A new study, the results of which were published in The Journal of Nutrition, suggests rethinking one of the key indicators of childhood obesity and resolves a long-standing scientific puzzle related to changes in children’s body weight.

The focus is on the so-called “adiposity rebound” — a stage when a child’s body mass index (BMI) first decreases in early childhood and then begins to rise again at around ages 5–6. For more than 40 years, it has been believed that an early onset of this increase is associated with a higher risk of obesity later in life.

However, the new study shows that this rise in BMI may be misleading. Scientists analyzed data from more than 2,400 children and adolescents and found that during the period when BMI begins to increase, another indicator — the waist-to-height ratio — continues to decline.

This means that the increase in BMI at this age reflects not the accumulation of fat mass, but the growth of “fat-free” tissues — primarily muscles and bones. In other words, it is a normal growth process rather than the onset of obesity.

Thus, the traditional interpretation of “adiposity rebound” as an early signal of obesity risk may be partially incorrect. Researchers emphasize that BMI cannot distinguish between fat and muscle mass, which may lead to inaccurate conclusions when used in children.

As a more accurate indicator, the waist-to-height ratio is proposed, as it better reflects the level of body fat, especially abdominal fat, which is associated with disease risks, including cardiovascular conditions and diabetes.

The authors describe the observed effect as a kind of “recalibration of body composition” — a shift toward active growth of muscle and bone tissue in childhood. This may be an important sign of normal development rather than a pathological process.