Body Fat Percentage from Body Density

Health care professionals like our RN friend Jolene understand that BMI provides a relatively quick way to asses body composition and gives providers and patients an easy method for monitoring changes, but it does not always accurately capture a person’s body composition. The errors common to the previously discussed skinfold method and the BMI can be somewhat avoided by actually measuring body density, which can then be used in empirical formulas that interpolate body fat percentage from body density:

Formulas used in calculating residual lung volume, body density, and body fat percentage. Image Credit: Measure Body Fat Via Under Water Weighing by MattVerlinich via Instructables

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Your lab for this unit might involve some of these formulas and if you are curious you can read more about those formulas, play with a simulation of hydrostatic weighing, check out a website that does the calculations for you, and see that different formulas have been developed for different population sets in an effort to increase accuracy.^[2]. Determining body fat percentage from body density is not something that Jolene would do on the MED floor, but athletic training facilities and clinics specializing in care associated with body composition might use this method.

Body Density

Mass and Volume

In order to understand density and how it might be measured, we need to know that volume (V) is the amount of space taken up by an object. Mass (m) is a measure how strongly an object attracts other objects by gravitation and resists changes in its motion. Atoms are the matter  that make up everyday objects like the body, and each type of atom exhibits a certain mass, so we sometimes speak of the mass as a measure of the amount of matter in the object. For example, carbon atoms will exhibit a mass of 12.011 grams. The number at the bottom of each square in the periodic table tells you the mass (in grams) exhibited by of that type of atom. This seemingly odd number is known as Avogadro’s Number.

Density

The SI units for volume and mass are cubic meters (m³) and kilograms (kg).  Mass Density (ρ), which we usually shorten to just density, for any object is defined as its mass divided by its volume. The same mass of different materials will have different volume, and thus different densities. For example 1 kg of foam takes up much more space than 1 kg of steel (in fact, about 80 times more).  This giant table of material densities is a useful reference (click the kg/m³ button to see the values in SI units).

Sometimes weight density is used instead of mass density, in which case weight (pull of gravity on an object) rather than mass is divided by the object volume.  The following chapters will explain how we measure the volume, weight, and mass of a body in order to calculate body density for use in determining body composition.