What the study did
A paper published in Nature on 11 November tested the obesity framework proposed by the Lancet Commission against the BMI cut-offs that clinics have used for decades. The authors pooled cohorts from several countries with measured height, weight, waist and hip data and followed participants for years, which let them compare how well each definition predicted death and disease.
The commission's approach is deliberately more complicated than a single number. A person counts as obese with a BMI of 40 or above, or with a BMI of 30 or above plus a raised waist circumference or waist-to-height ratio, or with a BMI between 25 and 29.9 plus both of those measures. Chinese thresholds are set lower.
What it found
People whose BMI fell in the overweight range but who met the added waist criteria had mortality risks close to those of people whose BMI was already in the obese range. Under the old system they would have been labelled overweight and sent home. The framework also drops overweight as a separate risk category and folds those patients into obesity when their measurements support it.
The paper separates clinical obesity, where excess fat is already damaging organs or limiting daily function, from preclinical obesity, where the risk is elevated but no organ damage has appeared. That split is meant to guide treatment: clinical cases need intervention now, preclinical cases need monitoring and lifestyle support.
The counterargument
Not everyone is convinced the extra measurements earn their place. A separate US analysis published this year compared definitions against DEXA body scans and found that a simple rule, BMI or waist circumference alone, had better specificity and fewer false positives than the full commission framework, which also asks for hip measurement that few clinics record.
That critique matters for adoption. Waist measurement is quick but poorly standardised, and adding steps to a primary-care visit is a real cost. The commission's answer is that BMI alone misclassifies muscular patients and misses central fat in people with a normal weight.
Why it matters
About one in eight people worldwide live with obesity, and the new class of GLP-1 drugs has made accurate diagnosis a spending question as well as a medical one. Insurers and health systems need a definition that identifies who will benefit. This study gives the commission's version its first large multinational test, and the result supports using it.