Body Mass
Index (BMI) is a simple index of weight-for-height
that is commonly used to classify underweight,
overweight and obesity in adults. It is defined as
the weight in kilograms divided by the square of the
height in metres (kg/m2).
For example, an adult who weighs 70kg and whose
height is 1.75m will have a BMI of 22.9.
BMI = 70 (kg) / 1.752 (m2)
= 22.9
Table 1: The International Classification of
adult underweight, overweight and obesity according
to BMI
|
Classification |
BMI(kg/m²) |
| |
Principal cut-off points |
Additional cut-off points |
|
Underweight |
<18.50 |
<18.50 |
| Severe thinness |
<16.00 |
<16.00 |
| Moderate thinness |
16.00 - 16.99 |
16.00 - 16.99 |
| Mild thinness |
17.00 - 18.49 |
17.00 - 18.49 |
|
Normal range |
18.50 - 24.99 |
18.50 - 22.99 |
|
23.00 - 24.99 |
|
Overweight |
≥25.00 |
≥25.00 |
| Pre-obese |
25.00 -
29.99 |
25.00 - 27.49 |
|
27.50 - 29.99 |
|
Obese |
≥30.00 |
≥30.00 |
| Obese class I |
30.00 -
34-99 |
30.00 - 32.49 |
|
32.50 - 34.99 |
| Obese class II |
35.00 -
39.99 |
35.00 - 37.49 |
|
37.50 - 39.99 |
| Obese class III |
≥40.00 |
≥40.00 |
Source: Adapted from WHO, 1995, WHO, 2000 and
WHO 2004.
BMI values are age-independent and the same for
both sexes. However, BMI may not correspond to the
same degree of fatness in different populations due,
in part, to different body proportions. The health
risks associated with increasing BMI are continuous
and the interpretation of BMI gradings in relation
to risk may differ for different populations.
In recent years, there was a growing debate on
whether there are possible needs for developing
different BMI cut-off points for different ethnic
groups due to the increasing evidence that the
associations between BMI, percentage of body fat,
and body fat distribution differ across populations
and therefore, the health risks increase below the
cut-off point of 25 kg/m2
that defines overweight in the current WHO
classification.
There had been two previous attempts to interpret
the BMI cut-offs in Asian and Pacific populations3,4,
which contributed to the growing debates. Therefore,
to shed the light on this debates, WHO convened the
Expert Consultation on BMI in Asian populations
(Singapore, 8-11 July, 2002)5.
The WHO Expert Consultation5
concluded that the proportion of Asian people with a
high risk of type 2 diabetes and cardiovascular
disease is substantial at BMI's lower than the
existing WHO cut-off point for overweight (= 25 kg/m2).
However, the cut-off point for observed risk varies
from 22 kg/m2 to 25
kg/m2 in different
Asian populations and for high risk, it varies from
26 kg/m2 to 31 kg/m2
. The Consultation, therefore, recommended that the
current WHO BMI cut-off points (Table 1) should be
retained as the international classification.
But the cut-off points of 23, 27.5, 32.5 and 37.5
kg/m2 are to be
added as points for public health action. It was,
therefore, recommended that countries should use all
categories (i.e. 18.5, 23, 25, 27.5, 30, 32.5 kg/m2
, and in many populations, 35, 37.5, and 40 kg/m2)
for reporting purposes, with a view to facilitating
international comparisons.
Discussion updates
A WHO working group was formed by the WHO Expert
Consultation5 and
is currently undertaking a further review and
assessment of available data on the relation between
waist circumference and morbidity and the
interaction between BMI, waist circumference, and
health risk.