A study led by King's College London adds to growing evidence that body mass index, or BMI, should play a smaller role in assessing obesity, finding that it misclassified some study participants.

The body mass index uses a simple formula based on a person's height and weight. A person with a BMI between 18.5 and 24.9 is considered to be at a healthy weight. Those with a BMI between 25 and 29.9 are considered overweight, while those with a BMI of 30 or higher are considered obese.

RELATED STORY | Why some people classified as obese may have lower heart risks than expected

But researchers say BMI does not account for differences in body composition or how weight is distributed throughout the body.

Researchers said BMI does not accurately measure body fat levels or fat distribution, cannot assess how excess fat affects organ function and may fail to distinguish between healthy individuals and those with obesity-related disease.

Researchers analyzed data from 844,108 adults in China, the United Kingdom, the United States and Spain. They found that BMI alone frequently misclassified obesity, missing obesity-level body fat in up to 8.6% of participants and incorrectly classifying up to 3.1% of people as having obesity.

Researchers suggest using BMI alongside measures such as waist circumference and waist-to-height ratio to confirm excess body fat. They also recommend assessing whether excess fat is already causing disease or primarily increasing the risk of future health problems.

"Body mass index alone cannot tell us whether someone is already ill or at risk of illness, or what care they need, leaving people vulnerable to both undertreatment and overtreatment," said Francesco Rubino of King's College London. "The new definition makes personalized, proportionate care possible, whereas a uniform BMI-based definition prevents it. Getting the scope of care right, treating existing disease or reducing risk, and matching its intensity to individual needs matters enormously, both for the billion-plus people affected and for health care systems. People with obesity, like everyone else, deserve care matched to their needs: no less, and no more."

RELATED STORY | Excess belly fat could reveal more about the lifespan of older adults than BMI

In 2023, the American Medical Association adopted recommendations calling for reduced reliance on BMI as a standalone measure in medicine. The organization recommends considering BMI alongside other factors, including visceral fat, body adiposity index, body composition, relative fat mass, waist circumference, and genetic and metabolic factors.

For someone who is 6 feet tall, a weight of about 184 pounds is considered overweight, while a weight of about 221 pounds is considered obese based on BMI calculations.

For someone who is 5 feet 6 inches tall, a weight of about 155 pounds is considered overweight, while a weight of about 186 pounds is considered obese.