Obesity Classification
There are various means through which you can screen for obesity, the most widely-used method being the Body Mass Index (BMI).
BMI is calculated as 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 World Health Organization (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).
| Classification | BMI Cut-Off Points (kg/m²) |
|---|---|
| Healthy Weight | 18.5-24.99 |
| Overweight (including obesity) | ≥25.00 |
| Obesity | ≥30.00 |
| Severe Obesity | ≥40.00 |
In children, different cut-off points are used. At World Obesity we generally use the International IOTF cut-off points, details of which can be found below and in this abstract. Other commonly used cut-offs for children include those provided by the WHO and Center for Disease Control and Prevention (CDC).
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.
Other methods
While BMI is a simple measure that is very useful for populations, it can only predict risk in individuals. Afterall, BMI is a measure of size not health and so has some limits as a diagnostic tool. For example, athletes are commonly misclassified due to their high muscle mass. In short, BMI is most useful at a population level and for determining risk (not diagnosis) at a individual level.
Other methods of classifying obesity include measurement of waist circumference, waist to hip ratio and the Edmonton Obesity Staging System (EOSS).
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 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.
On the other hand, waist to hip involves two measurements and but is also cheap and easy to use on large populations. The ratio highlights if excess weight is 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.
Increasingly, the EOSS is being used in clinical settings. The EOSS diagnoses and considers the severity of obesity based on a clinical assessment of weight-related health issues, mental health and quality of life. This is useful at an individual level and for decision-making for treatment, but it is not practical at the population level.
Finally, biometric impedance, Magnetic Resonance Imaging, Computed Tomography and Dual Energy X-ray Absorptiometry scans are also available, but most are normally too expensive to be used on large populations.
Ethnic variation
The relationship between BMI, percentage of body fat, body fat distribution and health risks differ for different populations. Evidence suggests that much of the Asian population have a higher risk of cardiovascular disease and type 2 diabetes at a much lower BMI than Caucasians. It has also been found that the relationship between body fat (percentage and distribution) and BMI is different for many Asian populations when compared to Caucasians. Specifically, it appears central obesity is particularly prevalent in Asian populations who tend to have relatively low BMIs but high levels of abdominal fat.
As a result of these ethnic variations, a WHO expert consultation came up with an alternative set of criteria for Asian populations (see below). Although these criteria are commonly used, many Asian countries are devising their own country-specific cut-offs for their populations.
| Classification | BMI (kg/m²) | Risk of co-morbidities | ||
| Waist circumference | ||||
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| Underweight | < 18.5 |
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| Normal range | 18.5 – 22.9 |
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| Overweight | ≥ 23 |
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| At risk | 23 - 24.9 |
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| Obesity Class I | 25 - 29.9 |
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| Obesity Class II | ≥ 30 |
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Reproduced from: Asia Pacific report (WHO, IASO, IOTF 2000)
Prevention of obesity
You've made it this far, so why not read our last section on about obesity: Prevention. Click the link below to find out more.
Prevention of obesity