Scientists Warn of a Common 40-Year-Old Mistake Regarding Childhood Obesity

SadaNews - A long-standing interpretation of children's growth patterns is being challenged by new research that questions whether early changes in body mass index reflect fat accumulation at all.

For years, scientists and doctors have pointed to a pattern in children's body mass index, which drops after infancy and then begins to rise again around the age of six. This shift, known as "fat rebound," is often viewed as an early sign of future obesity. However, new research suggests a surprising fact: this pattern may have little to do with body fat.

According to SciTechDaily, the study, led by Professor Andrew Agbaji from the University of Eastern Finland, was presented at the European Obesity Conference in Istanbul and published in the journal Nutrition. The study's results challenge the assumption that early body mass index changes reflect increased fat, instead suggesting a more fundamental process related to healthy growth.

Questioning the existence of the "fat rebound" phenomenon is crucial, as many doctors, including pediatricians, have treated it as a real biological process. Some have even tried to influence it through lifestyle changes to reduce the risk of future obesity.

This concept was first proposed in 1984 by French researcher Marie-Françoise Rolland-Cachera and her colleagues in a research paper published in the American Journal of Clinical Nutrition. Their research described a pattern where body mass index rises during infancy, then drops to its lowest point in early childhood, and then rises again.

They also noted a link between the timing of this rebound and later body fat levels. Children who experienced an early rebound, before the age of 5.5 years, were more likely to have higher body fat percentages by the age of 16 compared to those whose rebound occurred after the age of 7. Subsequent studies supported this relationship.

Typically, body mass index rises rapidly during the first year of life, then declines until about the age of four before rising again. By the age of six, body mass index often returns to the level it was at two years of age. This pattern occurs in all children, but researchers noted that early increases and decreases in weight could be linked to an increased risk of obesity later in life.

Questioning Biological Significance

Other growth stages, such as puberty, occur in all children as they reach adulthood. However, early puberty is associated with known health risks, unlike the so-called "fat rebound."

Professor Agbaji explains that "puberty is a pivotal moment in human biology that changes the entire body, but fat rebound is not; it's a normal growth process not associated with any problem, whether it’s an early or late rebound. Thus, previous correlations linking early fat rebound based on body mass index to later obesity in life are misleading analyses. Positive statistical correlations do not always equate to biological plausibility."

Over the years, numerous studies have tried to adjust this pattern. One long-term randomized clinical trial in Finland followed participants from 7 months to 20 years of age. The experimental group received guidance on a heart-healthy diet low in saturated fat and cholesterol, along with ongoing nutritional education. The control group did not receive any similar intervention.

The study found no difference between the two groups in the timing or pattern of body mass index changes. Professor Agbaji points out that "this is just one example showing that clinical trials cannot change what is termed "fat rebound," because it is simply a natural part of life and not a disease or risk factor."

A New Interpretation

To gain a better understanding of the factors driving these changes, Professor Agbaji used the waist-to-height ratio, a measure that estimates body fat percentage with an accuracy of approximately 90% compared to the gold standard (dual-energy X-ray absorptiometry).

He analyzed data from 2,410 children of multiple ethnicities aged 2 to 19 from the 2021-2023 National Health and Nutrition Examination Survey (NHANES), comparing body mass index and the waist-to-height ratio. The body mass index followed the expected pattern, decreasing between the ages of two and six before returning to its previous level.

The waist-to-height ratio presented a different picture. The average waist-to-height ratio at age two (0.54) was not reached again during childhood and adolescence. This ratio decreased until around age seven, then gradually increased, but did not return to previous levels.

These results suggest that fat mass does not "regain" its weight. Instead, it appears that the rise in body mass index during early childhood reflects an increase in muscle mass and lean tissue. Professor Agbaji explains: "Children are actually undergoing a body composition reset at around the age of four, preparing them for the growth stages that follow that age."

Misinterpretation of Body Mass Index

Professor Agbaji argues that the fat rebound theory is a "false discovery" driven by body mass index, similar to the "obesity paradox" observed in adults. In some cases, it seems that adults with higher body mass indices have a lower risk of mortality, particularly in cases of heart failure.

Subsequent research has shown that this effect is related to increased muscle mass rather than fat gain. When using the waist-to-height ratio instead of body mass index, the relationship between fat and heart disease becomes clear, with higher fat levels associated with worse outcomes. This suggests that the waist-to-height ratio may be a more accurate tool than body mass index for determining harmful fat levels in the body.

Implications for Child Health

Professor Agbaji states: "The growth of lean mass or muscle mass is likely the accurate physiological explanation for the body composition reset occurring in early childhood. It is a natural survival phenomenon that we have mistakenly considered a pathological process, attempting to treat or prevent it for 42 years. Therefore, the term "fat rebound" is incorrect, it is a fallacy derived from body mass index; it is simply the accumulation or growth of muscle mass."

He concludes by saying: "New analyses suggest that this fat rebound phenomenon is not an obesity issue, but an increase in muscle mass, which is beneficial for healthy and natural growth. There is no need for any medical intervention to address a problem that does not exist in children."