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Body Composition. Densitometry. The difference in weight in a person in air and after submerging in water can be used to work out volume This works because 1litre of water weighs 1 Kg Density can then be calculated

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The difference in weight in a person in air and after submerging in water can be used to work out volume

This works because 1litre of water weighs 1 Kg

Density can then be calculated

The density is important to help work out the proportion of fat a person has as fat is less dense than muscle.


Volume is worked out by weighing the person in air and again when submerged.

  • Volume ( l) = weight in air ( Kg)-weight in water ( kg)
  • The volume is then use to calculate body density using the following formula
  • Density= body mass(kg)
  • Body volume(l)
  • %body fat computed using the formula
  • % fat = 495/density - 450
  • Disadvantage is that being submerged under water may be difficult and produce some anxiety
An example of this calculation is:
  • % Fat = 495/density (g cmֿ³) – 450
  • A 60 kg person weighs 2 kg underwater. According to Achimedes’ Principle, the weight loss in water of 58 kg is equal to the weight of water displaced.
  • 58 kg of water = 58 litres, or 58,000 cm³
  • The density of this person calculated as weight/volume is therefore:

Density (g cmֿ³) = 60,000 g (60 kg)/58,000 cm³

= 1.0345 g cmֿ³

  • When this value is incorporated into the formula above, % body fat is:

% Fat = 495/1.0345 – 450

= 28.5%

  • New method known as the ‘Bod Pod’ uses air instead of water
bod pod
New method of measuring body density

Bod Pod uses air displacement instead of water to measure body volume

Subject sits in a small chamber (Bod Pod) and body volume is computed by initial vol. of chamber minus vol. with person inside

Bod Pod
Advantages are

short measurment time, non-submersion, used by elderly, obese, children etc

Disadvantages =

very high cost

skinfold thickness
Skinfold Thickness

This is the most widely used methods for estimating body composition and involves measuring the layer of fat under the skin (subcutaneous layer) at specific sites with a skinfold calliper.

Four sites are commonly used

  • Over the biceps muscle at the front of the arm
  • Over the triceps muscle at the back of the arm
  • Under the shoulder blade at the back
  • Above the hip bone at the side of the body

Shoulder blade

Triceps muscle

Digital skinfold calipers

Skinfold thickness taken

from the triceps muscle

Diagram of

muscle and

fat layer


The sum of the 4 skinfolds is used in a mathematical formula to predict body density and in turn % body fat

Advatages= non-invasive, cheap, portable, quick, accurate once skill is mastered

Disadvantages = errors with measurer skill, does not take into account unusual fat distribution, difficult in the very obese and the very lean

bioelectrical impedance analysis bia
BIA is based on the principle that the FFM (which is about 73% salty water) offers very little resistance to the flow of a small electric current whereas the FM (which is an insulator) conducts very little of the current.

Therefore, measuring the impedance of the body to the flow of the applied electric current can give an estimate of the lean/fat ratio in the body.

Bioelectrical impedance analysis (BIA)
Advantages of BIA:

Requires little or no technical skill by the operator

Takes less that a minute to perform

Unit is easily transportable

Only requires removal of a sock (unlike skinfolds)


Any disturbance in hydration level in the body (e.g. dehydration) will affect the accuracy of the result

Tends to over-estimate body fat in very lean, muscular people and under-estimate % fat in obese people

waist hip ratio
It is the distribution of fat rather than the quantity with regard to health risk

People can be classified as ‘apples’ (android) or ‘pears’ (gynoid) according to their fat distribution

‘Apples’ have extra abdominal fat and carry a greater risk of CHD, Type 2 diabetes than ‘pears’ (extra fat around hips and thighs)

Waist/hip ratio





A way of estimating fat distribution is to measure the ratio of waist circumference to hip circumference. Those with apple shape will have a higher ratio than those with pear shape.
  • ‘At risk’ values are a waist/hip ratio of greater than 1.0 for men and 0.8 for women.