Experts look beyond BMI to diagnose obesity

Experts have proposed a subtle approach to diagnosing obesity that shifts the focus from BMI alone. Instead, they urged using other body fat measures such as waist circumference or direct fat mass measurement and signs and symptoms of ill health to confirm excess body fat.

Obesity or excess adiposity is currently defined using a person’s BMI, calculated by dividing weight in kilograms by height in metres squared. A BMI over 30 kg/m² is considered obese. The blanket definition means too many people are being diagnosed as obese.

“However, BMI alone does not tell us whether a person has health problems related to excess body fat,” said the experts, who formed part of The Lancet Diabetes & Endocrinology Commission. “Athletes with a relatively high muscle mass, for example, may have a higher BMI. Even with a BMI over 30 kg/m², their higher weight is due to excess muscle rather than excess fatty tissue.”

Conversely, some people with excess body fat do not always have a BMI over 30. This may hold for tall people or those with abdominal fat but with a healthy BMI.

“Therefore, BMI is an imperfect tool for diagnosing obesity,” said the experts. “BMI should be used only as a surrogate measure of health risk at a population level for epidemiological studies or screening purposes, rather than as an individual measure of health.”

Obesity categories

Led by Professor Francesco Rubino from King's College London, UK, the Commission comprised 58 experts from various specialties and countries and was endorsed by 75 medical organizations. Together, they sought to develop an easier approach to define and diagnose obesity using available evidence. Two obesity categories—clinical and preclinical— were defined based on signs and symptoms of poor health due to excess adiposity. [Lancet Diabetes Endocrinol 2025;doi:10.1016/ S2213-8587(24)00316-4]

Clinical obesity is now defined as having signs and symptoms of reduced tissue or organ function and/or difficulty with activities of daily living (bathing, going to the toilet, eating, or dressing) due to excess adiposity. This can manifest as breathlessness, bone and joint abnormalities, fatty liver disease, and increased blood pressure. Clinical obesity can lead to severe end-organ damage, causing life-altering and potentially life-threatening complications such as heart attack, stroke, and renal failure.

On the other hand, pre-clinical obesity is defined as having high levels of body fat without evidence of reduced tissue or organ function due to obesity. These individuals can complete daily activities unhindered. However, they are generally at increased risk of developing clinical obesity, heart disease, certain types of cancers, mental disorders, and type 2 diabetes.

“People with clinical obesity may require more time-sensitive therapeutics like drugs and surgery while those with preclinical obesity may be better suited for prophylactic interventions like diet and exercise,” explained Rubino.

Shifting the focus

While BMI may still hold clinical utility, the commission said BMI can overestimate or underestimate body fat. Hence, obesity status should be confirmed with at least one additional anthropometric measure, such as waist circumference, waist-to-hip ratio, and waist-to-height ratio, or direct fat mass measurement by dual-energy x-ray absorptiometry (DEXA) or bioimpedance.

Excess body fat can be assumed in those with a BMI of >40 kg/m², and no further confirmation is required.

Clinical implication

Clinical obesity is a disease requiring effective intervention. “Our reframing acknowledges the nuanced reality of obesity and allows for personalized care without stigma or blame,” said Rubino.