A 42-year-old childhood obesity theory may be wrong

New findings challenge a 42-year-old theory about why children's body mass index (BMI) declines after infancy and then begins climbing steadily from about age 6. The pattern has long been described as the "adiposity rebound," based on the idea that body fat first decreases and then returns.

However, the new analysis suggests that the later increase in BMI is not caused by a rebound in fat. Instead, it may largely reflect the normal growth of muscle and other lean tissue.

The research, led by Professor Andrew Agbaje, a physician and associate professor of clinical epidemiology and child health at the University of Eastern Finland, Kuopio, Finland, was presented at this year's European Congress on Obesity in Istanbul, Turkey and published in The Journal of Nutrition.

Why the Adiposity Rebound Theory Matters

Questioning the existence of the "adiposity rebound" could have important consequences for childhood health care. Since the theory was introduced, some physicians, including pediatricians, have treated the pattern as a meaningful warning sign that might be prevented or modified through diet and lifestyle changes.

If the rise in BMI is actually a normal result of muscle development, however, efforts to stop it may be addressing a problem that does not exist.

The concept dates back to 1984, when French researcher Marie Françoise Rolland-Cachera and her colleagues described "adiposity rebound" in a paper published in The American Journal of Clinical Nutrition.

The researchers reported a connection between the age at which a child's BMI began rising again and the child's BMI adiposity at age 16 years. An early rebound (before age 5.5 years) was associated with greater adiposity later in adolescence than a rebound occurring after age 7 years. Some later studies reported similar associations.

How Children's BMI Normally Changes

BMI tends to follow a recognizable pattern during childhood. It rises quickly during infancy and reaches a high point at about age 1 year. It then gradually declines, reaching its lowest level at around age 4 years, before beginning to rise again.

By age 6 years, a child typically returns to the same BMI recorded at age 2 years. This pattern occurs across the childhood population and became known as the "rebound."

Researchers have often focused on the timing of the decline and subsequent rise. They proposed that children whose BMI begins climbing at a younger age may reach a higher BMI later in life and face a greater risk of BMI-obesity.

The theory was based partly on statistical models suggesting that when BMI reaches its lowest point earlier, it also begins rising earlier, potentially leading to a higher BMI in later childhood or adolescence.

A Statistical Link May Not Reflect Biology

Professor Agbaje argues that the BMI pattern should not be compared with major biological transitions such as puberty.

Puberty occurs in nearly every child who reaches adulthood, but unusually early puberty has been linked to health risks supported by known biological mechanisms. According to Agbaje, no comparable biological explanation supports the idea that an early "adiposity rebound" causes later obesity.

"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."

Over the past several decades, researchers have also conducted clinical trials intended to influence the timing of the supposed rebound. Agbaje says the results provide further evidence that the pattern is a fixed part of normal growth rather than a disease process.

A Long-Term Diet Trial Could Not Change the Pattern

One randomized controlled trial in Finland followed participants from 7 months of age until age 20 years.

Infants in the intervention group were introduced to a heart-healthy diet with a relatively low intake of saturated fat and cholesterol. Parents and children received dietary counseling and nutrition education from the time the children were 7 months old through early adulthood. The control group did not receive the intervention.

Despite the long-term nutritional program, there was no difference between the groups in the average age at which BMI declined and then began rising again by age 6.

"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."

Measuring Body Fat More Directly

To determine whether body fat truly rebounds, Agbaje examined a measurement that more closely reflects adiposity than BMI.

BMI is calculated using weight and height, but it cannot distinguish between fat, muscle, bone, and other tissues. A rise in BMI can therefore occur even when a person is gaining healthy muscle rather than excess fat.

The new study used waist circumference-to-height ratio (WHtR), which estimates body fat/adiposity with around 90% accuracy when compared with the gold standard (dual-energy X-ray absorptiometry) measure of fat mass.

Agbaje analyzed information from 2410 multiracial children aged 2 -- 19 years who participated in the US National Health and Nutrition Examination Survey (NHANES) 2021-2023 cycle. The analysis compared childhood patterns in both BMI and WHtR.

The average BMI at age 2 years was 17.1 kg/m2. After declining significantly between age 2 and age 6 years (see graph in full paper), BMI returned to that same average level by age 6 years. On its own, this result appeared to support the traditional adiposity rebound theory.

Body Fat Did Not Actually Rebound

The WHtR results told a different story.

The average WHtR at age 2 years was 0.54, but children never returned to that level at age 6 years or at any other point during childhood and adolescence.

WHtR continued to fall until about age 7 years. It then increased through the rest of childhood and late adolescence, but it never recovered to the level recorded at age 2 years.

According to Agbaje, this means there was no true rebound in fat mass. Instead, the increase in BMI seen between about ages 5 and 7 appears to be caused by the growth of muscle and other lean tissue.

"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.

A BMI Pattern Mistaken for Fat Gain

Agbaje describes the adiposity rebound theory as a BMI-driven "false discovery." He compares it with the so-called "obesity paradox" reported in some adult studies.

The "obesity paradox" refers to findings suggesting that people living with obesity may, under certain circumstances, have lower mortality rates than people with a BMI in the normal range. In studies of heart failure and mortality, BMI has sometimes produced a U-shaped pattern that appears to suggest that a higher BMI protects against heart disease.

Later research indicated that the apparent protection may come from greater muscle mass rather than excess body fat. Because BMI counts muscle as part of total weight, it can create misleading associations.

When WHtR was examined in randomized clinical trials involving heart failure, the relationship was linear. Greater fat mass was consistently associated with worse cardiovascular disease. Agbaje argues that this makes WHtR more useful than BMI for identifying excess fat and the health risks connected with it.

Researchers Call the Rebound a BMI Fallacy

"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."

Agbaje believes the findings could change how excess body fat is identified in young people.

"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."

Normal Muscle Growth May Need No Intervention

The analysis suggests that the familiar rise in childhood BMI is not necessarily evidence of an obesity-related process. It may instead represent a healthy transition as children build muscle and prepare for later stages of growth.

"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."

Agbaje also reports that his research team has released a freely accessible WHtR calculator designed to help detect excess fat in children and adolescents.

Reporting from ScienceDaily contributed to this story.