# 27 Body Density

# Body Fat Percentage from Body Density

Health care professionals like our RN friend Jolene understand that BMI provides provides a relatively quick way to asses body composition and and monitoring changes, but it does not always accurately capture a person’s body composition. Body fat percentage can provide a more complete picture of 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. Different equations have been developed for males and females in different populations in order to improve accuracy, but the following is once example: ^{[1]}. In order to understand those models we first need to understand density and how it can be measured.

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 typically 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 (**m ^{3}**) 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

*to see the values in SI units).*

**kg/m**button^{3}### Reinforcement Exercises

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.

### Reinforcement Exercises

- Siri, SE (1961), "Body composition from fluid spaces and density: analysis of methods", in Brozek J, Henschel A (eds.), Techniques for measuring body composition, Washington, DC: National Academy of Sciences, National Research Council, pp. 223–34 ↵
- "Under Water Weighing" by University of Vermont College of Medicine, Department of Nutrition and Food Science, ↵

method for measuring body fat percentage using specially designed calipers to measure the thickness of skinfolds that are pinched from several specific locations on the body as inputs to empirical equations

Body Mass Index (BMI) is a person’s weight in kilograms divided by the square of height in meters. BMI can be used to screen for weight categories that may lead to health problems but it is not diagnostic of the body fatness or health of an individual

relation between the amount of a material and the space it takes up, calculated as mass divided by volume.

mathematical explanation of the relation between measured values that is used for making predictions

refers to the closeness of a measured value to a standard or known value

a quantity of space, such as the volume within a box or the volume taken up by an object.

a measurement of the amount of matter in an object made by determining its resistance to changes in motion (inertial mass) or the force of gravity applied to it by another known mass from a known distance (gravitational mass). The gravitational mass and an inertial mass appear equal.