New Obesity Definition Counts Fewer Children But Far More Older Adults

New Obesity Definition Tells a Different Story for Kids and Older Adults

For decades, one number has largely decided who gets labeled as having obesity: body mass index (BMI), a formula built from height and weight. A new study of nearly 800,000 people found that the number tells a very different story depending on age. Among adults ages 65 to 74, 52% met a newer definition of obesity that accounts for excess body fat and related health problems, far more than the 34.4% flagged by BMI.

Published in eClinicalMedicine, a journal in the Lancet family, the analysis found the pattern flips at the other end of life. Young children were less likely to qualify under the new definition than BMI alone would indicate.

That gap matters because a label steers care. Under BMI alone, many older adults with weight-related health problems may go uncounted, while some children may be labeled before any problems show up.

Clinical Obesity Requires Excess Body Fat Plus Signs of Related Health Problems

A group of international experts, the Lancet Diabetes & Endocrinology Commission, proposed the definition, called clinical obesity, in 2025. It asks two questions. First, does the person carry excess body fat, confirmed by more than BMI, such as waist size? Second, does the person have signs of related health problems, such as high blood pressure, trouble with the heart, liver, or kidneys, or difficulty with everyday tasks like bathing and getting dressed? The approach treats obesity as a chronic disease rather than only a risk factor for other illnesses. Excess fat without those signs is called preclinical obesity.

A team led by the Murdoch Children’s Research Institute in Australia applied the definition to records from 11 existing health studies in Australia, Canada, Estonia, Finland, the Netherlands, the United Kingdom, and the United States, collected between 1996 and 2023. The records covered children and adults ages 4 and older, including 97,071 children. Until now, research on clinical obesity in children amounted to a single study of 111 kids in Mexico.

Clinical Obesity Overtook BMI-Defined Obesity Starting at Ages 55 to 64

Among young children, the new definition was the stricter one. At ages 4 to 5, 3.7% met the clinical criteria versus 8.8% by BMI. BMI counted more people through middle age. At ages 35 to 44, for example, 26.3% met the BMI cutoff versus 15.1% under the clinical definition, and preclinical obesity peaked there at 27.5%. Then clinical obesity pulled ahead in the 55-to-64 group, and the gap kept widening with age, even as clinical obesity itself peaked at ages 65 to 74.

Men and women followed different paths. Men had higher rates from the late teens through their mid-40s, including 18.6% versus 13.9% for women at ages 35 to 44, and women overtook them after age 55. At ages 65 to 74, 57.1% of women met the criteria compared with 44.1% of men. The authors say survival bias (people who live longest may differ from those who do not) and gaps in the data may partly explain women’s higher late-life numbers. UK Biobank, the largest data set, included no one older than 73. Education made a smaller difference, with slightly higher rates among people with less schooling.

Raised Blood Pressure Was a Frequent Sign of Clinical Obesity, Even in Children

Among people who met the clinical definition, raised systolic blood pressure, the top number in a reading, was common at every age and appeared in 87% of tested children ages 4 to 9. Other signs followed life stages: abnormal liver blood tests in early and middle adulthood, kidney-related findings in childhood and after age 75, and joint and muscle problems among the oldest participants.

In the adult groups with complete body measurements, nearly everyone with clinical obesity also had a waist measurement that was large relative to height. A tape measure may be a handy way to spot excess body fat, although waist size alone cannot establish clinical obesity.

Missing Measurements Mean the 52% Figure Is a Rough Estimate

No study had every measurement the definition calls for, so the authors describe their numbers as “preliminary rather than definitive.” Missing information on breathing, nervous system, and reproductive problems probably pushed counts down. Meanwhile, high blood pressure, joint trouble, and sleep apnea become common in later life regardless of weight, which could push counts up when one of them was a person’s only sign. Researchers also had to assume that abnormal lab results stemmed from excess body fat, although other conditions can cause them. The authors call for long-term studies that follow people over time and gather complete data.

Most records came from wealthy countries with mostly white populations, and most captured different people at one point in time rather than following anyone as they aged.

Nothing here proves the new definition will improve care. Still, the two measures disagree at nearly every age, with BMI counting more people before the mid-50s and fewer after. For children, a high BMI may be better treated as a reason to look closer than as a verdict. For older adults, pairing BMI with signs of real health problems looks like a more informative way to decide who needs treatment.

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Disclaimer: This article is based on a published research study and is intended for general informational purposes only. It is not medical advice. Consult a qualified healthcare professional with questions about personal health or before making any health decisions.

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