If you’re serious about improving your health and fitness, chances are you’ve focused on building muscle or losing fat at some point. While the scale can show you if you’re gaining or losing weight, it can also leave you guessing what exactly spurred this change. Was it your diet that caused you to lose fat, or are you losing precious muscle that you worked hard to build? Is your mass-building effort causing too much fat gain while you’re attempting to bulk up? The best way to truly know what is behind the changes in your body weight is by tracking your body composition, and there are several useful methods that we’re going to discuss in this article.

Historically, the first widely accepted methods of measuring body composition included surface anthropometry (skinfold caliper testing) and hydrostatic weighing (dunk-tank); however, these methods are susceptible to human error, require intrusive test procedures, and have limited accuracy. Over the past three decades, technological advancements have led to new methods that provide safe, non-invasive, accurate body composition measurements. These new methods include Differential X-Ray absorptiometry (DXA or DEXA); Air Displacement plethysmography or ADP (e.g., BOD POD); three-dimensional photo scanning (e.g., Fit3D, Styku); and Bioelectrical impedance measurement (e.g., Tanita, InBody, Seca).

If you’re going to spend time and money tracking your composition, it’s worth understanding the pros and cons of the different methods and select the one that is best for you.

In the first part of this series, we’re going to look at body composition models and the true gold standard in composition measurement. In future posts, we will look at how each measurement technology works, the type of accuracy you can expect, and the influence of test procedures on repeatability.


Body Composition Models:

Body composition models classify body matter into distinct groups. The most basic model divides the body into Fat Mass (FM) and Fat-Free Mass (FFM):

two compartment model of body composition
This two-compartment model requires direct measurement of body volume (or density) and body weight, and it uses constant FM and FFM densities to calculate FM and FFM. Both body volume and body weight can be measured with a high degree of accuracy; however, the major limitation with the two-compartment model is biological variability of the FFM density, which can cause significant errors in the calculated FM and FFM (Siri 1993). In the two-compartment model, everything that is not fat gets classified as FFM. This includes muscle, bone, organs, and fluid mass.

In the two compartment model, everything that is not fat gets classified as FFM. This includes muscle, bone, organs and fluid mass.

fat free mass
Given our individual differences in the level of bone mass, muscle mass, and fluid content, it’s easy to see how using a constant FFM density would lead to errors when using a two-compartment model. For example, one significant factor affecting FM and FFM density is an individual’s hydration state; variation in FFM hydration levels can lead to under- or over-estimation of FFM by as much as 10% (Brozek et. al. 1963).

To overcome some of the limitations of the two-compartment models, three-component models take into account the hydration state of the individual. Deuterium Dilution is an accurate method to measure total body water, allowing FFM to be divided into Water and Dry Mass.

total body mass
More recently, the four-compartment model has become widely regarded as the gold standard for measuring body composition (Withers et al. 1999). Using DXA technology, this model accounts for biological diversity by directly measuring bone mineral content. Furthermore, the four-compartment model is commonly used as a reference to assess the accuracy of new body composition technologies.
total body mass 2
So, where can you find this test? Well, the four-compartment model requires three highly specialized tests (ADP or Hydrostatic weighing, Deuterium Dilution, and DXA scanning). The test itself costs >$500 and takes about half a day to complete. Fortunately, for those interested in tracking body composition, there are other convenient and affordable options that offer great accuracy, precision, and repeatability.

In our next post, we will look at some of the more common methods of body composition testing to help you understand your test options and make the choice that is best for you.


Brozek, J., Grande, F., Anderson, J.T., and Keys, A. 1963. Densitometric Analysis of Body Composition: Revision of some quantitative assumptions. Annals of the New York Academy of Sciences 110:113-140

Siri, W.E. 1993 Body Composition from fluid spaces and density. Analysis of methods, 1961 Nutrition 9(5): 480-491

Withers, R.T., Lafogia, J., Heymsfield, S.B., 1999. Critical appraisal of the estimation of body composition via two-, three-, and four-compartment models. American Journal of Human Biology 11(2):175-185