Body composition vs BMI · St. Louis
Body composition, not BMI
Body mass index divides your weight by your height squared. It cannot tell muscle from fat, and in the people it matters most for, that is exactly the distinction that counts.
What does BMI miss about your body?
BMI is a ratio of two numbers, both of which you can measure with a scale and a tape. It is useful for describing populations. It is a poor description of an individual, because two people at the same height and weight can have completely different amounts of muscle, fat and body water.
Two failure modes matter clinically. A muscular person can be classified as overweight while being metabolically healthy. And a person of entirely normal weight can carry very little muscle and a great deal of visceral fat — a pattern with all the metabolic consequences of obesity and none of its visibility.
What does a body composition analysis measure?
Bioimpedance analysis separates weight into components rather than treating it as one number.
| Component | What it tells you |
|---|---|
| Body fat mass | The tissue most closely tied to metabolic risk |
| Lean body mass | Muscle, protein and mineral — the tissue that burns energy and keeps you upright |
| Total body water | Intracellular and extracellular water; extracellular excess can indicate fluid retention |
| Segmental distribution | Whether muscle loss is generalized or affecting the legs first |
The relationships are straightforward: intracellular plus extracellular water gives total body water; body water plus protein gives soft lean mass; soft lean mass plus minerals gives lean body mass; lean body mass plus fat mass gives body weight.
Why does leg muscle matter so much?
Muscle loss in the legs shows up before it shows up on a scale, and it is one of the strongest predictors of losing independence. It also interacts with everything else in this assessment: less leg muscle means worse balance, less glucose disposal, and less capacity to recover from a stumble.
How does body composition change the meaning of other test results?
Body composition is the denominator for measured metabolic rate — a resting energy expenditure only means something in relation to how much lean tissue is producing it. It also helps interpret metabolic markers, because a normal-weight person with low muscle mass has a very different risk profile from a normal-weight person with plenty of it.
If you are a clinician: the same subject written for the person ordering the test is at the full test description.
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Tell us what you are trying to find out and we will explain which Measura assessments answer that question, what each one involves, and how the results are reviewed with a clinician.
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Frequently asked questions
My BMI says I am overweight but I lift weights. Does that matter?
It is one of the two classic ways BMI misleads. Body composition answers it directly. Body composition is not the same as weight.
Is bioimpedance accurate?
It is reproducible under standardized conditions, which is what makes it useful for tracking change. Hydration, food and exercise beforehand all shift it, hence the preparation rules. How to prepare.
Does it hurt?
No. The current is far too small to feel. What the appointment feels like.
Can I have it if I have a pacemaker?
Tell us when you book — bioimpedance is generally avoided with implanted electrical devices and the rest of the assessment proceeds without it. What to tell us in advance.
What is more accurate, BMI or body composition?
For one person, body composition. BMI divides your weight by your height squared, which is useful for describing populations but cannot tell muscle from fat. Bioimpedance separates your weight into body fat, lean mass and body water, and shows whether muscle loss is general or starting in the legs. Measured under standardized conditions, it is reproducible, so it can track change over time.
What happens in a body composition analysis?
A bioimpedance measurement passes a current through your body that is far too small to feel. It splits your weight into fat mass, lean body mass, total body water and how your muscle is distributed. Because hydration, food and exercise beforehand all shift the reading, you follow simple preparation rules before the appointment so results can be compared from one visit to the next.
Why is BMI no longer used?
It still is, for describing populations. The problem is using it to judge one person. Two people with the same height and weight can carry completely different amounts of muscle, fat and body water. A muscular person can be labeled overweight while metabolically healthy, and a normal-weight person can carry little muscle and a great deal of visceral fat.
Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, medical director of Measura. Last reviewed .