Childhood BMI Changes Linked to Muscle Growth, Not Fat Gain

Early childhood body mass index (BMI) changes may reflect lean mass growth rather than fat changes, with waist-to-height ratio (WHtR) shown to be more accurate than BMI for assessing body fat in children.

The early drop in children’s BMI is due to increasing muscle mass, not a reduction in body fat, according to a new study.
New research to be presented at this year’s European Congress on Obesity in Istanbul and published in The Journal of Nutrition challenges a 42-year-old theory about children’s growth patterns (1^✔ ✔Trusted Source
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

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).

Adiposity Rebound: BMI Drop in Kids Linked to Strength Gains, Not Weight Loss

Previously, scientists believed that children’s BMI drops after infancy and then rises again around age six due to a decrease in body fat—a process called “adiposity rebound.” However, the new analysis suggests this is not because fat decreases, but because muscle mass increases during that period.

The study is by Professor Andrew Agbaje, physician and associate professor of clinical epidemiology and child health at the University of Eastern Finland, Kuopio, Finland.

Disproving the existence of ‘adiposity rebound’ is important because, since the theory was proposed, some doctors including pediatricians have believed it is a real phenomenon, and it is possible to intervene with lifestyle changes to prevent or mitigate its effect.

Is Early Adiposity Rebound Tied to Higher Obesity Risk?

It was in 1984 that French researcher Marie Françoise Rolland-Cachera and colleagues proposed the concept of “adiposity rebound” in a paper published in The American Journal of Clinical Nutrition. They observed the adiposity rebound and a relationship between the age at BMI adiposity rebound and final BMI adiposity (at age 16 years), and showed that an early rebound (before age 5.5 years) is followed by a higher adiposity level than a later rebound (after age 7 years) (2^✔ ✔Trusted Source
Adiposity rebound in children: a simple indicator for predicting obesity

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). Some subsequent studies confirmed this (3^✔ ✔Trusted Source
Early Adiposity Rebound and the Risk of Adult Obesity

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).

How Early BMI Changes Shape Future Weight?

In more detail, when a child is born, the child’s BMI increases rapidly by age 1 year, and then starts falling to the lowest level around age 4 years and subsequently begins to rise again. By age 6 years, the child regains the exact BMI he or she had at age 2 years. This ‘rebound’ happens to all children. However, the timing or age of this fall in BMI in early childhood has been associated with the risk of BMI-obesity in later years – experts hypothesised and simulated that if the BMI falls too early, then it will rise too early – and end up higher in those where it rises too early.

Adiposity Rebound is Just Growth, Not a Health Problem

Other biological processes also occur in all children who live to adulthood – for example, puberty. However, going through puberty at too early an age has been associated with biologically plausible health risks, unlike the ‘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”

BMI Rebound is Natural, Not Modifiable

Several trials have taken place in the intervening decades regarding this phenomenon that Prof Agbaje’s new evidence shows is non-existent. In one randomized controlled trial from Finland, starting at 7 months of age which continued until age 20 years, an intervention introduced infants to a heart-healthy diet, characterised by low proportional intake of saturated fat and cholesterol, by dietary counselling and nutrition education sessions to parents and children from the age of 7 months to 20 years, while the control group received no intervention. There was no difference between the intervention and control group regarding the ‘rebound age’ – the average decrease in BMI and the subsequent increase by age 6. 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.”

Childhood BMI Rebound Explained Using Waist-to-Height Ratio Analysis

To establish whether or not this phenomenon is real – or what is really the cause of it – Prof Agbaje in this new study instead used waist circumference-to-height ratio (WHtR) , which measures body fat/adiposity with around 90% accuracy compared with the gold standard (dual-energy Xray absorptiometry) measure of fat mass. He analysed data of 2410 multiracial children aged 2 – 19 years from the US National Health and Nutrition Examination Survey (NHANES) 2021–2023 cycle, using both BMI and WHtR measurements. The mean value of BMI at age 2 years (17.1 kg/m2) was regained by age 6 years after a significant decrease between age 2 and 6 years (see graph in full paper), which was consistent with the adiposity rebound theory.

Childhood BMI Rebound Explained: It’s Muscle Gain, Not Fat Return

However, the WHtR mean value at age 2 years (0.54) was never regained throughout childhood and adolescence, at age 6 years or any other age. Overall, WHtR falls until age 7 years, from which age it increases across childhood and late adolescence – but never recovering to the level it was at age 2 years. Thus, there is no true rebound in fat mass – Prof Agbaje says his results show that it is an increase in muscle/lean mass that causes the increase in BMI seen around age 5 to 7 years, which has been erroneously described as fat or adiposity. “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.

WHtR vs BMI: Why Waist-to-Height Ratio Better Predicts Heart Disease Risk

He suggests that the adiposity rebound theory is therefore a BMI-induced ‘false discovery’ similar to the “obesity paradox” in adults, explained as people living with obesity can have lower mortality rates in certain scenarios than people with normal weight. The BMI obesity paradox emphasises a U-shaped relationship with heart failure and mortality among adults, meaning those with higher BMI are protected from heart disease. However, subsequent research has established that it is the increased muscle mass within the BMI that is a protective factor. However, when WHtR was associated with heart failure in randomized clinical trials, the association was linear, meaning that the higher the fat mass, the worse the cardiovascular disease. WHtR is thus better than BMI at identifying fat mass and its associated risk.

Adiposity Rebound is Not a Disease, But Normal Muscle Growth

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 build up 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.”

Reference:

1. 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 – (https://www.sciencedirect.com/science/article/pii/S0022316626000866)
2. Adiposity rebound in children: a simple indicator for predicting obesity – (https://www.sciencedirect.com/science/article/abs/pii/S0002916523245657)
3. Early Adiposity Rebound and the Risk of Adult Obesity – (https://publications.aap.org/pediatrics/article-abstract/101/3/e5/61923/Early-Adiposity-Rebound-and-the-Risk-of-Adult)

Source-Eurekalert