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.

Abstract

Background: Rolland-Cachera et al. introduced the concept of "adiposity rebound" in a paper published in the American Journal of Clinical Nutrition in 1984. They observed that body mass index (BMI) increased during the first year of life and then decreased with a renewed rise at about age 6 y, which they termed "adiposity rebound," concluding that early rebound increased the risk of excess adiposity in later years. Although this concept has been vigorously criticized, an alternative explanation for this phenomenon has been lacking for 42 y. Moreover, BMI does not distinguish between fat mass and muscle mass.

Objectives: To examine whether a more accurate surrogate measure of adiposity, waist circumference to height ratio (WHtR), confirms or refutes BMI-based adiposity rebound.

Methods: In this study, 2410 children and adolescents' data aged 2-19 y from the US National Health and Nutrition Examination Survey 2021-2023 cycle were analyzed using a more accurate WHtR. Both raw values of BMI and WHtR and their z-scores were plotted to ascertain the trajectory of adiposity with increasing age.

Results: The mean value of BMI at age 2 y (17.1 kg/m₂) was regained by age 6 y (mean BMI 17.0 kg/m₂), while the mean BMI at age 7 y was 17.3 kg/m₂ after a significant decrease (adiposity rebound). The WHtR mean value at age 2 y (0.54) was never regained throughout childhood and adolescence (0.51). Although BMI-adiposity rebound seems to be completed by age 6 y, WHtR, which specifically assesses fat mass, continued decreasing. A body composition reset (BCR) at the intersection of BMI and WHtR trajectories at age 4 y until WHtR nadir at age 7 y was observed. The BCR is a post-infancy BMI increase after an initial decline that simultaneously corresponds to a continued WHtR-adiposity physiologic decrease, culminating at the lowest WHtR trajectory before a subsequent WHtR increase.

Conclusions: These novel findings establish that BMI-adiposity rebound is not physiologic but an epiphenomenon. I posit that "adiposity rebound" is a BMI-induced false discovery similar to the "obesity paradox" in adults. Therefore, fat-free mass or skeletal muscle mass anabolism is likely the accurate physiologic explanation for the BCR effect that occurs in early childhood.