A study published in The Journal of Nutrition and announced on Apr. 16 suggests that rising body mass index (BMI) in early childhood may be due to growth in lean tissue such as muscle and bone, rather than an increase in body fat. The research analyzed data from 2,410 children and adolescents aged 2 to 19 who participated in the U.S. National Health and Nutrition Examination Survey between 2021 and 2023.
This finding challenges a long-standing belief that an early rise in BMI, known as adiposity rebound around age six, is a warning sign for future obesity risk. Instead, the study found that while BMI begins to rise after an initial decline during this period, waist-to-height ratio—a measure more closely linked with body fat—continues to decrease.
The research was led by Andrew Agbaje, MD, MPH, PhD of the University of Eastern Finland. "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," Agbaje said.
For decades researchers have relied on BMI as a screening tool for obesity because it is calculated using height and weight; however, it cannot distinguish between fat mass and lean mass like muscle or bone. This limitation has led some scientists to question whether increases seen at adiposity rebound actually indicate excess fat accumulation or simply normal developmental changes.
The researchers described this phenomenon as a "body composition reset," suggesting that shifts toward lean tissue development support healthy growth rather than signaling increased obesity risk. They recommend clinicians consider waist-to-height ratio alongside BMI when assessing children's health status.
Agbaje's work has been recognized internationally; 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 next presentation of this award will take place at ASN's annual scientific meeting NUTRITION 2026.
