In the 1830s, a Belgian mathematician named Adolphe Quetelet set out to describe the average man. Not a patient, and not really a person at all, but a statistical figure assembled from the measurements of thousands of people. One of the tools he used was a simple ratio: weight divided by height squared.
Nearly two centuries later that ratio, known as BMI (body mass index) is how most people find out whether their weight is a problem.
It is how a lot of people get told “your weight is fine.” At a corporate health check, from a GP, from an app. It is usually said kindly, and it is usually where the conversation ends.
It should be where it starts.
What BMI can and cannot tell you
BMI still earns its place as a population tool. It is fast, it costs nothing and it is genuinely useful for spotting trends across millions of people. That is what it was built for.
The trouble begins when a screening number is treated as a diagnosis. In practice, BMI gets asked questions it has no way of answering. Am I metabolically healthy? Do I carry too much fat around my organs. Am I losing muscle? Should I lose weight?
“BMI tells me how heavy you are relative to your height,” says Dr. Samatha Tulla. “It does not tell me what that weight is made of or where the fat is located.”
BMI cannot tell you the full picture of your health
Those two things, composition and location, are most of what matters.
Normal BMI, but metabolically unhealthy
One of the patterns Dr. Samatha Tulla sees most often is someone with a BMI of 22 or 23 who has spent years being reassured. When the team looks underneath the number, they may find a large amount of visceral fat, which is fat stored around the organs rather than under the skin. They may find less muscle than expected for the person’s BMI, early insulin resistance, where the body needs more and more insulin to keep blood sugar steady, a liver beginning to store fat, arteries that have lost some of their elasticity or low cardiovascular fitness.
None of that shows up on a scale.
In one internal group of roughly 200 PMX members, 38 members had a BMI in the conventionally normal range. Among them, 74% showed signs of reduced insulin sensitivity, 37% carried more than 500 grams of visceral fat, 63% had an elevated body fat percentage, and 55% showed evidence of arterial stiffness.
This is our own clinical group, not population research. People who come to a longevity centre not a random sample of India, and we are not going to turn 38 people into a national statistic. But, what the numbers do show is how easily a normal BMI can reassure the wrong person.
The opposite happens too
Someone who strength trains regularly can land in the overweight range simply because muscle is dense and heavy. Their visceral fat may be low and their metabolic markers very good. BMI reads both people, the reassured one and the flagged one, with equal confidence, and it can be wrong about both.
Why BMI matters more for South Asians
South Asians tend to develop visceral fat, insulin resistance and type 2 diabetes at lower BMI levels than people of European descent. In the MASALA study, a long-running study of South Asian adults, 44 percent of participants with a normal BMI already had two or more metabolic abnormalities. Indian clinical literature has described the same tendency for years: more fat around the abdomen and inside organs, at a lower overall weight.
This is why obesity guidance for Asian populations now uses lower cut-offs, treating a BMI of 23 as the point where excess fat becomes worth screening for, rather than the 25 used for Western populations.
Dr. Samatha Tulla’s position is a careful one. BMI is still useful. It is the conventional Western thresholds that are less reliable as a standalone screen here. “Ethnicity changes how seriously I interpret a BMI of 23 or 24,” she says. “But I still prefer actually measuring body composition rather than simply lowering a cut-off.”
What we measure instead
There is no single replacement number, and anyone offering one is repeating the original mistake. What replaces BMI is a picture built from several angles.
Body composition. A DEXA scan is a low-dose scan that separates fat, muscle and bone, and shows where the fat sits. It gives total body fat, visceral fat, how lean mass is distributed across the body and bone density.
Simple central measures. Waist circumference and waist-to-height ratio are quick, cheap and surprisingly informative about fat around the middle.
What the tissue is doing. Blood markers show the metabolic effect of that fat: fasting glucose and insulin, HbA1c, which reflects average blood sugar over roughly three months, triglycerides, HDL cholesterol, ApoB, which counts the particles that carry cholesterol into artery walls, and liver markers.
What the body can do. Strength, cardiorespiratory fitness and how well someone moves. These tell a doctor something neither a scale nor a blood test can.
When weight loss is prescribed from BMI alone
Two things tend to go wrong.
The first is that a metabolically healthy person gets sent chasing a number they did not need to change.
The second is less obvious and more important. Weight loss that is not designed carefully removes muscle along with fat. With substantial weight loss, particularly in older adults and during treatment with weight-loss medication, a meaningful share of what is lost can be lean tissue, which is the tissue you most want to keep as you age.
Weight is an outcome
So the useful question after weight loss is not how many kilograms came off. It is which tissue did.
What to do with this
- If you have been told your BMI, ask what else was measured. If the answer is nothing, that is worth knowing.
- Measure your waist and compare it with your height. It is the cheapest upgrade to BMI available.
- If you are South Asian, treat a BMI of 23 or 24 as a reason to look further rather than a reason to stop.
- If someone recommends weight loss based on BMI alone, ask what you would be losing and how they plan to protect muscle.
- Treat strength and fitness as health measures, not gym measures.
BMI is a reasonable place to begin. As Dr. Samatha Tulla puts it, it starts the conversation. It should rarely finish it.