What a DEXA body composition scan shows that your BMI cannot

Most people who ask us about a DEXA scan have had a health screen come back unremarkable. Their BMI sits in the healthy range and nothing on the report is flagged. But their trousers fit differently than they did five years ago, or they have been training hard for months and the scale has not moved — or has moved the wrong way.

A DEXA body composition scan answers a question the scales cannot. It measures how much of you is fat, how much is muscle, and where the fat sits. Fat stored around your organs behaves very differently from fat stored under the skin. BMI cannot tell those apart. Neither can a mirror.

At Advantage Medical Group in Singapore, a DEXA body composition scan costs $120 and is done at our Bukit Timah clinic. It takes a few minutes and involves lying still on a table.

The reason this matters more here than in most places is that BMI was calibrated on European populations. At the same BMI, Asians carry roughly 3 to 5% more body fat — which is why Singapore uses lower cut-offs than the rest of the world. Even inside those tighter limits, BMI still misses people.

When the scale says you are failing but the scan says otherwise

What the scan measures

A DEXA scanner passes two low-dose X-ray beams at different energy levels through the body. Tissues of different density absorb them differently, which lets the scan separate fat from muscle from bone. What comes out is not a single number but a map.

It reports your total body fat percentage, how much lean mass you carry in each region — arms, legs, trunk — your bone mineral content, and an estimate of visceral fat, the fat packed around your internal organs.

The radiation dose is very low. A whole-body DEXA scan delivers less than 10 microsieverts — similar to the natural background radiation you receive over a single day at sea level, and roughly a tenth to a thirtieth of a chest X-ray, which is around 100. A long-haul flight exposes you to more than the scan does. It is still ionising radiation, and DEXA is not performed during pregnancy. If there is any possibility that you are pregnant, please tell the clinic before your appointment.

That regional detail is the point. Knowing you are 28% fat tells you something. Knowing where that fat sits, and how much muscle you have to work with, tells you what to do about it.

It is not a bone density scan

This confuses people often enough to be worth stating plainly. The same machine performs two different tests.

A body composition scan measures fat, muscle and fat distribution — the test this article describes. A bone density scan, often written DXA, measures bone mineral density at the hip and spine and produces a T-score used in diagnosing osteoporosis. Different protocol, different report, different clinical question.

Both cost $120 at our Bukit Timah clinic. If you book “a DEXA scan”, say which one you mean.

Why BMI misses people, and misses more of them here

BMI divides your weight by the square of your height. It cannot distinguish fat from muscle, and it has nothing to say about where fat is stored. The 2025 Lancet Commission on Clinical Obesity put it directly: BMI fails to differentiate fat mass from lean mass and systematically misclassifies adiposity risk.

Two people at the same BMI can be metabolically unalike. One carries excess visceral fat with little muscle; the other carries substantial muscle and very little visceral fat. Their risks differ considerably. Their BMI does not.

This is sharper in Asian populations. At a given BMI, Asians carry approximately 3 to 5% more body fat than Caucasians — so a Singaporean at BMI 24 may be at 24 to 25% body fat, already in the moderate-risk range, while a Caucasian at the same BMI sits nearer 20%. That difference is why Singapore’s thresholds are lower than the international ones.

Internationally, overweight starts at 25 and obesity at 30. Singapore’s cut-offs are lower because the same BMI carries more fat here.

And even within the tighter range, BMI still misses people. An estimated 5 to 10% of normal-weight adults carry excess visceral fat despite a healthy BMI — a pattern sometimes called TOFI, thin outside, fat inside. It cannot be seen, weighed, or inferred from a BMI. It has to be measured.

Visceral fat: where it sits matters more than how much

Fat around the organs is metabolically active in a way that fat under the skin is not. It sits closer to the liver, releases different signalling molecules, and is more strongly associated with cardiovascular disease, type 2 diabetes and fatty liver.

A Mendelian randomisation analysis — a design that uses genetic variation to test whether a relationship is likely causal rather than coincidental — found that genetically predicted increases in visceral fat mass were associated with a 57% higher risk of coronary artery disease and a 71% higher risk of heart failure.

For people at a normal weight carrying excess visceral fat, the studies find a 2 to 5 times higher risk of type 2 diabetes and a 2 to 3 times higher cardiovascular risk than normal-weight people without it.

Fatty liver follows the same pattern. Between 5 and 27% of lean individuals have non-alcoholic fatty liver disease — often undetected, because it does not match the profile anyone is looking for.

The threshold most commonly used for visceral obesity in Asian populations is a visceral fat area of 100 cm² or more.

Muscle: the part nobody thinks to measure

Body composition is not only about fat. DEXA measures lean mass by region, including appendicular lean mass — the muscle in your arms and legs — which is the reference measurement for assessing age-related muscle loss.

Muscle matters metabolically, not just for strength. Skeletal muscle is the body’s main destination for glucose after a meal, accounting for roughly 80% of insulin-stimulated glucose uptake. Less muscle means less somewhere for glucose to go, so the body produces more insulin to manage the same meal. That is how insulin resistance develops in people whose weight looks entirely normal — a pattern we cover in our article on insulin resistance.

The Asian Working Group for Sarcopenia sets low muscle mass at an appendicular skeletal muscle mass index below 7.0 kg/m² in men and below 5.4 kg/m² in women, measured by DEXA. These are Asian-specific thresholds, not the European ones.

One thing worth being clear about: a scan alone does not diagnose sarcopenia. The diagnosis requires low muscle mass plus reduced grip strength or slow walking speed. DEXA supplies the muscle measurement; the rest comes from functional testing in the clinic.

If you have lost weight on a GLP-1 medicine

Weight loss is not all the same. Published data from a large trial of a GLP-1 receptor agonist found that roughly a quarter to a third of the weight lost was lean mass rather than fat.

That is not an argument against these medicines. It is an argument for measuring more than weight while someone is taking one. A DEXA scan shows whether the weight lost came off as fat or took muscle with it — and if lean mass has fallen while the weight target has been met, the conversation shifts from losing more weight to rebuilding what was lost.

It is why the patients we see on GLP-1 treatment are reminded, at every visit, about protein intake and resistance training. Sometimes the signal runs the other way: weight falling faster than expected, which can suggest muscle is going with it. That prompts a look at what someone is eating straight away, rather than waiting for the next scan.

What changes when the scan shows something

A scan is only worth having if the result changes what happens next.

In our practice, a DEXA scan showing excess visceral fat in someone with a normal BMI typically prompts three things: testing insulin resistance with HOMA-IR, assessing the liver, and a targeted exercise prescription weighted towards resistance training. None of those would ordinarily be triggered by a normal BMI and an unremarkable screening result.

This is how we work rather than something guidelines require. The scan does not replace a consultation; it gives the consultation something specific to work with.

What it costs, and where

A DEXA body composition scan costs $120 as a standalone test, at our Bukit Timah clinic. A bone density DXA also costs $120 — a different test, as above. Neither requires preparation beyond arriving in clothing without metal.