Body Composition Models

The more traditional methods are based on a two-compartment model that simply divides the body in to fat and fat-free mass. Hydrodensitometry (underwater weighing) is based on the two compartment model. Newer, more sophisticated techniques, such as DEXA (dual energy x-ray absorptiometery) measure the body as multiple compartments.This approach improves the accuracy of the calculation for determining the real density of fat-free mass.

Reference Models
Often referred to as "gold standards", these are clinical techniques that have been validated through repeated scientific studies and against which other clinical and field method results are evaluated. The two main reference models today are Hydrodensitometry and DEXA.

Prediction Equations
In Vivo methods use equations to predict percentage of body fat, fat-free mass, muscle, hydration, etc. Using a form of statistics known as multiple regression analysis, this allows an unmeasurable component, such as body fat, to be predicted from one or more measured variable, where studies have proved there is a correlation.

For example, calipers use external skinfold measurements (a method that estimates fat found just under the skin) to calculate total body fat. BIA measures the body's impedance (resistance) to an electrical signal to estimate total body fat.

There are two basic body-composition models: (below)

1. The two compartment model - fat-free mass and fat

2. The four compartment model - bone/mineral, protein, water and fat.

Equations can be population specific (developed for specific types of people, including such categories as gender, age, ethnicity, fitness level, disease, etc.) or generalized to cover a wide range of people types. A given equation is validated according to how well the results match the results of the reference method.

It is important to note that the results of reference methods themselves do not agree 100 percent. Therefore, when comparing different methods or products, you should consider which reference method was used and the appropriateness of both the method and particular product for the body type being analyzed.

• Back: Body Composition Study, Part 1
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