Many of us have been through it: you stand on the scales, your height is recorded, and a figure is plotted on a chart. That figure is your body mass index, or BMI, and it is often treated as a quick signal of overall health.
For a long time, clinicians have relied on BMI to sort people into categories such as underweight, a healthy weight, overweight, or obese.
Because BMI is straightforward to work out, it became widespread. Yet simplicity is not the same as precision.
A growing body of research now argues that this long-standing number can overlook two crucial issues: where fat is stored and whether it is already damaging health.
The problem with BMI
BMI appears clear-cut because the calculation is basic: it sets body weight against height.
What it cannot do is distinguish whether that weight is fat, muscle, bone, or water. As a result, two individuals with the same BMI may have very different body composition and very different health risks.
Someone with a lot of muscle can be classed as obese despite having little spare fat. Meanwhile, another person may sit within the “healthy” BMI band while carrying risky fat deep within the abdomen.
“BMI is problematic because it does not specifically measure body fat and instead reflects total body weight, which includes muscle and bone,” said Dr. Brian P. Lee, a hepatologist and liver transplant specialist with Keck Medicine.
“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.”
A 200-year-old health tool
BMI did not begin life as a clinical test designed to assess an individual patient. The approach traces back to Adolphe Quetelet, a Belgian mathematician working in the 1830s.
His formula was intended to identify broad trends across populations, not to diagnose the person in front of a doctor.
In time, health systems adopted BMI because it was inexpensive and quick: a clinician needed only height and weight.
There was no need for imaging, blood tests, or specialist equipment. That convenience made BMI practical, but it also encouraged people to treat it as a more definitive measure than it truly is.
Where fat sits inside the body
Not all body fat behaves the same way. Fat stored beneath the skin-known as subcutaneous fat-contributes to body shape.
It can still affect health when present in large amounts, but it is not the main driver behind many diseases linked to obesity.
Greater concern centres on visceral fat. This form sits deep inside the abdomen, surrounding organs such as the liver, pancreas, and intestines.
Visceral fat can release inflammatory compounds and interfere with the body’s control of blood sugar, cholesterol, and blood pressure.
That is why a person can appear slim yet still carry significant health risk.
Visceral fat may not be obvious in the mirror. It may also fail to shift BMI enough to trigger alarm. Even so, it can cause harm internally.
A better measure of health
In 2025, an international panel of obesity specialists put forward an updated way to define obesity.
Their proposal, published by the Lancet Diabetes and Endocrinology Commission, places less emphasis on weight alone and more on body fat and its health consequences.
The framework relies on waist-based assessments, including waist circumference, waist-to-hip ratio, and waist-to-height ratio.
Using this model, a person needs to show excess fat on at least two of these measures and also show evidence that fat is affecting the body.
Such evidence might include joint issues, cardiovascular strain, metabolic changes, or other health problems associated with obesity.
Put simply, the newer definition asks a more relevant question: is excess body fat harming this person?
Hidden obesity appears
A research team at Keck Medicine set out to see how much this newer method would diverge from BMI.
They analysed information from around 5,600 adults in the National Health and Nutrition Examination Survey (NHANES) and compared outcomes under both approaches.
What they found was notable. One in four people whose BMI fell within the healthy range still met the newer definition of clinical obesity.
Among those described as overweight according to BMI, roughly half satisfied the updated obesity criteria.
In other words, many people who seem “fine” on a BMI chart may already be experiencing health effects associated with excess fat.
“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,” said Dr. Lee.
“Our findings show that millions of Americans may already have obesity-related health impacts and may be missing needed health interventions,” Lee added.
Health treatment depends on BMI
This is more than an argument about terminology. In many settings, BMI directly influences what treatment a person can access.
Eligibility for weight-loss medicines, surgery, or targeted medical counselling may depend on reaching a particular BMI threshold.
But consider someone with a “normal” BMI who has high visceral fat. They may not get support soon enough.
Their clinician may not view them as high risk. Insurance criteria may not fund care. In the meantime, the underlying problem may worsen before it is recognised.
Excess body fat is associated with heart disease, type 2 diabetes, high blood pressure, liver disease, and some cancers. Detecting risk earlier gives people more time to respond.
Waist size reveals more
A key takeaway from this work is that a tape measure can highlight risks that scales cannot.
Waist size and related ratios can help clinicians identify hidden danger, particularly when BMI provides false reassurance.
“The good news is that obesity can be treated,” said Dr. Lee. “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.”
A fuller picture of health
BMI is likely to remain part of clinical practice for years: it is familiar, quick, and useful as an initial screen. But it should not be treated as the entire assessment.
A more complete view of health should bring together waist measurements, where fat is distributed, relevant medical signs, and the person’s overall condition.
Scales can show how much someone weighs. They cannot reveal where fat is concentrated or what effects it is having.
For many people, that distinction could determine how soon risk is spotted, when treatment starts, and how long good health is maintained.
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