Researchers Debunk 42-Year-Old Theory Behind Childhood Obesity - Video

What if your child's rising BMI isn't a warning sign-but proof they're growing stronger?

A long-held belief about childhood weight gain may need a major rethink. New research suggests the well-known "adiposity rebound"—the rise in body mass index (BMI) that begins around age 5 to 6—may not reflect increasing body fat after all. Instead, it could simply represent healthy muscle and lean tissue growth. The study was presented at the European Congress on Obesity and published in The Journal of Nutrition.

For more than 40 years, doctors have believed that children's BMI rises again after early childhood because body fat "rebounds." Earlier rebound has even been linked to a higher risk of obesity later in life. However, Agbaje argues that this interpretation is based on BMI alone—a measure that cannot distinguish between fat, muscle, and bone.

To investigate further, researchers analyzed data from 2,410 children and adolescents aged 2–19 years who participated in the US National Health and Nutrition Examination Survey (NHANES) 2021–2023.

While BMI followed the familiar pattern of declining after infancy and rising again by age six, a more accurate measure of body fat—the waist-to-height ratio (WHtR)—told a different story. Body fat continued to decline until around age seven and never returned to the higher levels seen at age two.

The findings suggest the increase in BMI is largely driven by muscle and lean mass development, not fat accumulation.

The researchers say this challenges the decades-old adiposity rebound theory and questions whether the BMI rise should be treated as an early warning sign requiring intervention. Instead, they propose that the waist-to-height ratio may be a more reliable tool for identifying excess body fat in children.

Although further research is needed, the study suggests many children may simply be experiencing normal, healthy growth—not an early obesity problem.

REFERENCE: Andrew O Agbaje. Adiposity Rebound or Fat-Free Mass Anabolism in Children—Challenging a 42-Year-Old BMI Puzzle with Waist-to-Height Ratio: The ASNF-NNF 2025 Inaugural Flemming Quaade Award for Innovation in Childhood Obesity Lecture. The Journal of Nutrition, 2026; 156 (5): 101437 DOI: 10.1016/j.tjnut.2026.101437