New research is challenging the 42-year-old theory of “adiposity rebound,” which describes the decline in children’s body mass index (BMI) after infancy followed by a steady rise beginning at around age 6. The traditional explanation assumes that body fat decreases during early childhood and then rebounds.
However, a new analysis suggests that the childhood BMI increase may not reflect a rebound in body fat. Instead, it may largely be explained by the normal development of muscle and other lean tissues.
The research was led by Professor Andrew Agbaje, a physician and associate professor of clinical epidemiology and child health at the University of Eastern Finland in Kuopio, Finland. The findings were 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 implications for childhood health care. Since the theory was introduced, some physicians, including pediatricians, have viewed the timing of a child’s BMI increase as a potential warning sign that could be influenced through diet and lifestyle changes.
If the increase in BMI is primarily caused by normal muscle development, however, attempts to prevent it could be targeting a process that is not harmful.
The adiposity rebound concept dates back to 1984, when French researcher Marie Françoise Rolland-Cachera and colleagues described the pattern in a paper published in The American Journal of Clinical Nutrition.
The researchers reported an association between the age when a child’s BMI began rising again and the child’s adiposity at age 16. An early rebound, defined as occurring before age 5.5 years, was associated with greater adiposity during adolescence than a rebound occurring after age 7. Some later studies reported similar findings.
How Children’s BMI Normally Changes
Children’s BMI typically follows a recognizable pattern. It rises rapidly during infancy and reaches a peak at approximately age 1. BMI then gradually declines, reaching its lowest point at around age 4, before beginning to increase again.
By about age 6, children generally return to the BMI level recorded at age 2. Because this pattern occurs across the childhood population, it became known as the “adiposity rebound.”
Researchers have traditionally focused on when the BMI decline ends and the subsequent increase begins. They proposed that children whose BMI starts rising at a younger age may develop a higher BMI later in life and face a greater risk of obesity.
This theory was partly based on statistical models suggesting that children who reach their lowest BMI at an earlier age also begin gaining BMI earlier, potentially resulting in a higher BMI during later childhood or adolescence.
A Statistical Link May Not Reflect Biology
Professor Agbaje argues that the childhood BMI pattern should not be compared with major biological transitions such as puberty.
Puberty occurs in nearly every person who reaches adulthood, while unusually early puberty has been linked to health risks supported by established biological mechanisms. According to Agbaje, there is no comparable biological explanation showing that an early “adiposity rebound” directly 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 designed to influence the timing of the supposed rebound. Agbaje says the results provide additional evidence that the pattern may be 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 they reached 20 years old.
Infants in the intervention group were introduced to a heart-healthy diet containing relatively low amounts of saturated fat and cholesterol. Parents and children received dietary counseling and nutrition education from age 7 months through early adulthood, while the control group did not receive the intervention.
Despite the long-term nutrition program, the two groups showed no difference in the average age when BMI stopped declining and began rising again by approximately 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 from a person’s weight and height, but it cannot distinguish between fat, muscle, bone, and other tissues. As a result, BMI can increase when a person gains healthy muscle rather than excess body fat.
The study used the waist circumference-to-height ratio, or WHtR, as an alternative measure. WHtR estimates body fat and adiposity with approximately 90% accuracy when compared with dual-energy X-ray absorptiometry, or DXA, the gold-standard method for measuring fat mass.
Agbaje analyzed data from 2,410 multiracial children and adolescents aged 2 to 19 years who participated in the 2021–2023 cycle of the US National Health and Nutrition Examination Survey (NHANES). The analysis compared patterns in both BMI and WHtR during childhood.
The average BMI at age 2 was 17.1 kg/m2. After declining significantly between ages 2 and 6, BMI returned to the same average level by age 6. Viewed alone, this finding appeared to support the traditional adiposity rebound theory.
Body Fat Did Not Actually Rebound
The WHtR findings told a different story.
The average WHtR at age 2 was 0.54, but children did not return to that level at age 6 or at any other point during childhood and adolescence.
Instead, WHtR continued to decline until approximately age 7. It then increased throughout the rest of childhood and late adolescence but never returned to the level recorded at age 2.
According to Agbaje, these findings suggest that there was no true rebound in fat mass. The BMI increase seen between approximately ages 5 and 7 may instead be explained by the development of muscle and other lean tissues.
“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, in certain circumstances, people living with obesity may 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 cardiovascular disease.
Later research indicated that the apparent protection may be related to greater muscle mass rather than excess body fat. Because BMI includes muscle in its weight measurement, it can produce misleading associations.
When researchers examined WHtR in randomized clinical trials involving heart failure, the relationship was linear: greater body fat was consistently associated with worse cardiovascular outcomes. Agbaje argues that WHtR may therefore be more useful than BMI for identifying excess fat and related health risks.
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 health professionals identify excess body fat in children and adolescents.
“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 Not Require 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 developmental 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 assess excess body fat in children and adolescents.
Source: www.sciencedaily.com