A new international study led by the Murdoch Children's Research Institute has found that moving away from the body mass index as the sole measure of obesity changes clinical diagnosis rates across all age groups.
The research evaluated a revised definition of clinical obesity recently proposed by a Lancet commission. Under the new criteria, which examine how body fat actually affects health and daily function rather than just recording weight, fewer children meet the threshold for clinical obesity, while the opposite is true for older adults.
The findings draw on extensive data from eleven studies and databases, encompassing 786,557 participants across seven countries: Australia, Canada, Estonia, Finland, the Netherlands, the United Kingdom and the United States.
The body mass index, calculated using a person's weight and height, has long been the standard medical tool for monitoring population weight trends. However, the measurement has recognized limitations. It cannot indicate where fat is distributed in the body, it cannot distinguish between fat and muscle mass, and it does not show how fat affects an individual's internal organs or overall health.
In 2025, a commission from the Lancet Diabetes and Endocrinology, a leading peer reviewed medical journal, proposed a shift in how doctors diagnose the condition. The commission suggested that practitioners should consider additional measurements, such as waist circumference or direct body fat assessment, alongside weight and height.
Crucially, the commission stated that a clinical obesity diagnosis requires concrete evidence that excess body fat is causing organ or tissue dysfunction, or significantly restricting daily activities. These restrictions must be evaluated in the context of a patient's age and might involve difficulties with basic movement or personal self care.
Rates drop for youth
Applying this functional framework to children and adolescents, researchers recorded clinical obesity rates between 2.1 percent and 4.9 percent. When using traditional criteria based strictly on the body mass index, obesity rates for the exact same young demographic were notably higher, ranging from 7 percent to 10.8 percent.
This diagnostic picture inverted as participant age increased. From the 55 to 64 age group onwards, clinical obesity became more common than obesity defined merely by the body mass index. The highest rate of clinical obesity was recorded among participants aged 65 to 74, reaching a total of 52 percent.
The study also identified distinct differences between sexes. Men presented higher rates of clinical obesity in early and middle adulthood, whereas women showed higher rates in the older age brackets.
Focusing on function
Dr Danielle Longmore from the Murdoch Children's Research Institute noted that this more comprehensive medical assessment could influence decisions on when clinical intervention is necessary. She indicated that the diagnostic shift could affect dietary recommendations and influence the use of pharmaceutical medications prescribed to treat obesity.
The Murdoch Children's Research Institute is the largest child health research institute in Australia. Based in Melbourne, the organisation focuses on discovering the causes of childhood conditions and developing preventive treatments.
The researchers emphasised that the lower diagnostic rates in children using the new criteria do not mean that young people have lost weight or that excess fat no longer matters for their future health.
Instead, the Lancet commission distinguishes clearly between clinical and preclinical obesity. In preclinical cases, an individual carries excess body fat but maintains normal organ function without the specific physical limitations required for a clinical diagnosis.
Preclinical care
Preclinical obesity still carries significant health risks. The proposed framework includes ongoing patient monitoring, evidence based health advice, and appropriate medical care designed to reduce the likelihood of future complications.
By focusing on physical symptoms, daily functionality, and the actual needs of the patient alongside standard measurements, the diagnostic process in a doctor's clinic becomes much broader. The ultimate goal of the new approach is to provide highly targeted care. This involves intervening early when a patient's health is already affected, while focusing on lifestyle prevention when the medical risk remains in the future.
The researchers cautioned that this study evaluates a proposed diagnostic framework using data exclusively from seven high income nations. The published results do not automatically represent global diagnostic rates, nor do they mean that existing hospital treatment protocols will change immediately. However, the study suggests that the traditional scale will remain a useful medical tool, provided it is used as just one part of a much more complete health examination.
