BODY FAT COMPOSITION

Body Composition is the technical term used to describe the different components that, when taken together, make up a person's body weight. Now you must keep in mind that body composition and body weight are two entirely different concepts, and they are not interchangeable. Evaluation of body composition is a common and important component of overall physical fitness assessment. It is well established that excess body fat is harmful to health, but many misconceptions exist regarding the assessment and interpretation of such data.
The accurate calculation of percent body fat is the true definition of fitness and obesity. The accurate measurement of Lean Body Mass is now the most rational basis for nutritional and exercise prescriptions. The importance of clinical body composition is now being recognised. There is evidence that research and interest in body composition was explored centuries ago by Archimedes, though most of the research data that is available on human body composition has been completed in the last forty years. With the recent interest in personal health, nutritional status and fitness, several methods of estimating body fat have been developed and used in clinical settings. Various body composition analysis techniques are reviewed in this article including:

� Hydrostatic Weighing.
� Height/Weight Tables.
� Body Mass Index.
� Skin Fold Measurements.
� Anthropometric Measurements.
� Infrared Interactance.
� Bioimpedance Assessment (BIA).
� ElectroLipoGraphy (ELG).
� Dual-Energy x-ray Absorptiometry (DEXA)

Hydrostatic Weighing:
Hydrostatic Weighing is currently considered the "Gold Standard" of body composition analysis. Hydrostatic measurements are based on the assumption that density and specific gravity of lean tissue is greater than that of fat tissue. Thus, lean tissue will sink in water and fat tissue will float. By comparing a test subject's mass measured under water and out of the water, body composition may be calculated. The "Gold Standard" of body composition is a mathematical prediction.
There are several limitations to the hydrostatic weighing technique. The equipment required to perform hydrostatic measurements is bulky and maintenance intense. A large tank of water, usually 1000 gallons, must be maintained at a constant temperature. Equipment to measure residual lung volume must be utilised. A calibrated autopsy scale or its electronic equivalent, connected to an "under-water chair" is also required. Test subjects are asked to exhale as much air as possible from their lungs and be immersed for 10 to 15 seconds for an underwater weight measurement to be taken. This procedure is repeated 7 to 10 times. Total test procedures may require 45 minutes to one hour. (Cohn, 1981).
Test Re-Test
The accepted test re-test of hydrostatic weighing is � 2.5% for comparison of consecutive tests with the same subject and the same technician. In non-controlled settings the test re-test variable may be significantly higher. Most of the variance is accounted for from a lack of subject co-operation and a lack of technician discipline and/or experience. Many tanks have not critically examined their test/re-test results. (Lohman, et al, 1981) (Katch, et al, 1984)
Bone Density
Hydrostatic weighing methodology assumes that the density of bone in humans is constant. Thus, differences in bone density will create test errors. African-Americans and trained athletes are now known to have a higher bone density than non-athlete Caucasians. The elderly and Asians have considerably lower bone density. Specialised equations have been developed for use with African-American populations. (Harsha, et al, 1978)
Convenience
Fear of immersion, fear of infection, obesity and infirmity are additional barriers to the Hydrostatic measurement of accurate body composition analysis.

Height/Weight Tables
In 1953 the Metropolitan Life Insurance Company developed the first height/weight tables to calculate the degree of individuals over or under weight status. The data was based on "averages" from its client base for both men and women. In 1983 the tables were revised based on updated data.
Frame size is an important, subjective factor utilised in the development of the tables with small, medium and large frame determinations changing the "ideal weight" recommendation. Improvement on frame size determinations were implemented in 1986 with the elbow breadth or wrist circumference measurements used to classify frame size.
The use of the Metropolitan height/weight table gives no indication as to the degree of either obesity or leanness on an individual basis. In the individual clinical setting, height/weight tables can provide grossly inaccurate conclusions about an individual's health risk. The validity of estimation of percent body fat and density by height and weight measurements when compared to the Hydrostatic tank is very poor with correlation coefficients in the range of .31 to .43. (Girandola et al 1989)

Body Mass Index (BMI)
Body Mass Index has recently been used to quantify an individual's obesity level. BMI is derived from a ratio equation of height squared divided by weight. Here again, only an individual's height and weight are used and no indication of actual lean or fat mass can be determined. Thus, BMI offers little advantage over the existing Metropolitan tables.

Skin Fold Measurements
The test methodology for body fat estimation with skin fold measurements requires the use of a "calliper device" to measure the thickness of substantial fat stores. The assumption is that substantial fat is proportional to over all body fat and thus by measuring several sites total body fat may be calculated.
There are many site measurements where skin fold measurements can be taken. Currently over 100 different equations are available to estimate body fat with the use of skin fold callipers. The wide variety of equations reflects the problem with the accuracy of this methodology.
There are many limitations with the Skin Fold measurement technique. The validity of skin fold measurements is at best �6% compared to the hydrostatic tank. Because of the inaccuracy associated with skin fold callipers, many credible organisations such as the U.S. Army and the Los Angeles Police Department have abandoned the use of them. (Stevens 1983)

Skin Fold Measurements-Inter-operator Error
The estimation results obtained from skin fold measurements vary widely from technician to technician. The "art" of skin fold measurements requires the technician to properly identify a site measurement and pinch the skin gathering only the fat store and no other tissue. The error of estimate between technicians has been reported to be �8%. (Smith, 1977)
Fat Storage
The assumption that 50% of human body fat is located in subcutaneal tissues and the remaining 50% is found in intra-muscular and essential fat (around organs) is not universally valid. Body fat distribution and health risk varies depending on genetics, exercise and nutritional patterns. (Cooper, et al, 1978)
Fat Thickness and Density
The obese population represents unique limitations for skin fold measurements. Skin fold callipers cannot open wide enough to measure the total fat thickness, thus tends to grossly under estimate body fat percentage in the obese population. Also of concern, especially in the obese population, is the compression of fat by the calliper due to variances in fat density. Again, this tends to inaccurately estimate percent fat in the obese, the population where accuracy is most important.

Anthropometric Measurement
The use of Anthropometric Measurement (girth and length) is a quick, easy and inexpensive method to estimate body composition. Using a standard calibrated cloth tape, girth and length measurements are taken from specific points on the body.
The methodology is based on the assumption that body fat is distributed at various sites on the body such as the waist, neck and thigh. Muscle tissue on the other hand is usually located at anatomical locations such as the biceps, forearm and calf. The subject's weight, height, girth size and ratios of various site comparisons are utilised in the calculations of percent body fat. Although the use of anthropometric measurements provides a reasonably reproducible value and gives a topographical assessment of an individual, the established accuracy for the prediction of body fat is at least �5% compared to the hydrostatic tank.

Near Infrared Interactance
The use of Infrared (IR) light to measure fat is not a new technique. The U.S.D.A first developed the technique to measure the fat contained in 1 cubic centimetre sections of beef and pork carcasses after slaughter. In the human device, a "wand" from the device emits an IR light source at about 900 nanometers into the biceps area.
The methodology is based on the ability of fat tissues to "absorb" more IR light than lean tissue, which can then be measured as a change in the infrared level.
The only commercially available unit to predict human body composition is manufactured by Futrex. Since the Futrex device was first marketed for clinical use, many research articles have been published stating that the device is not accurate and is not recommended for clinical use in the assessment of body composition.
The original application of this IR technology was developed on "skinned" carcasses. No research is available about IR penetration through the skin. The actual contribution of the IR wand measurement and input into the height and weight calculations used in the device's program have also been questioned. This device has not been approved by the FDA for use. (Israel et al 1989) (Davis et al 1989)

Bioelectrical Impedance
The use of bioelectrical impedance was first documented in 1880 (Kalvin), as a potentially safe, convenient and accurate technique to measure conductivity in the body. The method is based on the fact that the lean tissue of the body is much more conductive due to its higher water content than fat tissue. A bioimpedance meter is attached to the body, at the extremities, and a small 500-800 micro-amp, 50-kilohertz, signal measures the body's ability to conduct the current.
The more lean tissue present in the body the greater the conductive potential, measured in ohms. (Thomasett, 1963)
Linear Regression Formulas
In the early 1980's, the first commercial bioimpedance units were available to measure human body composition (RJL, Valhalla, Space Labs etc.). These bioimpedance units utilise "linear regression" formulas to predict body fat based on biological data input into a single equation.
A review of the literature indicates that bioimpedance units which utilise these linear regression equations tend to be somewhat valid for a "normal" population, but under-predict body fat for obese subjects and over-predict body fat of lean subjects. The standard errors of estimate for these equations are �5% to �6.4% in normal populations when compared to the hydrostatic tank. (Jackson et al 1988) (Segal et al 1988)

ElectroLipoGraphy (ELG)
In 1985, the first validated algorithmic equations to interpret bioimpedance measurements were developed and patented by Bio|Analogics. The use of the algorithmic equations instead of linear regression allows for population specific variables for lean, obese, elderly and paediatric subjects. The National Institutes of Health (NIH) conference on bio-impedance analysis (1994) concluded that to obtain valid predictions of percent body fat in humans, population specific equations must be applied. The algorithmic equations developed and patented by Bio|Analogics are population specific and validated on all types of subjects. The current validation studies contain more than 1000 subjects with an error factor of �3.3% and a correlation coefficient of 0.88 compared to the criterion hydrostatic tank.
As mentioned earlier, the accepted test/re-test variance with hydrostatic measurements is 2.5% when compared to itself. The accepted test/re-test variance for bioimpedance analysis is less than �.5%. (Girandola 1987)

Dual-Energy x-ray Absorptiometry (DEXA)
The DEXA instruments differentiate body weight into the components of lean soft tissue, fat soft tissue and bone, based on the differential attenuation by tissues of two levels of x-rays. equipment required: DEXA machine.
DEXA measurements are based on a three compartmental model rather than two compartment as in most other methods. It can also distinguish regional as well as whole body parameters of body composition. However the disadvantages are that the equipment is expensive, and often requires trained radiology personnel to operate, and if not for the limiting price of measurement, DEXA would be considered the criterion method of body composition analysis.

Conclusion:
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.

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 (non-functional mass) in runners. Because of the errors that are inherent in body composition measurement, caution should be exercised when interpreting or giving results. Elite athletes require very precise information and feedback about their physical condition and performance in order to adjust or continue their level of training. It would therefore be appropriate to suggest that elite athletes would benefit from hydrostatic weighing, the most accurate technique of body composition measurement.
Most elite athletes now have access to extensive facilities provided by their coaches and sports scientists, and having funding for such facilities, it would therefore be more economically viable for the elite athlete to use the more accurate methods of measuring body fat composition.
For the general member of the public, it is important to keep in mind that they do not have access to the extensive facilities provided for the elite athlete, and that there may also be a substantial financial cost for the most accurate methods. Skinfold measurements are the most widely used field method of body composition assessment due to its practicality and cost effectiveness. The test methodology for body fat estimation with skin fold measurements requires the use of a "calliper device" to measure the thickness of substantial fat stores, it provides a fairly accurate method of measurement and is relatively inexpensive.
The obese population represents unique limitations for skin fold measurements. Skin fold callipers cannot open wide enough to measure the total fat thickness, thus tends to grossly under estimate body fat percentage in the obese population. Also of concern, especially in the obese population, is the compression of fat by the calliper due to variances in fat density. Again, this tends to inaccurately estimate percent fat in the obese, the population where accuracy is most important.
Because measuring a person's body fat is tricky, doctors often rely on other means to diagnose obesity. Two widely used measurements are weight-for-height tables and body mass index. While both measurements have their limitations, they are reliable indicators that someone may have a weight problem. They are easy to calculate and require no special equipment.

Obesity has been linked to several serious medical conditions, including diabetes, heart disease, high blood pressure, and stroke. It is also associated with higher rates of certain types of cancer. Obese men are more likely than non-obese men to die from cancer of the colon, rectum, and prostate. Obese women are more likely than non-obese women to die from cancer of the gallbladder, breast, uterus, cervix, and ovaries.
For those who are extremely over weight it may important to consult their GP or to visit their local hospital for a more accurate method of body composition measurement, such as (DEXA) or Dual-Energy x-ray Absorptiometry, where a more clinical measurement will be taken and appropriate treatment may be given.

References:

Adams, J.A. (1985). Symposium on human body composition. Ross Medical Publications.

Cohn, D.A.; Kay, T.P.; Tatsch, R.F.; Thies, C.F.; (1981). Comparison of methods for estimating body fat in normal subjects and cancer patients. American journal of clinical nutrition, vol. 34, 2839-2847.

Durin, J.U.G.A.; Wormersly, J.; (1974). Body fat assessment from total body density and its estimation from skinfold thickness: Measurement of 481 men and women aged 16-72. British journal of nutrition, vol 32, 77-97.

Katch, F.I.; Katch, V.L.; (1980). Measurement and prediction errors in body composition assessment and the search for the perfect equation. Research Quarterly, vol. 51, no. 1, 249-260.

Israel, R.G.; Houmard, J.A.; O'briens, K.F. (1990). Validity of NIR for estimating human body composition. Medicine and science in sports and exercise, vol 21, no.2, s103.

References:

Adams, J.A. (1985). Symposium on human body composition. Ross Medical Publications.

Cohn, D.A.; Kay, T.P.; Tatsch, R.F.; Thies, C.F.; (1981). Comparison of methods for estimating body fat in normal subjects and cancer patients. American journal of clinical nutrition, vol. 34, 2839-2847.

Durin, J.U.G.A.; Wormersly, J.; (1974). Body fat assessment from total body density and its estimation from skinfold thickness: Measurement of 481 men and women aged 16-72. British journal of nutrition, vol 32, 77-97.

Katch, F.I.; Katch, V.L.; (1980). Measurement and prediction errors in body composition assessment and the search for the perfect equation. Research Quarterly, vol. 51, no. 1, 249-260.

Israel, R.G.; Houmard, J.A.; O'briens, K.F. (1990). Validity of NIR for estimating human body composition. Medicine and science in sports and exercise, vol 21, no.2, s103.