What is obesity and why does it matter?
Obesity refers to excess body fat. Traditionally, the medical establishment has defined obesity as body mass index of 30 or more. However, this definition has several well-known problems. First, most people have a hard time visualizing body mass index, defined as body weight measured in kilograms divided by height measured in meters squared. Second, certain athletes and others who have hefty muscle and bone mass relative to their height have obesity according to this definition. Third and most importantly, the location of excess body fat matters greatly in terms of health; belly fat and fat infused in internal organs, such as liver, pancreas, and muscle, predict significantly worse health than fat stored under the skin elsewhere in the body, such as the hips.
A new definition of clinical obesity
In January 2025, the Lancet Diabetes & Endocrinology Commission published a report in which the 58 Commissioners proposed a definition and diagnostic criteria for clinical obesity. Previously, obesity was considered a risk factor rather than a disease. The Commissioners defined obesity as a condition of illness that arises directly from the effects of excess body fat on the function of bodily organs and tissues. Specifically, persons with clinical obesity exhibit either: 1) body mass index of 30 or more (the traditional cut point for obesity) and at least one elevated anthropomorphic measure (such as elevated waist-to-height ratio, elevated waist circumference, elevated waist-to-hip ratio), or 2) body mass index below 30 and the presence of at least two elevated anthropomorphic measures.
The Commissioners also developed diagnostic criteria for clinical obesity that doctors could use in their practices. Clinical diagnosis of obesity requires evidence of the following: 1) reduced organ function (signs and symptoms of organ system or tissue abnormality) due to obesity, or 2) substantial age-adjusted limitations of daily activities that reflect effects of obesity on mobility or on other activities of daily life. The Commissioners hope that the proposed definition will help doctors and policy makers identify people who most need therapeutic help to lessen the harmful effects of obesity.
Testing the new definition of clinical obesity using US subjects
Publication of proposed new definition and diagnostic criteria for clinical obesity by the Lancet Diabetes & Endocrinology Commission in 2025 offered an opportunity to apply the new definition and diagnostic criteria to the All of US cohort. The study used cross-sectional and longitudinal data from 301,026 participants with a median age of 54 years with a median follow-up of 4 years. At baseline, 43 percent of the participants had obesity according to the traditional definition of obesity (body mass index of 30 or more). But 69 percent of the participants had obesity according to the new definition, a 37 percent increase. The increase arose due to inclusion of participants with anthropometric obesity (meaning at least one elevated measure of body fat location, such as waist-to-height ratio). In addition, obesity at baseline was more prevalent (79 percent) in participants aged 70 years or more according to the new definition.
Clinical obesity (which requires evidence of bodily organ or tissue malfunction) was present in 36 percent of participants at baseline according to the new definition and was much more prevalent (54 percent) in participants aged 70 and older. After 4 years of follow-up and after adjusting for confounding factors, the presence of clinical obesity at baseline predicted a whopping five-fold increase in risk of developing diabetes and a nearly five-fold increase in risk of having a heart attack or acute heart failure or stroke compared to participants with no obesity or organ failure. Preclinical obesity also predicted significantly increased risks of diabetes and heart attack, acute heart failure, and stroke. Implementing the new definition of clinical obesity may greatly change the obesity treatment landscape and hopefully direct treatment and preventive measures to patients who will benefit most.
Testing the new definition of clinical obesity using UK subjects
In 2025, the Lancet Diabetes & Endocrinology commission proposed a new, hopefully more clinically meaningful definition of obesity than simply body mass index of 30 or greater. Clinical obesity was defined as body mass index of 30 or more and at least one objective sign of obesity-related illness. Preclinical obesity was defined as body mass index of 30 or more without evidence of obesity-related illness.
Researchers in Europe used data from 290,664 participants with an average of 55 years in the UK Biobank to test whether participants with clinical or preclinical obesity would have different risks of mortality. Fifteen percent of participants had clinical obesity, while 36 percent had preclinical obesity and 49 percent had no obesity. Clinical obesity predicted significantly higher risks of all-cause and cause-specific mortality compared to pre-clinical obesity or no obesity during a median follow-up of 14 years. For example, compared to no obesity, the risks of dying of any cause during follow-up were significantly 18 and 56 percent higher for participants with preclinical and clinical obesity, respectively. The all-cause mortality risk for participants with obesity per the traditional definition of obesity was 36 percent higher than for normal weight participants. Thus, the new definition of clinical obesity appears to identify persons with higher risk of dying compared to the traditional definition of obesity. Thus, treatments for obesity can be directed preferentially to persons with clinical obesity and who have the highest mortality risk.
What to do
If the medical establishment, insurance companies, and governments accept this new definition of obesity, clinical obesity, and preclinical obesity, the rules relating to treatment of obesity will likely change substantially. For example, patients with pre-clinical obesity might not be eligible for insurance-covered and high-cost GLP-1 agonist drugs. Be alert for news of changing obesity-treatment insurance coverage.
