body mass index (BMI) starts to rise in early childhood during a stage known as adiposity rebound, when BMI increases after an initial decline, the waist-to-height ratio, a measure that better reflects body fat, continues to drop. This suggests that the BMI increase at this age mainly reflects growth in lean tissues such as muscle and bone, rather than extra body fat.
The findings come from an analysis of 2,410 children and adolescents ages 2 to 19 who participated in the 2021-2023 U.S. National Health and Nutrition Examination Survey (NHANES). The study was led by Andrew Agbaje, MD, MPH, PhD, associate professor of Clinical Epidemiology and Child Health at the University of Eastern Finland and head of the urFIT-child (Understanding FITness and cardiometabolic health in little darlings) research group. Researchers found that BMI followed the classic pattern of declining in early childhood and rising again around age 6, while waist-to-height ratio continued to decrease. This suggests that the increase in BMI during this period mainly reflects growth in lean tissue, not extra body fat, challenging the long-held idea that an early adiposity rebound signals increased obesity risk.
"Recent global consensus statements on redefining and diagnosing obesity have recommended that obesity should not be diagnosed with BMI alone but confirmed with non-invasive measures such as waist-to-height ratio," Agbaje said. "This new study buttresses the misleading use of BMI in children whose body composition rapidly changes during growth and the potential for attributing physiological functions to pathology, which might lead to unnecessary interventions. Waist-to-height ratio should be incorporated as the first inexpensive measure in diagnosing pediatric obesity with BMI used as a confirmatory tool due to its imprecision."
For more than 40 years, researchers have described adiposity rebound as the point in childhood, usually around age 6, when BMI starts to rise again after an early decline. BMI is calculated using a person's height and weight and is commonly used as a screening tool for obesity. An earlier rebound has often been considered a warning sign for future obesity.
However, BMI cannot distinguish between fat mass, such as body fat, and fat-free mass, including muscle and bone. Because of this, some scientists have questioned whether the apparent rebound in BMI reflects a gain in body fat or just normal changes in children's body composition as they grow.
In the new study, researchers reexamined this issue using waist-to-height ratio, which compares a child's waist to their height to estimate abdominal fat. Abdominal fat is linked to a higher risk of health problems, including heart disease, type 2 diabetes, high blood pressure, liver disease, and bone fractures. Because waist-to-height ratio reflects fat distribution and is less affected by normal muscle growth, it may provide a clearer picture of body fat in children.
The researchers describe the mismatch between BMI and waist-to-height ratio as a "body composition reset," a shift toward lean tissue development that may support healthy growth.
This research also adds to growing evidence that BMI alone may not accurately reflect body composition in children, whose bodies change rapidly during growth. Incorporating waist-to-height ratio could 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 inaugural recipient of the American Society for Nutrition Foundation-Novo Nordisk Foundation Flemming Quaade Award for Innovative Approaches to Childhood Obesity. The award recognizes early-career physicians who have made exceptional contributions to obesity prevention or management. Now in its second year, the Flemming Quaade Award will next be presented at ASN's annual scientific meeting, NUTRITION 2026, July 25-28, 2026, in National Harbor, Maryland, just outside Washington, D.C.
Citation: Adiposity Rebound or Fat-Free Mass Anabolism in Children—Challenging a 42-Year-Old BMI Puzzle with Waist-to-Height Ratio: The American Society of Nutrition Foundation's 2025 Inaugural Flemming Quaade Award for Innovation in Childhood Obesity Lecture. Andrew O. Agbaje. J Nutr 2026;101437.
