You can be thin, but still clinically obese: BMI underestimates obesity levels, says study

Most people assume that being thin equals being healthy. But that’s not true. In fact, you can be skinny and still obese, according to experts. Scientists at the University of Southern California found that body mass index (BMI) can drastically underestimate obesity levels.
For decades, BMI has been the gold standard for understanding whether a person is underweight, healthy, overweight, or obese. However, a new study suggests that BMI may often miss cases of obesity. The findings of the study are published in the Annals of Internal Medicine.​

Medical experts have been calculating obesity using body mass index (BMI), a calculation based on a person’s height and weight. However, a new study reveals that this practice allows a subset of people with obesity-related health risks to slip through the cracks. In the past year and a half, many experts have shifted from BMI to a newer measure known as clinical obesity, which identifies obesity based on fat around the abdomen coupled with weight-related health issues.
The new study from Keck Medicine of USC shows that one quarter of people considered to have a normal (healthy) BMI meet the standard of clinical obesity. It also found that 50% of those considered overweight by BMI would be reclassified as obese.
“Many people assume that if their BMI says they are not obese, they don’t have to worry about the many health problems linked to obesity. Our findings show that millions of Americans may already have obesity-related health impacts and may be missing needed health interventions,” Brian P. Lee, MD, MAS, a hepatologist and liver transplant specialist with Keck Medicine and principal investigator of the study, said.
People with normal or overweight BMIs do not currently qualify for pharmacological or surgical treatments for obesity. Also, physicians may not necessarily ask them to make lifestyle modifications that could improve their health, Lee added.

Until now, obesity has often been determined by BMI. People are categorised into four groups depending on their weight-to-height ratio: a BMI under 18.5 is considered underweight; a BMI between 18.5 and 25 is considered normal, or healthy; a BMI between 25 and 29.9 is considered overweight; and a BMI of 30 or over is categorised as obese.
“BMI is problematic because it does not specifically measure body fat and instead reflects total body weight, which includes muscle and bone. So, a muscular person can have a very high BMI but not have excess fat, while someone without much muscle can have a normal BMI but have excess fat, causing health problems,” Lee said.

Clinical obesity, on the other hand, examines the fat around the waist, known as adipose fat. Adipose fat accumulates deep in the abdomen, causing dangerous inflammation in the body’s organs, unlike under-the-skin, or subcutaneous, fat, which is found in other parts of the body. This term was coined in 2025 by the Lancet Diabetes and Endocrinology Commission, an international group of obesity experts. According to Lee, clinical obesity is a more accurate way of determining obesity and obesity-linked health problems._​

Clinical obesity is determined through three measurements: a person’s waist circumference, their waist-to-hip ratio, and their waist-to-height ratio.
“If a person is found to have excess fat by at least two of the three measurements and has evidence of organ, tissue, or other health issues associated with excess fat, such as heart disease or chronic hip or knee pain, they are considered clinically obese,” Lee said.
Obesity is linked to many illnesses, including heart disease, diabetes, high blood pressure, high cholesterol, liver disease, and certain cancers, making it one of the leading causes of preventable death in the country.

The researchers looked at data from some 5,600 adults from the National Health and Nutrition Examination Survey, a nationally representative survey of health and nutrition. The participants had an average age of 49. Their BMI and hip and waist circumferences were measured. When they compared the BMI results with clinical obesity status, they found that approximately 26% of people with a normal BMI and 50% of people with an overweight BMI qualified as clinically obese.
The researchers are hoping that more physicians start to diagnose obesity using the clinical obesity definition rather than BMI, to better inform people of any health risks they may be facing.
“The good news is that obesity can be treated. Whether through lifestyle changes, medication or both, we have effective ways to reduce excess body fat and lower the risk of future health problems. The earlier we identify people at risk, the better chance we have of improving long-term health and quality of life,” Lee said.