New research to be presented and published at this year’s European Congress on Obesity in Istanbul, Türkiye (12-15 May) Journal of NutritionA 42-year-old theory as to why children’s body mass index (BMI) decreases after infancy and then increases steadily from the age of six – ‘adiposity rebound’ – has been debunked using new analyzes – with the real cause being proposed as increased muscle mass, rather than reduced body fat. The study has been conducted by Professor Andrew Agbaje, physician and associate professor of clinical epidemiology and child health at the University of Eastern Finland, Kuopio, Finland.
It is important to deny the existence of ‘fat rebound’ because, since the theory was proposed, some doctors, including pediatricians, believe 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 François Rolland-Cachera and colleagues proposed the concept of “adiposity rebound” in a paper published in 1984. The American Journal of Clinical Nutrition. They looked at the relationship between the age of fat rebound and BMI fat rebound and final BMI adiposity (at age 16), and showed that early rebound (before 5.5 years of age) resulted in higher fat levels than later rebound (after 7 years of age). Some later studies confirmed this.
In more detail, when a child is born, the child’s BMI increases rapidly until the age of 1 year, and then begins to fall to the lowest level around the age of 4 years and begins to rise again later. By the age of 6 years, the child attains the exact same BMI as he had at the age of 2 years. This ‘rebound’ happens with all children. However, the timing or age of this decline in BMI in childhood is linked to increased BMI-obesity risk in later years – experts hypothesized and simulated that if BMI falls too quickly, it will rise too quickly – and will be greater in those where it rises too quickly.
Other biological processes also occur in all children who live to adulthood – for example, puberty. However, going through puberty at a very young age is associated with potential health risks, biologically speaking, as opposed to ‘fat rebound’. Professor Agbaje explains: “Puberty is a pivotal moment in human biology that transforms the entire body, but fat rebound does not occur; it is a natural developmental process unrelated to any problem, whether early or late rebound. Therefore previous associations relating early BMI-based fat rebound to later life obesity are misleading analyses. Positive statistical relationships do not always equate to biological plausibility”
There have been several trials regarding this phenomenon in the intervening decades, but new evidence from Professor Agbaje suggests that it is non-existent. In a randomized controlled trial in Finland, starting at 7 months of age and continuing until age 20 years, an intervention provided parents and children ages 7 months to 20 years with dietary counseling and nutrition education sessions to introduce the infants to a heart-healthy diet that included a low proportional intake of saturated fat and cholesterol, while the control group received no intervention. There was no difference between the intervention and control groups with respect to ‘rebound age’ – the average decrease in BMI followed by an increase at 6 years of age. Professor Agbaje explains: “This is just one example that shows that clinical trials cannot change so-called ‘adiposity rebound’ because it is simply a normal part of life and not a disease process or risk.”
To establish whether or not this phenomenon is real – or what exactly causes it – Professor Agbaje in this new study instead used the waist circumference-to-height ratio (WHTR), which measures body fat/fat with approximately 90% accuracy compared to the gold standard (dual-energy X-ray absorptiometry) measurement of fat mass. They analyzed data from 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 the age of 2 years (17.1 kg/m2) was regained by 6 years of age after a significant decrease between 2 and 6 years of age (see graphs throughout the paper), which was consistent with the fat rebound theory.
However, the mean value of WHtR at age 2 (0.54) was never regained throughout childhood and adolescence, at age 6, or at any other age. Overall, WHtR falls by age 7, from this age it increases through childhood and late adolescence – but never returns to 2-year-old levels. Thus, there is no true rebound in fat mass – Professor Agbaje says his results show that it is the increase in muscle/lean mass that causes the increase in BMI seen around 5 to 7 years of age, which is erroneously described as adiposity or fat. He explains, “In fact, children’s physical structure resets to a plateau around age 4, preparing them for the developmental stages after that age.”
They suggest that the fat rebound theory is a BMI-induced ‘false finding’ similar to the “obesity paradox” in adults, which is explained by the fact that obese people may have lower mortality rates in some scenarios than normal weight people. The BMI obesity paradox emphasizes a U-shaped relationship with heart failure and mortality in adults, meaning that people with higher BMIs are protected from heart disease. However, subsequent research has established that it is the increased muscle mass within BMI that is a protective factor. However, when WHtR was associated with heart failure in randomized clinical trials, the relationship was linear, meaning the greater the fat mass, the worse the heart disease. Thus WHtR is superior to BMI in identifying fat mass and its associated risk.
Professor Agbaje says: “We do not need to pursue the fat rebound theory in the pediatric literature because this is not a real disease state or critical period that requires clinical intervention. It is a statistical anomaly. Fat-free mass or lean mass increase is probably the accurate physiological explanation for the body composition reset that occurs in childhood. It is a natural phenomenon necessary for survival, which we have mistakenly considered a disease process, and we have been trying to treat or prevent it for 42 years. Trying. The term ‘adiposity rebound’ is a misconception, it’s just muscle building or growth.”
This is an important moment in history in the definition and accurate diagnosis of excess body fat in childhood, with the possibility of adopting the WHtR as a practical and clinically useful universal tool in the diagnosis of excess body fat in children and adolescents.
Andrew Agbaje, University of Eastern Finland
They concluded: “Our new analysis shows that this fat rebound phenomenon is not an obesity problem; it is an increase in muscle mass, and that is a good thing for healthy, normal development. No clinical intervention is needed to solve a non-existent problem in children. Let’s let children grow in peace.”
He further said that his team has published a freely accessible WHtR calculator to detect excess fat in children and adolescents.
