Research suggests childhood “adiposity rebound” may reflect lean muscle growth, not increased body fat.
A new study published in The Journal of Nutrition offers new insights into a phenomenon known as adiposity rebound, where body mass index (BMI) starts to rise in early childhood after an initial decline. The study showed that despite the BMI increase, the waist-to-height ratio, a measure reflecting 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.
Calculated using a person’s height and weight, BMI cannot distinguish between fat mass and fat-free mass, including muscle and bone. Waist-to-height ratio has been shown to better estimate abdominal fat.
The new 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). While BMI followed the classic pattern of declining in early childhood and rising again around age six, waist-to-height ratio continued to decrease.
Adiposity rebound was identified more than 40 years ago, and an earlier rebound, occurring before the age of around six, has often been considered a warning sign for future obesity.
According to the author, Docent Andrew Agbaje from the University of Eastern Finland, the new study challenges this long-held idea, and the BMI increase around the age of six is more likely to reflect 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 provides a clearer picture of body fat and helps clinicians avoid misclassifying normal developmental changes as obesity risk, Agbaje adds.
Agbaje 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 recognises early-career physicians who have made exceptional contributions to obesity prevention or management.
American Society for Nutrition press release
Research article:
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. https://doi.org/10.1016/j.tjnut.2026.101437
