American Fitness Professionals & Associates

By Michael Darnley, RD

The term, “body composition” refers to the constituents that provide for structure, movement, and metabolic functions within the body. Body fat is one component of body composition and is often expressed as a percentage of total body weight.
Body composition can not be measured directly in living humans.

Thus, all methods of body composition measurement are only estimations, and these estimations are only as valid as the techniques that have been developed for this purpose. There are some common misconceptions regarding the application and measurement of body fat. In this article I will discuss several methods of measuring body composition and the relationship of body fat to performance.

2-Compartment and 4-Compartment Models

Methods of measuring body composition either use a 2-compartment model in which the body is divided into fat mass (FM) and fat-free mass (FFM), or a 4-compartment model, in which the FFM is further divided into water, protein, and bone mineral. Most of the field methods of body composition assessment are based upon the criterion methods used in a clinical setting.

These criterion methods are densitometry (underwater weighing), hydrometry (total body water), and potassium spectroscopy. Statistics (regression analysis) is used to develop an equation for calculation of % FM (and % FFM) from field methods that are based upon the results of the criterion methods.

It is important to understand the principles behind some of these measurement techniques and the general assumptions that they are based upon. Of the criterion methods, densitometry (currently the gold standard) is the most widely used method of body composition measurement. This method is used to determine the density of the body by measuring the total volume of fluid that is
displaced by the body. From this measurement, % fat can be calculated by assuming constant densities for FFM (1.1 g/cc) and FM (0.9 g/cc). Total body water is based on the assumption that there is a constant average water content of FFM in the body. FFM, estimated from potassium spectroscopy, assumes that there is a constant amount of potassium per kilogram of FFM in the body.

Because bone mass and muscle mass are not separated in the 2-compartment model, measurement errors are introduced when subjects are measured with different FFM densities than what is assumed. Deviations from the norm in mineral content (bone) are possible in swimmers, amenorrheic runners, and bodybuilders (1).

Although most new methods of body composition assessment are based upon the above 3 criterion methods, dual-energy x-ray absorptiometry (DXA) may one day be considered the gold standard for body composition measurement.  DXA is similar to an x-ray machine in that the body is scanned from above while a person lies on the table surface of the machine while minimal amounts of
radiation are emitted. DXA uses a 4-compartment model, which measures body water, body fat, body mineral content, and lean body mass. The limitations of this method are the cost of the machine and the space required. A certified technician is also needed to operate the machine, since radiation is used. This method can be more accurate in estimating body composition because all 4
compartments are measured.

Field Methods

Bioelectrical impedance analysis (BIA) is a convenient method of body composition assessment because it is noninvasive, quick, and does not require a lot of space. Electrodes are placed on the skin of the wrist and ankle, while a low-dose electrical current is passed through the body. The electrical conductivity of muscle is greater than that of adipose or bone due to the greater electrolyte content (2), and a leaner individual will provide less of an impedance to this current. FM and FFM can then be estimated using prediction equations.
The use of this method is of questionable value in athletes if hydration status is not controlled (3), since excessive water intake or dehydration can affect the conductance of the body. BIA should be performed under the same controlled conditions each time a measurement is performed. This method tends to overestimate FM in lean individuals and underestimate FM in obese individuals.

Near infrared interactance (Futrex 5000 is currently used) uses a fiberoptic probe that is positioned over the biceps while an electromagnetic radiation wave is reflected through soft tissue and conducted off the bone back into the probe.
This light beam is either reflected or absorbed and the interactance is measured by the energy of the light beam as it returns to the probe after being reflected through the tissue. NIR is very convenient, but there are very few studies that validate this method in comparison to criterion methods of body composition assessment. NIR also tends to overestimate FM in lean individuals and underestimate FM in obese individuals.

Skinfold measurements are the most widely used field method of body composition assessment. This method assumes that the thickness of the subcutaneous adipose tissue is proportional to the total amount of fat throughout the body. Equations for predicting FM from skinfolds have been developed based upon results of densitometry. It is beyond the scope of this article to discuss specific skinfold equations for predicting % fat, but it is very important to understand that there are hundreds of skinfold equations for different populations and ethnic groups (for predicting % fat in active people of all ages, refer to reference 4).

After skinfold thickness is measured, % fat should be estimated using an equation that was developed with a similar sex, age group, and activity level to the individual being measured. Equations using 7 sites are more accurate than those that use only 3 sites, but 3-site equations that use age in the calculation are widely used for the general population. Harpenden, Lange, and Slim Guide skinfold calipers are acceptable to use and have similar jaw compressions.

Body fat estimates can be given only in ranges; the typical error with most field methods is 3-5% even when a skilled practitioner performs the measurements.

This means that when body fat is measured, results should only be given in terms of a range. Error can be introduced in many ways with any of these methods.

For instance, when using skinfolds to predict FM, an appropriate prediction equation must be utilized, but the sites must also be measured correctly. Even very skilled practitioners usually measure each site 3 times and take an average of these to calculate the results. Error can also be introduced by failing to use a similar skinfold caliper than was used to develop the prediction equation.

Because it is often difficult to find an appropriate regression equation that fits the exact population being measured, the sum of skinfolds is another objective measure of body fat that can be used. Body composition changes can be monitored by the loss or gain in skinfold measurements. Personally, I prefer to use sum of skinfolds to monitor progress because it is very objective and not
subject to as much interpretation as % fat.

Body Fat and Performance

There is a tremendous amount of variation in the body fat of different groups of athletes. Percent fat in athletes can range from 5-20% in males and 10-20% in females depending upon the specific sport or activity. Athletes competing in sports where body weight is supported, such as swimming or kayaking, tend to have higher levels of body fat, whereas athletes involved in very high intensity
anaerobic (sprinting) or endurance events (marathon running) tend to have lower body fat levels (5).

Having more or less body fat can be an advantage or a disadvantage depending upon the activity. For instance, having more body fat can be an advantage for contact sports such as blocking in football or playing rugby. Having less body fat is an advantage when the main goal is to propel the body through space, as in long-distance running. Aerobic performance is negatively affected when body mass is increased (nonfunctional mass) in runners (6).

Because of the errors that are inherent in body composition measurement, caution should be exercised when interpreting or giving results. Remember these points when performing body composition assessments:

    · Learn as much as possible about the device or method that you will use.
     · After you are familiar with the method, practice performing it.
     · Research your population and find an appropriate prediction equation to use.
     · Consider giving results as sum of skinfolds if it is appropriate.
     · Remember that body composition results should only be expressed as a range.

References

1. Houtkooper, LB & Going, SB (1994) Body composition: How should it bemeasured? Does it affect sport performance? Gatorade Sport Science Exchange #52, 7(5).
2. Segal, KR, et al. (1985) Estimation of human body composition by electrical impedance methods: a comparative study. J. Appl. Physiol., 58, 1565-1571.
3. Jackson, AS, et al. (1988) Reliability and validity of bioelectrical impedance in determining body composition. J. Appl. Physiol., 64, 529-534.
4. Gatorade Sport Science Exchange (1994) Guidelines for body composition assessment. SSE #52 Supplement.
5. Fleck, SJ (1983) Body composition of elite American athletes. Am. J. Sports Med., 11(6), 398-403.
6. Cureton, KJ, et al. (1978) Effect of experimental alterations in excess weight on aerobic capacity and distance running performance. Med. Sci. Sports Exerc., 15:218-223.