Biomarker
BMI: what body mass index does and does not tell you
Body mass index is weight in kilograms divided by height in metres squared. It is a population screening tool that got adopted as an individual verdict, and understanding that gap is the point of this page.
What it measures
BMI estimates whether your weight is proportionate to your height. It cannot distinguish muscle from fat, nor visceral fat from subcutaneous fat — which is why a muscular athlete and someone with substantial abdominal fat can share a BMI while carrying very different metabolic risk.
Reported in: kg/m²
Reference ranges
WHO adult categories. Lower thresholds apply for South Asian, Chinese and other Asian populations.
Underweight
Below 18.5
Associated with nutritional deficiency and reduced bone density.
Healthy weight
18.5–24.9
Lowest all-cause mortality in most population studies.
Overweight
25.0–29.9
23.0–27.4 in many Asian populations, where risk rises at a lower BMI.
Obesity class I
30.0–34.9
27.5 and above in many Asian populations.
Obesity class II–III
35.0 and above
Substantially increased cardiometabolic and joint risk.
Laboratory reference ranges vary between analysers and populations. Always read the range printed on your own report.
What moves the number
- Energy balance across months, not days
- Muscle mass — resistance training raises BMI while lowering risk
- Ethnicity, which shifts the risk threshold rather than the arithmetic
- Age-related loss of lean mass, which hides fat gain behind a stable BMI
- Fluid retention from heart, kidney or liver conditions
How Vitals uses this measurement
Vitals calculates BMI from the height and weight in your profile, keeps it beside your waist circumference and waist-to-hip ratio rather than in isolation, and uses weight and height as direct inputs to the FRAX-style fracture model.
Frequently asked questions
Is BMI useless for individuals?
Not useless, but incomplete. It performs reasonably at the extremes and poorly in the 25–30 band, where body composition decides whether the number means anything. Pair it with a waist measurement.
Why are the Asian cut-offs lower?
At any given BMI, South Asian and East Asian populations tend to carry more visceral fat and show higher rates of type 2 diabetes, so the WHO recommends action thresholds of 23 and 27.5.
What should I track instead?
Waist circumference and waist-to-hip ratio track visceral fat far better, and they change measurably even when the scale does not.
Related biomarkers
Sources
- World Health Organization, Obesity and overweight fact sheet
- WHO Expert Consultation. Appropriate body-mass index for Asian populations, The Lancet 2004
Educational information only, not medical advice. Discuss your own results with a qualified healthcare professional.
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