In new research to be presented at this year’s European Obesity Conference (May 12-15) in Istanbul, Turkey. nutrition journala 42-year-old theory about why children’s body mass index (BMI) decreases from infancy onwards and then rises continuously from the age of 6 – the ‘obesity rebound’ has been refuted using new analysis – the real reason is proposed to be increased muscle mass, rather than decreased body fat. The study is by Professor Andrew Agbaje, a physician and associate professor of clinical epidemiology and child health at the University of Eastern Finland in Kuopio, Finland.
It is important to disprove the existence of “fat rebound.” This is because, ever since this theory was proposed, some doctors, including pediatricians, have believed that it is a real phenomenon and that it is possible to intervene with lifestyle changes to prevent or reduce its effects.
It was in 1984 that French researcher Marie-Françoise Laurent-Cachera and her colleagues proposed the concept of “obesity rebound” in a paper. American Journal of Clinical Nutrition. They observed a relationship between obesity rebound and age at BMI obesity rebound and final BMI obesity (16 years) and showed that early rebounds (<5.5 years) were followed by higher obesity levels than subsequent rebounds (7 years and beyond). Several subsequent studies confirmed this.
To explain in more detail, once a child is born, the child’s BMI increases rapidly by around 1 year of age, drops to its lowest value around 4 years of age, and then begins to rise again. By age 6, children have regained their exact BMI at age 2. This “rebound” happens to all children. However, the timing and age at which BMI declines in childhood are associated with the risk of obesity later in life, and experts have hypothesized and simulated that if BMI falls too quickly, it will rise too quickly, and if it rises too quickly, it will ultimately lead to a higher BMI.
Other biological processes also occur in all children who live to adulthood, such as adolescence. But unlike “obesity rebound,” hitting puberty too early is associated with biologically plausible health risks. Professor Agbaje explains: “While puberty is a defining moment in human biology that changes the entire body, obesity rebound is not. It is a natural growth process unrelated to any problems, whether early or late. Therefore, previous associations linking obesity rebound based on early BMI to later obesity are misleading analyses. A positive statistical association does not necessarily equate to biological plausibility.”
Although there have been several court cases in recent decades regarding this phenomenon, Professor Agbaje’s new evidence indicates that there is no such thing. In a randomized controlled trial conducted in Finland, infants were introduced to a heart-healthy diet characterized by low proportions of saturated fat and cholesterol through dietary counseling and nutrition education sessions for parents and children from 7 months to 20 years of age, with an intervention starting at age 7 months and continuing until age 20 years, with no intervention in the control group. There was no difference between the intervention and control groups in terms of ‘rebound age’, or mean decrease in BMI and subsequent increase up to age 6 years. Professor Agbaje explains: “This is just one example of how clinical trials have failed to change the so-called ‘obesity rebound’ because it is simply a normal part of life and not a disease process or risk.”
To ascertain whether this phenomenon is real or what its real cause is, Professor Agbaje used waist circumference to height ratio (WHtR) instead in this new study. It measures body fat/obesity with approximately 90% accuracy compared to the gold standard (dual energy X-ray absorptiometry) measurement of fat mass. He analyzed data from 2,410 multiethnic children ages 2 to 19 from the 2021-2023 cycle of the U.S. National Health and Nutrition Examination Survey (NHANES) using both BMI and WHtR measurements. The mean BMI at age 2 (17.1 kg/m2) decreased significantly between ages 2 and 6, and then recovered by age 6 (see graph in full paper). This is consistent with obesity rebound theory.
However, mean WHtR at 2 years of age (0.54) did not recover throughout childhood and adolescence at 6 years of age or any other age. Overall, WHtR declines until age 7 years, after which it increases from childhood to late adolescence, but never recovers to 2-year-old levels. Therefore, there is no true rebound in fat mass. Professor Agbaje says his results show that the cause of the increase in BMI seen around the age of five to seven years is an increase in muscle/lean body mass, which is incorrectly described as fat or obesity. “In fact, children reset their body composition during a plateau around the age of four, preparing them for growth stages beyond that age,” he explains.
He suggests that the obesity rebound theory is therefore a “false discovery” caused by BMI, similar to the “obesity paradox” in adults, explaining that people living with obesity may have lower mortality rates than normal-weight people in certain scenarios. The BMI-obesity paradox highlights a U-shaped relationship with heart failure and mortality in adults, meaning that people with a high BMI are protected from heart disease. However, subsequent studies have proven that increased muscle mass within BMI is a protective factor. However, when randomized clinical trials find an association between WHtR and heart failure, the association is linear, meaning that higher fat mass is associated with worse cardiovascular disease. Therefore, WHtR is better than BMI in identifying fat mass and its associated risks.
Professor Agbaje said: “There is no need to push the obesity rebound theory further in the pediatric literature, as this is not an actual pathology or a critical period that justifies clinical intervention. It is a statistical anomaly. Gains in lean mass or lean body mass “This is likely an accurate physiological explanation for the body composition reset that occurs during infancy. This is a natural survival phenomenon that we mistakenly think of as a disease process and have been trying to treat or prevent for 42 years.” Therefore, the term ‘obesity rebound’ is incorrect and a fallacy regarding BMI and is simply an increase or growth in muscle mass. ”
This is a pivotal moment in history in the definition and accurate diagnosis of excess body fat in childhood, and has the potential to adopt WHtR as a practical and clinically useful universal tool in diagnosing excess body fat in children and adolescents. ”
Andrew Agbaje, University of Eastern Finland
He concluded: “Our new analysis suggests that this obesity rebound phenomenon is not a problem of obesity. It’s an increase in muscle mass, which is good for healthy, normal growth. We don’t need clinical intervention to address a problem that doesn’t exist in our children. Let’s let our children grow up in peace.”
He added that his team has published a freely accessible WHtR calculator to detect excess fat in children and adolescents.
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European Obesity Research Association
