There are various means in which you can measure obesity, but the most widely-used method of measuring and identifying obesity is Body Mass Index (BMI).
This is weight in kilograms divided by the height in metres squared. 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 a 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 below while the abstract is here.
|Classification||BMI Cut-Off Points (kg/m²)|
|Overweight (including obesity)||≥25.00|
Extended International (IOTF) Body Mass Index Cut-Offs for Thinness, Overweight and Obesity in Children
The revised international child cut-offs are available corresponding to the following body mass index (BMI) cut-offs at 18 years:
- 16 thinness grade 3
- 17 thinness grade
- 18.5 thinness grade 1
- 23 overweight (unofficial Asian cut-off)
- 25 overweight
- 27 obesity (unofficial Asian cut-off)
- 30 obesity
- 35 morbid obesity
The cut-offs are given for exact ages by month from 2 to 18 years.
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 on an individual basis.
Other methods include measurement of 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. The National Institute for Health and Care Excellence (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 have identified levels of risk combining both BMI and WC. These are shown below.
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 noninsulin-dependent diabetes mellitus (NIDDM), hypertension and coronary heart disease (CHD). As a consequence of these different distributions of body fat an alternative set of criteria were devised.
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 waist to height ratio >1.0 and Females with a weight to height ratio >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, but are normally too expensive to be used on large populations.
|Classification||BMI (kg/m²)||Risk of co-morbidities|
|Normal range||18.5 – 22.9||
|At risk||23 - 24.9||
|Obese I||25 - 29.9||
|Obese II||≥ 30||
Reproduced from: Asia Pacific report (WHO, IASO, IOTF 2000)