Have you debated about eliminating body fat testing all together? Maybe you have gone to the opposite extreme and apply a very extensive assessment protocol for clients. If you are not confident with the method of body fat testing you are using, or if you have the purchasing power and authority to decide which tests should be used, before you pinch one more inch or sit your clients down into an egg-shaped machine, read this article for the facts on the most common feasible methods of body testing available today.
Hand-held BIA devices have gained popularity because of their low cost and ease of use. Measurements of body fat are determined by a voltage drop incurred when an individual creates a closed circuit by holding onto the BIA device with both hands. Because water conducts the current faster, and muscle mass carries the majority of water, a lower voltage drop (or better conductivity) should mean a lower percent body fat value. Conversely, fat does not store much water; therefore, an individual with a high body fat percentage should have a greater resistance to the current (or lower conductivity). Sounds good in theory, but there are some major limitations to this method. First, the current only passes through the upper body, not the whole body. Second, bone tissue is considered a part of fat-free mass yet is resistive and ends up in the wrong category (with fat). Third, hydration levels can have a significant effect on the result. Lastly, high skin temperatures can give erroneous readings.
Obviously, there are a few things that can be done to limit the sources of error. For example, perform BIA testing at the same time of day and before workouts to control for hydration level and the error associated with elevated body temperature. The limitations that cannot be controlled for, the measurement of the upper body only and the resistance attributed to bone tissue, should be considered when selecting this method to assess body composition. Depending on the manufacturer and model used, the standard error of measure for these devices is maybe as high as ±8%.
The Two-Component Model
The other methods of assessing body composition addressed in this article are based on what is called the component method. The most popular is the two-component model, which separates the body into fat and fat free mass (the two components). There is a major limitation to this model because fat-free mass does not just consist of muscle but also includes total body water and bone mineral. Errors in the estimated density of fat-free mass is a result of how the most commonly used formula, calculated by Dr. Siri more then four decades ago, was created.
The Siri equation first has to assume a density value of fat mass separately and for bone mineral, total body water and protein collectively. The assumed distribution of fat-free mass tissues by the Siri equation is roughly six percent bone mineral, 73% total body water and 21% protein. In combination, the density values and distribution of the various tissues results in a fat density (0.9 g/cm3) that is accurate for every population but a density for fat-free mass (1.1 g/cm3) that does not take into account the variability in the distribution of bone mineral and total body water from person to person or population to population. With this inter-person and inter-population variability, an accurate estimate of muscle mass increase or decrease is difficult, if not impossible, to assess. Not surprisingly, the inaccurate estimate of fat-free mass density produces most of the error in using techniques such as skinfold thickness, underwater weighing and the Bod Pod.
Skinfold testing involves pinching and measuring folds of subcutaneous (below the skin) fat at various well-researched locations on the body. Common skinfold protocols use a three-site or seven-site assessment. Specific sites used to assess men are the chest and abdomen, while female-specific sites include the triceps and suprailiac. Both genders are assessed at the subscapularis, midaxillary, mid-thigh and potentially the bicep and calf. Once the sum of the skinfolds is determined, a population-specific formula is used to determine body density. It is very important to select the formula that closely matches your clients ethnic background, gender and fitness status. The selected formula produces a density value which can then be used in the Siri equation or an equation like it to convert density into a percentage of body fat value (See figure below). Some error will still be introduced even if the appropriate equation is used to take the sum of skinfolds to body density. Of course, as already mentioned, even more error occurs when taking the body density value through the Siri or Siri-like equation to find percent body fat. However, skinfold assessments do provide a way to avoid all this error. Its simple: Dont use the formulas.
An effective way to use skinfold assessments is to take the values of the individual skinfold sites or the sum of skinfolds (in millimeters) and use that value or values to gauge changes in fat mass. For example, if the triceps fold changed from eight millimeters to six millimeters, fat mass has decreased, providing that all the other sites stayed the same or also decreased. Of course, a major source of error when doing skinfold testing is the test administrator, so practice on anybody that will let you. Measurements should be done on the right side of the body and done in a rotational manner, assessing each site at least twice. If any measurement is more then two millimeters apart, that site should be assessed again. The old saying is, You are not really proficient at skinfolds until you have successfully assessed over 100 people. From experience, I believe that to be true (See Advanced Fitness Assessment and Exercise Prescription, Heyward, 2006 or The American
Underwater Weighing and the Bod Pod
The last two methods of assessing body fat use essentially the same premise, except one method uses water and the other uses air. Underwater weighing works on the assumption that fat floats and thus your weight underwater, taking into account factors such as the water temperature, lung and GI air volume, will provide a body density measure. The Bod Pod, using air displacement, essentially works the same way but uses a pressure volume relationship generated in an air-tight compartment to find body density. The Bod Pod in particular is sensitive to body heat (dont exercise the day of the assessment) and requires that the participant is in a fasted state three hours leading up to the test. For both methods, once a density value is calculated, error is introduced by using the Siri or Siri-like equation to determine the percent body fat value.
A real problem in research you may notice is that underwater weighing in particular is commonly referred to as the gold standard, the test that all other tests are validated on; however, there is a two to four percent range of error (depending on what study you read) for underwater weighing. Validating field tests, such as BIA, with one of these gold standards does not produce an accurate representation of the true range of error associated with the field test in question. True gold standard methods are the four-component model (starting to appear in the literature) and other more intense assessments, such as exposing the body to high levels of radiation to determine molecular composition.
With all this potential error associated with common body composition measures, what good are they? Just remember: The percent body fat value is an estimate, not an exact measure of body composition. Expect error to be part of any measure, and understand that a testing protocol probably will not detect the one percent change your client has worked so hard to achieve. Differences in pre- and post-measures will need to be larger then the error associated with the selected body composition assessment. Thus, the value in body composition testing is detecting large changes (greater than ±4%, depending on the assessment used).
Search the current literature for assessment that best fits your client. For athletic clients, it may be skinfolds; for clients that are obese, perhaps just circumference measures are enough. A simple body assessment method I use is tracking one number: waist size. If a client is or is not losing weight but is dropping pant sizes, you dont need a Bod Pod, underwater tank or skinfold caliper to prove that body composition has changed.
Jason Miller, MS, CSCS, is a doctoral candidate in exercise physiology at the