Historically, researchers began to examine the chemical composition of the human body in the 1950-1970’s. These studies make up the chemical analysis of the body and is probably the only means of accurately determining body fat. All the other methods attempt to predict the body fat based upon a measurement of something else and certain assumptions. While research methods are available that allow for pretty good estimates, most methods are simply good guesses.
We must first understand that there are reference methods and prediction methods. A reference method is a method by which some component(s) of the body are directly measured and the body fat percent (%BF) is then determined. The prediction method is an indirect measure of something that is compared to the reference method with hopes of predicting the same value as that reference method. Examples of reference methods include underwater weighing (UWW), Dual-energy X-ray absorptiometry (DXA) and the four (4) compartment method (4C). Prediction methods include skin fold measurements (SK), body mass index (BMI), body circumferences, and bioelectric impedance assessment (BIA). As one researcher summarized:
“The measurement of body composition in the truest sense allows for the estimation of body tissues, organs, and their distributions in living persons without inflicting harm. It is important to recognize that there is no single measurement method that is error-free. Furthermore, bias can be introduced if a measurement method makes assumptions related to body composition proportions and characteristics that are inaccurate across different populations. Some methodologic concerns include hydration of fat-free body mass changes with age and differences across ethnic groups; the density of fat-free body mass changes with age and differences between men and women; total body potassium decreases with age and fatness and differences between African Americans and Caucasians; the mass of skeletal muscle differences across race group; and visceral fat differences across sex and race groups, independent of total adiposity.”
What are compartments?
A body compartment is a type of tissue. The two most common compartments used are the fat mass (FM) and fat-free mass (FFM). Other compartments that can be combined with FM is lean body mass, body potassium, or body water. Most methods of measuring body fat use FM and FFM.
Compartmental models of body composition. From left to right the columns represent a 2 compartment model, a 3 compartment model, a 4 compartment model and a 6 compartment model. ICF = intracellular fluid, P = protein, G = glucose + glycogen, ECF = extracellular fluid, M = mineral mass, FFM = fat free mass. For each model, fat is represented as 25% of weight. Mineral is 4% in the 3, 4 and 6 compartment model. Protein is 22%.
In these models the attempt is to measure the FFM. Then the FM is derived by subtracting this from the total body weight. The classic method is UWW. Two nuclear methods were also derived at that time – radioactive potassium counting 40K, and radioactive water. The assumption was that FFM has a constant hydration of 0.732 liters/kilogram and that FFM has 68.1 meq/kilogram of potassium. These assumptions are fine for young, healthy adults, but it fails for young children, seniors, different ethnic groups, or persons with certain diseases.
To improve accuracy the 2-C model was expanded to the 3-C model requiring that the UWW measurement include a measure of total body water. This model assumes the density of water, fat, and body solids. However, patients with certain diseases may be protein depleted or have less bone mineral mass making the assumptions inaccurate. Thus, the estimated values for the density of these solids would be incorrect and the final estimate of total body fat mass would also be inaccurate.
Four-compartment method (4C)?
Now we just extend the 3-C model by one more level. FFM is divided into three compartments – protein, mineral and water – from which we predict the fourth compartment, FM. Again, we start with UWW. We assume the density of protein and mineral to be 1.34 and 3.075 kilograms per liter. To measure the amount of mineral (bone) present (its mass) we use DXA, which is commonly available. However, to measure total protein, requires neutron activation analysis and there are only 8 research centers with this ability in the world. So, we assume that protein mass is proportional to the bone mineral mass, independent of age and gender. In the short term, for any individual, this is pretty accurate. However, over a long time or with substantial weight loss, the protein to mineral ratio does change.
Another 4-C model is available that does not require the UWW measurement. The body’s FFM is divided into the body cell mass (BCM), extracellular water (ECW), and extracellular solids (ECS). The BCM is measured by measuring whole body potassium, as this is the dominant body storage for potassium. The ECW compartment is measured by a dilution method with bromide or sulfate. Lastly, the ECS is defined as total body calcium or bone mineral. The main problem with this model is that the measurements (and their errors) are cumulative and effect the final estimate for body fat mass.
As of today, this 4C method is the most accurate method researchers have to measure body fat and is the best method compare others against.