Hydrodensitometry

Hydrodensitometry is a method for determining percentage body fat. The Archimedes' principle is applied by comparing the mass of a subject in air and underwater. It usually involves the use of a specially constructed tank in which the subject is seated on a suspended chair or placed on a cot attached to a weighing device. Once the body volume (V) of the subject has been determined, body density can be calculated using the mass in air (Ma, kg) and the weight while submerged (Ws, kg) with allowances being made for residual gas in the lungs (Vr) and GI gas (100 mL) and the density of water (Dw) at the temperature of submersion. Density is calculated using the following equation:

The value body density is of little use. Instead, it is used to determine percentage body fat. Entering body density into either Brozek or Siri's equations does this. These equations are based on the principle that the average densities of fat mass and lean mass are [Page 374]0.90 g cm–3 and 1.01 g cm–3, respectively. Brozek's equation states that

The equation developed by Brozek for the conversion of body density to percentage body fat was based on the chemical composition of a reference male. It is based on a young, nonathletic adult male.

While hydrodensitometry is considered a reliable technique for determining percentage body fat, there are some problems to consider with the measure. The procedure is not for everyone, because the subject must be submerged; therefore, confidence in water is important. In addition, most tanks are raised above ground level, which makes it difficult for morbidly obese, pregnant, elderly, and disabled subjects to access them.

The measurement is also sensitive to residual air in the lungs and gut; thus, total expiration is necessary. Most measuring devices are sensitive to movement and temperature. Often, multiple measurements are taken. The volume of gas in the intestine is usually included in the calculation as being 100 milliliters, but this volume may need to be increased for large adults and decreased for children. Measurements are most reliable if taken during the fasting state. Dehydration and hyperhydration also cause changes in the body density calculation.

The calculations are based upon the principle that the body consists of two homogeneous components. There are several limitations of this model, however; the main limitation is individual variation in the components of bone and muscle components. Athletes tend to have denser bones and muscles, while the elderly have lesser bone density especially those with osteoporosis. Furthermore, preadolescents have different bone mineral content than adults.