Body Mass index

As an alternative to using tables of weight and height, it is possible to calculate a simple numerical index from height and weight, and to use this to establish acceptable ranges. The most commonly used such index is the body mass index (BMI), sometimes also called Quetelet's index, after Quetelet, who first demonstrated its usefulness in nutritional studies.

Body mass index is calculated from the weight (in kilograms) divided by the square of the height (in metres) — i.e. BMI = weight (kg)/height2 (m). The desirable range, associated with optimum life expectancy, is between 20 and 25. As discussed in section 8.2, values of BMI below 20 are associated with undernutrition. Table 6.1 shows the classification of overweight and obesity by BMI. For older people, there is some evidence

Figure 6.2 Weight for height as an index of overweight and obesity.

150 160 170 180 190 200

height (cm)

Figure 6.2 Weight for height as an index of overweight and obesity.

that a higher body weight is associated with better health and survival; Table 6.2 shows the desirable ranges of BMI at different ages.

6.1.2 MEASUREMENT OF BODY FAT

Although BMI is widely used to assess overweight and obesity, what is important for health and life expectancy is the body content of fat, and it is important to be able to determine the proportion of body weight that is fat. This is termed adiposity; a number of techniques are available for assessing adiposity, although most of them are research techniques and are not appropriate for routine screening of the general public.

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