The so-called “adiposity rebound,” a long-standing concept in pediatric growth measurement, may reflect increases in muscle mass rather than fat mass, according to data to be presented at the European Congress on Obesity in May and published online in The Journal of Nutrition.

Developers of the concept noted in a 1984 study that BMI increases during the first year of life, and then decreases, only to begin to rise again at about 6 years of age in what they dubbed the adiposity rebound. An earlier rebound increases the risk for excess adiposity in later years, they wrote.

However, the current study found that when waist-to-height ratio (WHtR), a more specific measure of body fat, was used, there was no evidence of a rebound in adiposity during early childhood. Instead, the rise in BMI observed around ages 5-7 years appears to reflect increasing lean mass.

“Adiposity rebound is not a real disease state or critical period,” said the study’s author, Andrew Agbaje, PhD, associate professor of clinical epidemiology and child health at the University of Eastern Finland in Kuopio, Finland. “The term ‘adiposity rebound’ is wrong. It’s a BMI fallacy. It is simply muscle mass build-up or growth,” he said.

Although widely used, BMI can misclassify adiposity in children because it reflects both fat and lean mass, whereas WHtR shows almost 90% agreement with DEXA measures of fat mass, Agbaje told Medscape Medical News.

The study findings indicate that WHtR should be used as a primary measure of excess fat, rather than BMI, in children and adolescents, Agbaje said.

Adiposity rebound is a nonexistent problem in children, so no clinical intervention is needed, Agbaje stated. “Let’s allow children to grow in peace,” he added.

BMI Rebound Without Fat Rebound

Since the 1984 paper’s publication, multiple studies have established BMI’s limitations, including sex, age, and ethnic or racial variations; inability to distinguish fat mass from lean mass; and overestimation or underestimation of overweight and obesity prevalence, Agbaje noted. BMI’s shortcomings prompted him to examine whether using WHtR would confirm or refute the concept of adiposity rebound.

Using data from the US National Health and Nutrition Examination Survey 2021-2023, he compared the BMI and WHtR trajectories in 2410 children aged 2-19 years.

Mean BMI followed the pattern laid out in the adiposity rebound theory. It decreased from 17.05 at 2 years to 16.43 at age 5 years, began to rise at age 6 (16.93), and continued to climb steadily at age 14 (25.12).

By contrast, mean WHtR fell from age 2, when it was 0.54, to age 7, when it was 0.48. It began to rise at age 8 (0.49) but did not return to age 2 levels in the teen years, ending at 0.51 at age 19.

“The apparent rebound in BMI is not a rebound in fat,” Agbaje said. “It reflects a normal developmental increase in fat-free mass.”

Children appear to undergo a body composition reset around age 4 years that prepares them for subsequent growth stages, he added.

Previous associations drawn between early BMI-based adiposity rebound and later-life obesity are misleading, Agbaje said. “Positive statistical associations do not always equate to biological plausibility,” he said.

Implications for Measurement and Treatment

The current study’s findings raise broader questions about relying on BMI in pediatric obesity research and practice. “The continued use of BMI, even with its acknowledged limitations, reflects its widespread adoption rather than its precision,” Agbaje said.

Clinicians should use caution in interpreting early BMI trajectories as indicators of adiposity or future risk, Agbaje said. The study found that two thirds of children classified as overweight by BMI had normal fat when assessed using WHtR, he noted.

WHtR, which reflects central adiposity and correlates more closely with cardiometabolic risk, may offer a more informative alternative or complement, he said. Agbaje has created a WHtR calculator for measuring for excess fat among children older than 5 years. 

“Treating BMI-based overweight risks wasting resources, exposing healthy children to unnecessary interventions, and increasing stigma and mental health harms,” Agbaje said.

The study findings may be particularly relevant in the context of emerging obesity treatments. Trials of GLP-1s in young people report reductions in weight and BMI, but they typically do not assess body composition. Evidence suggests that up to 40% of weight loss may reflect lean mass, raising concerns about potential impacts on growth, metabolism, and long-term health in youth.

“Skeletal muscle development is critical in children and adolescents,” Agbaje said. Longer-term GLP-1 studies with more precise adiposity measures are needed for this study population, he said.

Expert Urges Balanced Approach

Asked for comment, Jennifer L. Baker, PhD, head of research in lifecourse epidemiology at Frederiksberg Hospital, Frederiksberg, Denmark, and president-elect of The European Association for the Study of Obesity, urged caution.

“I think we all want the same thing, that is to understand children’s health as accurately as possible and support them in the best way we can,” she told Medscape Medical News.

“BMI isn’t perfect, and it’s right that we continue to question and improve how we assess body composition. At the same time, BMI is a simple and useful tool that has helped us understand growth and risk over many years,” she said.

WHtR is useful, especially for measuring central body fat, but it also has limitations, Baker said. “This study highlights the value of using the best combination of tools, rather than replacing one with another,” she added. 

Agbaje reported financial support was provided by Novo Nordisk Foundation. Baker disclosed consulting for Novo Nordisk, with fees paid to her institution.