Measurement of Obesity

There are various ways in which to measure different aspects of obesity.

Body Mass Index (weight [kg]/height [m]2)

The most common method of measuring obesity is the Body Mass Index (BMI). BMI is calculated by dividing body weight (kilograms) by height (metres) squared. An adult BMI of between 25 and 29.9 is classified as overweight and a BMI of 30 or over is classified as obese (Table 1).

Table 1: World Health Organisation BMI classification system for adults

BMI is the most widely used approach in the UK, but it is important to note that it is not a direct measure of body fat mass or distribution, and BMI measures may be skewed by high muscle mass. The relationship between BMI and health also varies with ethnicity.

In children and adolsecents BMI varies with age and sex, which makes it harder to classify children as obese, overweight or underweight. For this reason a growth reference must be used. In England, the British 1990 growth reference charts are used to classify the weight status of children for the National Child Measurement Programme and Health Survey for England.

To check out your own BMI, click HERE

NOO has produced a Q and A style briefing paper on BMI. Click here to view paper

Skin Fold Thickness – biceps, triceps, subscapula, suprailiac

This refers to the measurement of subcutaneous fat located directly beneath the skin by grasping a fold of skin and subcutaneous fat and measuring it using calipers. It is used mainly to determine relative fatness and the percentage of body fat. Measurement requires callipers and some basic training.

Waist circumference and waist to hip ratio

The circumference of the waist is sometimes used as a simple measure of body fatness, though it can be subject to measurement error. Adult waist circumference cut points are:

• Increased risk of health problems: Men≥ 94cm Women ≥ 80cm
• Greatly increased risk of health problems: Men ≥ 102cm Women ≥ 88cm

Waist to hip ratio examines fat distribution and in practice is used less frequently, given the established links between waist circumference alone and health risk.

Bio-impedance

This measures the impedance or opposition to the flow of an electric current through the body fluids contained mainly in the lean and fat tissue. As lean mass is made up of 73% water and fat has no water content, this method estimates lean tissue mass (which acts as a conductor) and fat mass (which acts as an insulator), through changes in voltage. Home machines are available for bio-impedance measurement though these can be inaccurate as they often estimate from the legs only.