Densitometry

• Measurement of body density by underwater weighing

• 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 cm ֿ³) – 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 cm ֿ³) = 60,000 g (60 kg)/58,000 cm³ = 1.0345 g cm ֿ³

• 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

• Advantages are

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

• Disadvantages =

very high cost

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

1. Over the biceps muscle at the front of the arm

2. Over the triceps muscle at the back of the arm

3. Under the shoulder blade at the back

4. Above the hip bone at the side of the body

Triceps muscle

Shoulder blade

Digital skinfold calipers

Skinfold thickness taken from the triceps muscle

Diagram of muscle and fat layer

Calipers

• 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.

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)

Disadvantages:• 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)

Apple(android)

Pear(gynoid)

• 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.