A new global definition of childhood obesity is challenging the traditional reliance on body mass index, shifting attention from body size alone to the health effects associated with excess body fat. The approach asks doctors to consider whether a child is actually experiencing physiological problems or limitations in daily life.

Research from the Murdoch Children's Research Institute in Australia found a striking difference between the revised definition and standard BMI calculations. Under the new criteria, 2.1% to 4.9% of children and adolescents qualified as clinically obese.

By comparison, standard BMI measurements classified 7% to 10.8% of young people as obese. The contrast suggests that many children identified by BMI do not have the health complications required for a clinical obesity diagnosis under the revised model.

The definition was developed by an international panel of medical experts convened by the medical journal The Lancet. It requires evidence of both excess body fat and related health problems or limitations affecting everyday activities.

Those health indicators can include elevated blood pressure, abnormal blood sugar, impaired mobility and poor sleep. Doctors using the new model would evaluate these factors alongside body size rather than treating BMI as the sole measure.

Dr. Danielle Longmore, a pediatrician at the Melbourne institute, discussed the revised definition during an appearance on "Fox & Friends." She described the change as part of a broader effort to understand how body fat affects health.

"There's a push to really understand the impact of body fat on health," Longmore said. "BMI is really just measuring size. So, it doesn't really have a way of capturing whether somebody is well or unwell."

Traditional BMI calculations compare a person's weight with their height. The measurement can identify body size, but the new definition focuses on whether excess body fat is accompanied by medical complications or functional limitations.

Longmore noted that many children classified as obese under BMI standards remain active, sleep well, attend school and are otherwise healthy. Under the revised definition, those children may not meet the requirements for clinical obesity if they have no related health problems.

The diagnostic model also reflects assessments that many pediatricians already conduct in clinical practice. These evaluations can include checks of blood pressure, blood sugar, sleep quality and physical mobility, as well as measurements related to body size.

"I think this definition just really clarifies what people have been doing in actual clinical practice ," Longmore said. She noted that physicians typically look for underlying complications rather than relying exclusively on scale weight.

Although the revised definition produced substantially lower clinical obesity rates among children and adolescents, the pattern changed sharply with age. Clinical obesity became more common during adulthood as health problems associated with excess body fat accumulated.

The study examined nearly 800,000 people across their lifespan. Its findings showed clinical obesity rates rising through adulthood and reaching their highest reported level among adults between 65 and 74 years old.

In that age group, the clinical obesity rate reached 52%. The increase reflected the greater prevalence of health conditions linked to excess body fat among older adults under the new diagnostic framework.

The findings arrive as prescription weight loss medicines, particularly GLP-1 receptor agonists, continue to gain popularity. However, Longmore cautioned that the records analyzed in the study were collected before the use of GLP-1 treatments reflected in the current landscape.

That timing means the long term effects of those medicines were not captured in the dataset. Their potential impact on clinical obesity rates remains an area for future research rather than a conclusion addressed by the study.

"This is really just the start of understanding what this new diagnostic entity means, and we really need more research," Longmore said. The revised definition may refine which children receive a clinical diagnosis, but Longmore emphasized that early health interventions and lifestyle prevention remain important for children at risk.