A person's body mass index (BMI) reflects the relationship between their weight and height. It is used to classify people into four weight categories based on the ratio of their weight-to-height.

A person has been classified as underweight if they have a BMI under 18.5. A person with a BMI between 18.5 and 25 is considered normal weight, and one with a BMI between 25 and 29.9 is classified as overweight. A BMI of 30 or more has long been categorized as obese.

The body mass index may not be the best way to determine if someone is obese, however. A study led by researchers at the University of Southern California found that BMI can mischaracterize people who are heavy because they have a great deal of muscle, or not classify as obese those with long, slender legs but excessive and dangerous fat deposits around their waists.

“Clinical obesity is this whole concept of get rid of the scale and get out the tape measure.”

Clinical obesity, a term coined last year by an international panel of obesity specialists, may be a more meaningful way to measure body fat and identify those who have obesity. It is determined based on two ratios and the measurement of a person's waist circumference rather than the one, height-to-weight ratio, that is the basis of the body mass index. It involves gathering a waist-to-hip ratio; a waist-to-height ratio; as well as waist circumference.

The new definition emphasizes the health impact of excess body fat as much as weight itself. The fat around the waist, known as adipose tissue, accumulates deep in the abdomen and causes inflammation. This inflammation increases the risk of obesity-related diseases such as diabetes, hypertension and high cholesterol.

In the British medical journal, The Lancet Diabetes & Endocrinology, the panel defined clinical obesity as, “a chronic, systemic illness characterised by alterations in the function of tissues, organs, the entire individual, or a combination thereof, due to excess adiposity.”

The USC researchers analyzed health information from 5,600 people with an average age of 49 who were enrolled in the National Health and Nutrition Examination Survey. The data included participants' BMI and their waist and measurements of their waist and hip circumference.

They found that about 26 percent of people with a normal BMI and 50 percent of those who were overweight based on their BMI would be considered clinically obese according to the new definition.

“Clinical obesity is this whole concept of get rid of the scale and get out the tape measure,” Brian Lee, principal investigator on the study, told TheDoctor. Patients may want to discuss clinical obesity and its measurements with their doctors.

“I would say, anecdotally, the focus is on BMI,” said Lee, a hepatologist and transplant specialist at Keck Medicine at the University of Southern California. Convenience may be behind that focus: Medical assistants in a busy practice will measure patients' height and weight to calculate their BMI, so providers can determine if counseling or treatment for obesity are necessary.

Clinical obesity may be a more accurate way to measure body fat and identify those who have obesity and are at risk of obesity-related health problems.

Providers using the body mass approach may miss a lot of people who have obesity according to the newer definition. They may also misclassify as obese people with a BMI over 30 whose weight may be more muscle and bone than fat, making it less important to counsel them to lose weight.

Because clinical obesity is a new concept, the researchers want to know the best way to identify and manage patients with a normal BMI but anthropometric measurements associated with obesity. Providers can then screen and treat these patients for obesity-related conditions.

The study is published in the Annals of Internal Medicine.