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.