Comparing Methods For Measuring Body Fat, Page 2 of 2

Comparing Methods For Measuring Body Fat, Page 2 of 2

Hydrodensitometry Weighing

Most people now understand that to have a healthier body means to have a leaner body. These people are ready for Body Composition Analysis (BCA.) These pages will attempt to acquaint you with the most popular methods of BCA used in gyms, homes, and professional practices.

The Dunk Tank - Underwater weighing modelHydrodensitometry Weighing (Underwater Weighing) - This method measures whole body density by determining body volume. There is a variety of equipment available to do underwater weighing ranging in sophistication from the standard stainless steel tank with a chair or cot mounted on underwater scales, to a chair and scale suspended from a diving board over a pool or hot tub.

This technique first requires weighing a person outside the tank, then immersing them totally in water and weighing them again. The densities of bone and muscles are higher than water, and fat is less dense than water. So a person with more bone and muscle will weight more in water than a person with less bone and muscle, meaning they have a higher body density and lower percentage of body fat. The volume of the body is calculated and the individual's body density is determined by using standard formulas. Then body fat percentage is calculated from body density using standard equations (either Siri or Brozek ).

The underlying assumption with this method is that densities of fat mass and fat-free mass are constant. However, underwater weighing may not be the appropriate gold standard for everyone.


For example, athletes tend to have denser bones and muscles than non-athletes, which may lead to an underestimation of body fat percentage. While the body fat of elderly patients suffering from osteoporosis may be overestimated. To date, specific equations have not been developed to accommodate these different population groups.

An important consideration in this method is the amount of air left in a person's lungs after breathing out. This residual lung volume can be estimated or measured, but it is established that a direct measure is desirable and it should be taken in the tank whenever possible. Another consideration is that the water in the tank must be completely still; there can be no wind or movement.

Although this method has long been considered the laboratory "gold standard", many people find it difficult, cumbersome, and uncomfortable, and others are afraid of total submersion or cannot expel all the air in their lungs. Clinical studies often require subjects to be measured three to five times and an average taken of the results.

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