Science for a Healthier Tomorrow
A newer “clinical obesity” definition requires excess body fat (adiposity) plus clinical or laboratory evidence that the fat is already affecting health.
A multi-cohort, cross-sectional analysis pooled 786,557 records from 11 population cohorts in seven high-income countries — 97,071 of them children, ages 4 and older — to ask what a newer “clinical obesity” definition counts compared with the long-used measure, body mass index (BMI). The definition, proposed by a 2025 Lancet Commission, requires excess body fat (adiposity) plus clinical or laboratory evidence that the fat is already affecting health. By that definition, clinical obesity peaked at 52.0% among adults aged 65–74, versus 34.4% by BMI. Among children aged 4–5 the pattern reversed: 3.7% met the clinical definition against 8.8% by BMI. The authors describe the estimates as preliminary rather than definitive, because no cohort captured every criterion the definition requires — and the study is descriptive, so it can count and compare groups, not show that body fat caused any particular health problem.
What this means
Applying a preliminary “clinical obesity” definition to nearly 800,000 people across 11 high-income cohorts counted far more older adults than BMI — 52% versus 34% at ages 65–74 — but fewer young children. The numbers come from a cross-sectional analysis: they describe how many people met each definition at a single point in time, not what causes what. They are labeled PRELIMINARY because no cohort had all the measurements the definition requires; sparse data on some criteria probably undercount clinical obesity, while common late-life conditions used as the sole qualifying sign may overcount it. The figures also come mainly from high-income, majority-White cohorts that were healthier and less deprived than the general population, and UK Biobank, the largest dataset, included no one older than 73.
What the analysis counted, and how
The paper compares two ways of defining obesity across the life course. The comparator is BMI-defined obesity, using CDC cut-points. The newer measure is the Lancet Commission’s 2025 “clinical obesity” definition: excess adiposity plus clinical or laboratory evidence of obesity-related health problems (sequelae). The Commission also proposed a second category, “preclinical obesity” — excess fat without those signs.
The pooled records came from 11 cohorts in Australia, Canada, Estonia, Finland, the Netherlands, the UK and the US, with data collected between 1996 and 2023. Most participants appear once, at a single point in time; in a minority of the underlying cohorts, some participants contributed data at more than one point — so the analysis is cross-sectional and cannot show how individuals’ status changes.
How the definition shifted the count across ages
Clinical obesity was the lower of the two counts through childhood and early-to-mid adulthood, then overtook BMI-defined obesity from age 55–64 onward. In childhood and adolescence, clinical obesity ranged from 2.1% to 4.9%, against 7.0% to 10.8% by BMI — the clinical definition counted roughly half as many children. At ages 4–5, 3.7% met the clinical definition versus 8.8% by BMI.
The gap narrowed through early and mid adulthood and then reversed. At ages 65–74, the peak, 52.0% of adults met the clinical obesity definition against 34.4% by BMI — a gap of about 18 percentage points; the paper warns its operationalization choices are likely to shift these estimates.
Sex differences ran in opposite directions across life. In early-to-mid adulthood, clinical obesity counted 18.6% of men versus 13.9% of women at ages 35–44; by ages 65–74 it counted 57.1% of women versus 44.1% of men. The paper also reports an education gradient: clinical obesity spanned 1.3–48.2% in the most-educated group and 3.6–55.0% in the least-educated, only marginally lower at the top.
Why the authors call the estimates preliminary
No cohort carried every measurement the definition calls for. Missing or sparse data on central nervous system, respiratory, lymphatic and reproductive criteria probably under-count clinical obesity; at the same time, common late-life conditions — high blood pressure, musculoskeletal problems, sleep apnea — may have raised the count where such a condition was the sole qualifying sign.
Three further limits matter. First, researchers assumed abnormal lab results stemmed from excess body fat, but could not verify that adiposity caused any individual’s finding. Second, cohorts differed in how they measured problems — some used diagnostic codes, others questionnaires — so threshold choices may shift the estimates. Third, the children’s BMI comparisons used CDC growth charts, which differ from other international standards.
The population boundary is narrow. The cohorts were predominantly high-income and majority-White, and participants were healthier and less deprived than the general population. UK Biobank, the largest single dataset, included no one older than 73, so survival bias may inflate the late-life figures, especially for women. The analysis also reaches only down to age 4: it says nothing about infants or toddlers.
What qualified people, clinically
Among people who met the clinical definition, the signs varied by age. Raised systolic blood pressure was common at every age — present in 87% of children aged 4–9 tested. Liver-test abnormalities were frequent in early and mid adulthood, kidney findings appeared in childhood and after age 75, and joint and muscle problems dominated in the oldest groups.
What this says — and does not say — about BMI
The paper reports a definition comparison, not a clinical directive. It does not show that BMI should be abandoned, and it does not establish that clinical obesity is the better guide to anyone’s care. Its own field results are mixed: a separate 2026 analysis of 642 people with fatty liver disease found the clinical-obesity definition flagged more advanced disease than BMI did, while a 2025 study concluded that waist circumference still predicted insulin resistance better than the new definition. The Commission definition it applies dates to 2025 and is still debated.
Funding and access
The paper states no study-specific funding. Its Declaration of interests, read from the open-access article, opens by declaring “No study specific conflicts of interest to declare” and then lists named author disclosures; we do not enumerate them here.
Terms explained
- Body mass index (BMI): a measure built from height and weight. BMI-defined obesity here uses CDC cut-points; it does not distinguish fat from muscle or say where fat sits.
- Clinical vs preclinical obesity: clinical obesity requires excess body fat plus clinical or laboratory signs that it is already affecting health; preclinical obesity is excess fat without those signs.
- Adiposity: the amount of body fat a person carries.
- Sequelae: the health problems that follow from a condition — here, the conditions a definition counts as consequences of excess fat.
- Cross-sectional study: a study that measures people at a single point in time. It can show how common something is and how groups differ, not how individuals change or what causes what.
