Obesity may be more common than we thought

Scientists have long estimated that about 40 percent of adults in the United States have obesity. But a new study suggests that number could be much higher. The study is based on a newer, updated definition of what obesity actually means. Researchers say the tools doctors have been using to measure obesity may be leaving out a lot of people who actually have the disease.

For many years, doctors have used something called body mass index, or BMI, to decide if a person has obesity. BMI is a number that compares a person's weight to their height. It is easy and cheap to calculate, which is why so many doctors have relied on it. But experts have grown more and more critical of BMI because it cannot tell the difference between fat, muscle, and bone.

In 2025, an international group of scientists came up with a new definition of obesity. They called it 'clinical obesity' and described it as a long-term illness caused by too much body fat that damages organs and tissues. This new definition goes beyond just a number on a scale. It also looks at whether a person's organs are working properly and whether excess fat is getting in the way of everyday activities.

A doctor named Brian P. Lee from the University of Southern California led the new study. He and his team studied health data from 5,600 adults across the United States. Instead of only using BMI, they also measured waist size and looked for signs of organ problems. Their results were surprising: about half of adults who are considered 'overweight but not obese' by BMI standards might actually have clinical obesity.

Not everyone agrees with the study's conclusions, though. Dr. Francesco Rubino, who helped lead the international commission that wrote the new definition, said the study may have counted too many people as having obesity. He pointed out that some health problems the study linked to obesity can also be caused by other things, like a viral infection or another illness. The study did not always separate those other causes, he said.

Some experts say BMI is still useful, even if it is not perfect. Elizabeth Selvin, a professor at Johns Hopkins University, said there is a 'very strong correlation' between BMI and actual body fat. She also noted that doctors do not use BMI alone — they consider other information too. Using extra tests to confirm obesity can be expensive and take a lot of time, she added.

Other doctors say BMI has real limits that patients and doctors should understand. Catherine Varney, a family doctor at the University of Virginia, said BMI does not account for a person's sex, age, or ethnicity. It also does not show where fat is stored in the body, which matters a great deal for health. Waist measurements can help fill in those gaps, but they also have problems of their own.

What doctors do agree on is that too much body fat — especially fat stored around the belly — is dangerous. Fatima Rodriguez, a professor at Stanford University, explained that belly fat releases chemicals that can harm the heart and affect blood sugar levels. Over time, too much fat can lead to high blood pressure, diabetes, and kidney disease. These conditions raise the risk of heart attack, heart failure, and stroke.

The new definition separates people into two groups. Those who have too much fat but no organ damage yet are called 'preclinically obese.' Doctors worry that this label might make people feel like their condition is not serious enough to treat. It could also make it harder to get insurance companies to pay for treatment. Many younger adults fall into this preclinical group, and experts say it is important to help them before their health gets worse.

Doctors say there are now more treatments for obesity than ever before, including new medicines that have shown strong results. Some experts argue that waiting until someone's organs are already damaged is the wrong approach. Acting early, they say, gives patients the best chance at staying healthy for the long term.