How is obesity measured?
BMI
The most widely-used method of measuring and identifying obesity is Body Mass Index (BMI).
BMI = weight in kg/height m2
In adults Overweight, or pre-obesity, is defined as a BMI of 25-29.9 kg/m², while a BMI ≥ 30 kg/m² defines obesity. These BMI thresholds were proposed by WHO expert report and reflect the increasing risk of excess weight as BMI increases above an optimal range of 21-23 kg/m², the recommended median goal for adult Caucasian populations (WHO/NUT/NCD, 2000).
In children different cut off points are used. At World Obesity we generally use the International IOTF Cut off points and details can be found here
|
Classification |
BMI kg/m² |
|
|
|
Principal cut off points |
Additional cut off points |
|
Underweight |
< 18.5 |
<18.5 |
|
Severe thinness |
<16.00 |
<16.00 |
|
Moderate thinness |
16.00-16.99 |
16.00-16.99 |
|
Mild thinness |
17-18.49 |
1700-18.49 |
|
Normal range |
15.5-24.99 |
18.5-22.99 23.00-24.99 |
|
Overweight |
≥25.00 |
≥25.00 |
|
Pre-Obese |
25.00-29.99 |
25.00-27.49 27.5-29.99 |
|
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: WHO website (http://www.who.int/bmi)
While BMI is a simple measure that is very useful for populations, it should be considered a rough guide for predicting risk in individuals. Athletes are commonly misclassified as are those who are particularly short or tall. In brief BMI is most useful on a population not individual basis.
Other methods include waist circumference, waist to hip ratio and biometric impedance.
Waist circumference (wc) is a cheap and easy method of measurement. Waist circumference is considered a reasonable indicator of intra-abdominal or visceral fat. This fat is closely associated with increased risk of comorbidity. NICE cut off points suggest Males with wc >94cm or Females with wc => 85cm are considered to be at increased risk. The World Health Organisation classify have identified levels of risk combining both BMI and WC these are shown below
WHO classification for risk of obesity-related ill health
|
Classification |
BMI (kg/m²) |
Waist circumference (cm) |
||
|
|
|
Men: |
94-102 |
>102 |
|
|
|
Women: |
80-88 |
>88 |
|
Underweight |
<18.5 |
|
No increased risk |
No increased risk |
|
Healthy weight |
18.5-24.9 |
|
No increased risk |
Increased risk |
|
Overweight |
25-29.9 |
|
Increased risk |
High Risk |
|
Obesity |
>30 |
|
High risk |
Very high risk |
Ref:http://www.noo.org.uk/uploads/doc/vid_5187_MEASURESOFCENTRALADIPOSITYAugust%2009_updated%20Feb%202010.pdf
Ethnicity
Populations differ in the level of risk as fat is distributed differently in different populations. The importance of central obesity is clear in populations (e.g. Asian) who tend to have relatively low BMIs but high levels of abdominal fat, and are particularly prone to NIDDM, hypertension and coronary heart disease (CHD). As a consequence of these different distributions of body fat an alternative set of criteria were devised.
Co-morbidities risk associated with different levels of BMI and suggested waist circumference in adult Asians
|
Classification |
BMI (kg/m²) |
Risk of co-morbidities |
|
|
|
|
Waist circumference |
|
|
|
|
<90cm (men) <80cm (women) |
≥ 90cm (men) ≥ 80 cm (women) |
|
Underweight |
< 18.5 |
Low (but increased risk of other clinical problems) |
Average |
|
Normal range |
18.5 – 22.9 |
Average |
Increased |
|
Overweight: |
≥ 23 |
|
|
|
At risk |
23-24.9 |
Increased |
Moderate |
|
Obese I |
25-29.9 |
Moderate |
Severe |
|
Obese II |
≥ 30 |
Severe |
Very severe |
Reproduced from: Asia Pacific report (WHO, IASO, IOTF 2000)
Waist to hip involves two measurements and is cheap and easy to administer on large populations. The ratio highlights if excess weight is again stored around the waist resulting in increased risk of comorbidities. Males with a w:h >1.0 and Females with a w:h >0.85 are considered to be at increased risk.
Additionally Magnetic Resonance Imaging, computed tomography and dual energy x-ray absorptiometry (MRI, CT, DEXA) scans are available, however, too expensive to be used on large populations.