Determination of body density

The density of body fat is 0.9 g/mL, whereas that of the fat-free body mass is 1.10 g/ mL. This means that, if the density of the body can be calculated, then the proportion of fat and lean tissue can be calculated.

Density is determined by weighing in air, and again totally submerged in water (the density of water = 1.0 g/mL), or by determining the volume of the body by its displacement of water when submerged. Neither procedure is particularly pleasant for the experimental subject, and considerable precision is necessary in the measurements; at 10% of body weight as fat, which is extremely low, density = 1.08 g/mL, while at 50% fat, which is very high, density = 1.00 g/mL.

Although direct determination of body density is the standard against which all the other techniques listed below must be calibrated, it is clearly a research technique and not appropriate for general use.

Table 6.1 Classification of (overweight and obesity by body mass index

BMI

Excess weight (kg)

Per cent of desirable weight

Desirable

20-25

-

100

Acceptable but not desirable

25-27

< 5

100-110

Overweight

25-30

5-15

110-120

Obese

30-40

15-25

120-160

Severely obese

> 40

> 25

> 160

Table 6.2 Desirable ranges of body mass index with age

Age (years)

Desirable BMI

19-24

19-24

25-34

20-25

35-44

21-26

45-54

22-27

55-64

23-28

> 65

24-29

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