How to "Measure" Body Composition: An Overview | Liberate Breakfast
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How to “Measure” Body Composition: An Overview

Curiosity killed the cat but it made the humans smarter. When you start focusing on getting healthier, more fit, or losing weight eventually you get curious about your body fat percentage. There are a few things to note before even looking into which test to get, if any at all.

Measuring Body Composition: An Overview | Liberate Breakfast

Curiosity Noooo!

All tests are wrong to a certain degree.

Huh? Why would anyone use a test that is inaccurate? The thing is, what may be inaccurate on an individual level can be very useful on a population level when statistical significance is taken into account. Take BMI for instance. The Body Mass Index (BMI) is often called out as a ridiculous tool because you can be jacked with single digit body fat, but according to the BMI, you are overweight (or worse, obese). That type of classification doesn’t bode well for someone with body dysmorphia (I’m looking at you, bodybuilders). When applied to the population as a whole, the BMI is a very useful tool for determining the overall health of the country. Outliers like bodybuilders are a small percentage that barely affects the overall outcome of calculations.

Think of it like target practice at a shooting range (is there any analogy more American than combining obesity and guns into one?) From a distance, all your shots are generally around the bullseye, so it looks pretty good. After taking a walk to your target you realize there is one round that went high and right (probably pulling too hard with your trigger finger….anyway…) That one shot is an outlier that is not representative of the overall accuracy of your shooting. Those people who are in excellent shape but still fall into the overweight category (I’m guilty of this) or obese category (I’m bordering on this) are outliers who are not indicative of the entire population.

What can you do about it?

Whichever method of body fat test you choose will be somewhat inaccurate. So what should you do about it?

Stay consistent with your measurements or chosen test location and administrator. You may not get a perfectly accurate reading on whatever test you choose, but if you remain consistent with your measurements, over time you will be able to rather accurately show change and progression. The same rings true for measuring your weight on a scale: always measure yourself under the same conditions with the same scale in order to maintain consistency of error.  

Now, let’s take a cursory look at the science behind calculating body composition, before we talk about the actual tests that you could have administered.


The Compartment Models

Body composition tests can be broken down into various categories that generally correlate to the level of accuracy. The first and simplest category is the Two-Compartment Model.

Measuring Body Composition: An Overview | Liberate Breakfast

The Two-Compartment Model breaks the human body into two distinct compartments: fat mass (FM) and fat-free mass (FFM), i.e. muscle, bone, water, etc. Certain values are assumed for the density of FM and FFM respectively. Typical methods for the Two-Compartment Model are Hydrodensitometry “underwater weighing” and Air Displacement Plethysmography (ADP). A typical issue with Two-Compartment Model methods is that there is no room for individual variability in fat-free mass, which is quite common from human to human.

Measuring Body Composition: An Overview | Liberate Breakfast

The Three-Compartment Model is the next category, and is more accurate than the Two-Compartment Model. The Three-Compartment Model breaks the body down into three compartments: FM, FFM, and a third, either Total Body Water (TBW) or, less frequently, bone density. The model that measures TBW rather than bone density may not take into account variations in FFM that present in individual bodies, such as bone mineral density and actual lean muscle tissue. Even still, the Three-Compartment Model is the most accurate single iteration tests you can get. DEXA, for example, is a Three-Compartment model test.

The final category is the Four-Compartment Model. In addition to fat mass, fat-free mass, and body water, the Four-Compartment Model also takes into account mineral mass. There is currently no single test that can do the entire Four-Compartment model on its own. Multiple tests must be conducted that can measure fat mass, body water, and fat-free mass broken down into bone mass and the residual components. The only remaining component of the body left unknown by these tests is the value of the residual, un-measurable fat-free mass (such as DNA mass, etc.)., but as is taught in 5th grade algebra, it’s pretty easy to solve for one unknown variable when all others are known. In this way, the Four-Compartment Model can make a pretty accurate assessment of body fat percentage.

The Four-Compartment test is the Gold Standard. You will see that most companies claim their machines to be the Gold Standard in body composition measurements; DEXA, BIA, and BOD POD are three culprits in this marketing scheme. The only true Gold Standard, however, is the combination of multiple tests that give enough variables to calculate the Four-Compartment Model.


The Tests

Now that you have a general understanding of what is actually being measured in these tests, let’s look at the actual individual tests to see which is the most practical and realistic for you. The tests are broken down into three categories: indirect methods (observational, noninvasive methods), direct methods (include some type of physical penetration), or criterion methods (big fancy accurate machines, AKA most expensive).

Indirect methods:

Measuring Body Composition: An Overview | Liberate Breakfast Measuring Body Composition: An Overview | Liberate Breakfast

The most common indirect methods are known as Anthropometric measurements, including measuring weight, stature (height), and Body Mass Index (BMI). Techniques such as Circumference Measurements and Skinfold Techniques fall into Anthropometry. Both of these measurements take skill, but little else. You can learn how to measure your own body circumference measurements here. Skinfold measurement techniques are an art that is slowly dying due to the newer more accurate machines mentioned below. Though if you are able to find someone with a steady hand and experience in measuring skinfolds, you may be able to get a fairly accurate reading with little hassle or financial burden.

Measuring Body Composition: An Overview | Liberate Breakfast

Bioelectric Impedance Analysis (BIA) is another indirect method of measurement. BIA machines send an electrical current through the body, and based on resistance, they estimate the total body water (TBW), fat-free mass (FFM), and fat mass (FM). BIA tends to be more accurate on a population level than on an individual level (this is true of all methods because of fancy statistical analysis, but especially true of methods that tend to have greater thresholds of error for the individual). BIA machines are calibrated for certain populations that assumptions are made for, and because of this, if you are not in that population the machine was calibrated for, the resulting body composition estimate can be quite wrong. When using a BIA, try to find the type that uses both your hands and feet for the electric current, they tend to be more accurate and consistent than the machines that only use your feet.

Direct methods:

Total Body Water is a direct method in which a person consumes a certain isotope that can be measured in the body. The isotope dilutes throughout the body over a set period of time into the body’s water. It is then measured to find out how much total water is contained in the body. Once this is measured, it is used to predict the amount of fat-free mass in the body. Simple subtraction then allows for the calculation of fat mass. There are two important assumptions to note in this method: one, that the relationship between total body water and fat-free mass is constant, and two, that the time of measurement after the isotope is fully diluted is correct and not premature or too late.

Measuring Body Composition: An Overview | Liberate Breakfast

Two other less known and used direct methods of measuring body composition are Total Body Counting and Neutron Activation. Total body counting measures the amount of naturally radioactive potassium in the body. From the measured potassium levels, fat-free mass can then be estimated. This method isn’t really used that often because there are few machines that are capable of doing it, nothing to be said of its accuracy or potential for error. Neutron activation is some crazy Bruce Banner Incredible Hulk shit. A person is exposed to Gamma radiation which somehow affects the constitution of cell nuclei. Needless to say this method isn’t widely used because of the high levels of radiation that are blasted into, through, and around subjects going through the procedure.

Criterion methods:

These methods are considered more accurate and reliable than the direct and indirect methods.

Measuring Body Composition: An Overview | Liberate Breakfast

Underwater Weighing: Looks like fun…

Measuring Body Composition: An Overview | Liberate Breakfast

Bod Pod… AKA Escape Pod


Body density: Both Hydrodensitometry (underwater weighing) and Air Displacement Plethysmography (ADP) estimate body composition by measuring body weight, volume, and residual lung volume. Basically, how much water or air you displace can be used to calculate your fat mass from your fat free mass. These methods both have their own associated issues. For instance underwater weighing is difficult for certain patients because it is hard to fully submerge them underwater, you know like old people, children, and the physically incapable. ADP, often called the Bod Pod, makes certain assumptions about tissue density which can yield inaccurate results if the assumptions are off. Issues aside, both of these methods are a fairly accurate method for measuring your body composition; just ensure that you continue to go to the same machine for follow-on assessments to ensure consistency.

Measuring Body Composition: An Overview | Liberate Breakfast

Dual-Energy Xray Absorptiometry (DEXA) is fast, relatively harmless, and easy to use. The DEXA machines blast two low-energy level waves through the subject which have the ability to discriminate between adipose tissue, soft tissue, bone mineral content, and bone mineral density. Some studies have shown that DEXA has a tendency to overestimate fat-free mass; this means you may think you are more muscular than you actually are. If you have an overly honest friend or relative they should help keep your head from getting too big from this slight over estimation. All DEXA machines vary, just because one machine reads one body fat percentage, another might read something different. All machine models and manufacturers make differing assumptions in their equations that they input into the machines at programming. There is no coherence across machines. So again, frequent the same establishment for your follow-on tests.

Measuring Body Composition: An Overview | Liberate Breakfast

Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are also known to be used to get body composition readings. Both machines are in high demand for other tests and procedures, and for this reason they aren’t generally used for body composition. In addition, technicians need to train on how to conduct body composition procedures and upgrade their machine’s software. If you are able to find a place to conduct one of these tests for you, consider yourself lucky…. Also keep into account that because these methods aren’t widely used, there is potential error in their measurements that are not yet fully realized.


The Gold Standard

The Four-Compartment Model is the gold standard for measuring body composition. The idea that there is a singular Gold Standard  is a misnomer. There is no one technique or machine that measures in the Four-Compartment model. Multiple tests are needed in order to calculate the four compartment equation. Typically this is conducted by combining DEXA for bone mineral mass, Underwater weighing or BOD POD for body density and body fat, and Total Body Water dilution to measure total body water.

If the Gold Standard is really required for scientific analysis then multiple tests must be conducted and then combined to get the most accurate possible reading. This makes sense actually when you put it in simple terms. As an individual it would be nice to get a Gold Standard reading but it is often outside of reasonable expectations. Many funded broad scope scientific experiments can’t even get Gold Standard testing for their research. If you can find a place to do it expect to pay a premium.


 A whole lot of words for something that isn’t that necessarily necessary.

Ultimately body fat percentage is a number you don’t need. You can live a perfectly happy and healthy life and never get your body composition measured. Shit, you can lead a perfectly happy, healthy, and successful figure competitor or lingerie model career and never get your body composition measured. What truly matters for health is how you function, how you perform, and how you look at the world. If you aren’t chronically ill and have no issues digesting your food, then you function well. If you are able to do the types of exercise and recreation that you enjoy, as well as get through everyday chores without pain, then you perform well. If you can find the silver lining in life’s challenges and have relationships that make you a better person, then your world view is probably quite alright.

For those quantitatively-minded folks who enjoy experimenting with their bodies, AKA biohacking (ugh I find this term pretty nerdy) then getting your body composition measured is a great way to get some before and after metrics for whatever you are implementing into your life.



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