BMI is universal; the cut-offs are not
Body mass index is weight in kilograms divided by height in metres squared, and the formula is the same everywhere. What varies is the set of thresholds used to sort that number into categories. The World Health Organization publishes an international classification, and also recognises that the relationship between BMI and health risk differs across populations, leading to additional cut-offs for many Asian groups.
The two threshold systems side by side
The table compares the WHO international cut-offs with the lower Asian-Pacific cut-offs the WHO identified for many Asian populations, where cardiometabolic risk can rise at a lower BMI.
| Category | WHO international | Asian-Pacific (WHO) |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Increased risk / overweight | 25.0 – 29.9 | 23.0 – 27.4 |
| Obesity | 30.0 and above | 27.5 and above |
Why the same BMI can be classified differently
Because the bands differ, an identical BMI can land in different categories. A BMI of 24.0 is a healthy weight under the WHO international standard but falls in the increased-risk range under the Asian-Pacific cut-offs. A BMI of 28.0 is overweight internationally but reaches the obesity threshold under the Asian-Pacific system. The number has not changed — the risk threshold applied to it has.
The reason is that, at the same BMI, some Asian populations show higher body-fat percentage and higher rates of type 2 diabetes and cardiovascular disease than the populations the original international cut-offs were based on. Lower thresholds are intended to flag that elevated risk earlier.
The limits of BMI itself
Whichever thresholds are used, BMI is a screening tool, not a diagnosis or a measure of body composition. It does not distinguish muscle from fat, so very muscular people can register a high BMI without excess fat, and it says nothing about where fat is stored — waist measurement adds information BMI cannot. BMI is most useful at the population level and as a first-pass screen, with clinical judgement and additional measures used for individual assessment.
よくある質問
What is a healthy BMI range?
Under the WHO international standard, 18.5–24.9 is a healthy weight. Under the Asian-Pacific cut-offs the WHO identifies, the healthy range is 18.5–22.9.
Why are Asian BMI cut-offs lower?
At the same BMI, many Asian populations show higher body-fat percentage and higher rates of type 2 diabetes and cardiovascular disease, so lower thresholds flag increased risk earlier.
Can the same BMI be healthy and overweight at once?
Under different standards, yes. A BMI of 24 is healthy under the WHO international cut-offs but increased-risk under the Asian-Pacific cut-offs.
Is BMI a diagnosis?
No. BMI is a screening tool. It doesn't distinguish muscle from fat or show fat distribution, so it should be combined with other measures and clinical judgement.
参考文献
- World Health Organization — Body mass index (BMI) classification. https://www.who.int/data/gho/data/themes/topics/topic-details/GHO/body-mass-index
- WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157-163. https://doi.org/10.1016/S0140-6736(03)15268-3
- U.S. Centers for Disease Control and Prevention — About Adult BMI. https://www.cdc.gov/healthyweight/assessing/bmi/