An extended-accepted rationalization of childhood development patterns is now being challenged by new analysis that questions whether or not BMI changes in early life reflect fat accumulation at all.
A decades-old idea about how children grow may be built on a misunderstanding.
For years, scientists and doctors have pointed to a pattern in childhood body mass index (BMI), where it drops after infancy and then begins rising again around age six. This shift, called the “adiposity rebound,” has often been treated as an early warning sign for future obesity. But new research suggests something surprising: the pattern may have little to do with body fat at all.
The study, led by Professor Andrew Agbaje at the University of Eastern Finland, was presented at the European Congress on Obesity in Istanbul, Turkey and published in The Journal of Nutrition. His findings challenge the assumption that early BMI changes reflect fat gain, instead pointing to a more fundamental process tied to healthy growth.
Questioning the existence of the “adiposity rebound” matters because many clinicians, including pediatricians, have treated it as a real biological process. Some have even attempted to influence it through lifestyle changes to reduce future obesity risk.
Origins of the Theory
The concept was first introduced in 1984 by French researcher Marie Françoise Rolland-Cachera and colleagues in a paper published in The American Journal of Clinical Nutrition. Their research described a pattern in which BMI rises during infancy, declines to a low point in early childhood, and then increases again.
They also reported a link between the timing of this rebound and later body fat levels. Children who experienced an earlier rebound, before age 5.5 years, were more likely to have higher adiposity by age 16 than those whose rebound occurred after age 7. Later studies appeared to support this association.
Typically, BMI increases rapidly during the first year of life, then declines until about age four before rising again. By age six, BMI often returns to the level seen at age two. This pattern occurs in all children, but researchers have suggested that earlier declines and rebounds may be linked to a higher risk of obesity later in life.
Questioning Biological Significance
Other developmental milestones, such as puberty, also occur in all children who reach adulthood. However, early puberty is tied to well-established health risks, unlike the so-called adiposity rebound.
Prof Agbaje explains: “Puberty is a defining moment in human biology that alters the whole body, but adiposity rebound is not; it is a natural growth process unattached to any problem, whether it is early rebound or late. So the previous associations relating early BMI-based adiposity rebound to later life obesity are misleading analyses. Positive statistical associations do not always equate to biological plausibility.”
Over the years, multiple studies have tried to modify this pattern. One long-term randomized controlled trial in Finland followed participants from 7 months to 20 years. The intervention group received guidance on a heart-healthy diet low in saturated fat and cholesterol, along with ongoing nutrition education. The control group received no such intervention.
The study found no difference between the groups in the timing or pattern of BMI changes. Prof Agbaje explains: “This is just one example showing clinical trials could not change the so-called ‘adiposity rebound’ because it is simply a normal part of life and not a disease process or risk.”
A New Explanation: Muscle, Not Fat
To better understand what drives these changes, Prof Agbaje used waist circumference-to-height ratio (WHtR), a measure that estimates body fat with about 90% accuracy compared with the gold standard (dual-energy Xray absorptiometry).
He analyzed data from 2,410 multiracial children ages 2–19 years in the US National Health and Nutrition Examination Survey (NHANES) 2021–2023 cycle, comparing BMI and WHtR. BMI followed the expected pattern, dropping between ages 2 and 6 before returning to its earlier level.
WHtR told a different story. The average WHtR at age two (0.54) was never reached again during childhood or adolescence. It declined until about age seven and then gradually increased, but it did not return to earlier levels.
These findings suggest that fat mass does not “rebound.” Instead, the rise in BMI during early childhood appears to reflect gains in muscle and lean tissue. “Children in effect undergo a body composition reset at the plateau around age 4 years, which prepares them for the growth stages after that age,” he explains.
BMI Limitations and Misinterpretation
Prof Agbaje argues that the adiposity rebound theory is a BMI-driven “false discovery,” similar to the “obesity paradox” seen in adults. In some cases, adults with higher BMI appear to have lower mortality risk, particularly in heart failure.
However, later research has shown that this effect is linked to greater muscle mass rather than excess fat. When WHtR is used instead of BMI, the relationship between fat and heart disease becomes straightforward, with higher fat levels linked to worse outcomes.
This suggests that WHtR may be a more accurate tool than BMI for identifying harmful levels of body fat.
Implications for Childhood Health
Prof Agbaje says: “We do not need to push the adiposity rebound theory in pediatric literature any further because it is not a real disease state or a critical period that warrants clinical intervention. It is a statistical anomaly. Fat-free mass or lean mass growth is likely the accurate physiological explanation for the body composition reset that occurs in early childhood. It is a natural phenomenon for survival, which we have erroneously considered a disease process, and we have been trying to treat or prevent it for 42 years. So, the term ‘adiposity rebound’ is wrong, it is a BMI fallacy, it is simply muscle mass buildup or growth.”
He adds: “This is a pivotal moment in history in the definition and accurate diagnosis of childhood excess body fat, with the possibility of adopting WHtR as a practical and clinically useful universal tool in diagnosing excess fat in children and adolescents.”
He concludes: “Our new analysis suggests that this adiposity rebound phenomenon is not an obesity problem; this is an increase in muscle mass, and it is a good thing for healthy, normal growth. No clinical intervention is needed to address a non-existent problem in children. Let’s allow children to grow in peace.”
He adds that his team has published a freely accessible WHtR calculator for detecting excess fat in children and adolescents.
Reference: “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” by Andrew O Agbaje, 10 March 2026, The Journal of Nutrition.
DOI: 10.1016/j.tjnut.2026.101437
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