Determination of Body Composition

Transcription

1 Determination of Body Composition Introduction A variety of methods have been developed for assessing body composition, including isotopic determination of total body water, whole body 40 K counting, radiography, electrical conductance and impedance, etc. Two of the most common methods of assessing body composition, however, are hydrostatic weighing and determination of skinfold thicknesses. Although we won t be doing hydrostatic weighing as part of the lab activities, the method is important for you to understand. The hydrostatic or underwater weighing method is based upon the assumption that the body is composed of two components or compartments. The components are fat-free or lean mass (FFM), which is assumed to have a density of 1.10 kg/l, and a fat component, which is assumed to have a density of 0.90 kg/l. The density of the whole body, therefore, will depend upon the relative size of these two components. If the body density is known, it is possible to convert this to a % body fat using the following equation, which was derived by Siri: % fat= (495/body density)-450 Although the concept involved in determining body composition from body density is relatively simple, actually measuring body density can be difficult. By definition, density is the mass of an object divided by its volume (D=M/V). Although it is easy to determine the mass of an object using scales, it is very difficult to determine the volume of an object that has an irregular shape such as the human body. It is possible to measure the volume of the human body by submerging a person in water, and measuring their weight. The decrease in their body weight when submerged is equal to the weight of water they displace. By knowing the density of water, it is then possible to calculate the volume of water they displace, and therefore determine the volume of their body. This relatively simple concept is complicated by the fact that the volume of the whole body also includes certain volumes of gas, which have a very low density and therefore tend to cause a person to float. The two most important of these are the air in the lungs and the air in the gastrointestinal tract. The first of these is corrected for by having the subject exhale as much air as possible from their lungs before determining their weight underwater, and by measuring or estimating the amount of air they cannot exhale (residual volume). The air in the GI tract is assumed to be approximately 0.1 L when a person is fasted. Skinfolds Determination of body composition from skinfold measurements is based on the fact that a large proportion of total body fat is stored directly underneath the skin. Therefore, by measuring body composition by underwater weighing in a large number of people and by obtaining skinfold measurements in these same subjects, it is possible to develop equations to predict % body fat from simple skinfold measurements. There are some issues using skinfolds to estimate % body fat for a number of reasons, including distribution of body fat varies with age, sex, race, and athletic activity. Reasonably accurate prediction of % body fat from skinfolds therefore requires that the equation used for a given person to have been developed using a similar subject population. This has led to a huge number of population specific equations, many of which are probably based on too few subjects to be of any value. In an attempt to remedy this problem, Jackson and Pollock and Jackson, Pollock, and Ward have provided generalized equations that have been validated for various age groups and both athletic and non-athletic populations. These equations are: Men: D= (x) (x 2 ) (y) Women: D= (x) (x 2 ) (y)

2 Where x=sum of triceps, chest, and subscapular skinfolds (in mm) for men, and the sum of triceps, suprailium, and abdominal skinfolds for women, and y =age in years. Bioelectrical Impedance Bioelectrical impedance analysis to estimate body composition is based on the following simple principle. A small, alternating current flowing between two electrodes passes more rapidly through hydrated fat-free body tissues and extracellular water compared with fat or bone tissue because of the greater electrolyte content of the fat-free component. Impedance (resistance) to the electrical current flow relates to the amount of body water, which in turn relates to the amount of fat and FFM. With BIA, the person receives a painless, localized electrical current. The impedance measurement is determined based on the resistance to the current flow. This can then be converted to body density, adding body weight and height, gender, age, and sometimes race, level of fatness, and some girth measurements. We will be using the Tanita body composition scale to take measurements of impedance. Bioelectrical impedance analysis requires measurement under standardized conditions, particularly hydration status, recent food and beverage intake, skin temperature, and recent physical activity. Hydration affects BIA accuracy. For example, impedance is lower when there is significant body water loss as a result of exercise of fluid restriction. This would produce a lower body fat percentage. Hyperhydration would have the opposite effect. Skin temperature also affects impedance measurements. A warm environment may result in less impedance to electrical flow, and thus, a lower percent body fat. Even under optimal conditions, BIA measurements may be questionable compared to underwater weighing. BIA tends to overpredict body fat in lean and athletic subjects, and underpredict body fat in obese subjects. Girth Measurements Girth measurements offer an easily administered, valid and attractive alternative to skinfolds. Girth measurements should be taken at the following sites using the Gulick tape. The sites commonly used for girth measurements are: upper arm (biceps), forearm, abdomen, hips (buttocks), thigh, and calf. The equations we will use to estimate % body fat using girth measurements are designed for young and old men and women, provided the individual s physical characteristics resemble the original validation group. The equations should not be used for individuals who appear excessively fat or thin, or who participate regularly in strenuous sports or resistance training that often increases girth without changing subcutaneous body fat. Along with predicting % body fat, girth measurements can also be used to analyze patterns of body fat distribution. Goal Body Fat Percentage and Target Weight The following method can be used to determine goal body fat percentage (GBF%) and target weight (TW). 1. Multiply total body weight (TBW) by the body fat percentage (BF%) to determine fat weight (FW). 2. Subtract FW from TBW 3. The remaining weight is the fat free mass (FFM) 4. Determine an appropriate and reasonable GBF% 5. Divide the LBM by the (1-GBF%) 6. The answer will be the TBW at the predetermined GBF% 7. Subtract the TW from the TBW to determine the amount of weight loss (WL) required to achieve GBF% Step 1: TBW x BF%-FW Step 2: TBW FW=LBM

3 Step 3: LBM/(1-GBF%) = TW Step 4: TBW TW = WL Lab Activity: 1. With a partner, practice using the skinfold calipers at sites listed in your ACSM book. Take the measurements on the right side of the body. a. Place the calipers 1 cm away from the thumb and finger, perpendicular to the skinfold and halfway between the crest and base of the fold. b. Maintain the pinch while reading the calipers c. Wait 1-2 seconds before reading the calipers d. Take duplicate measures on each site e. Retest if measurement do not fall within 1-2 mm f. Rotate through the sites g. Record the measurements on a data sheet h. Calculate your subject s estimated percent body fat using the 7-site formula and one of the 3 site formulas listed on page 63 of the ACSM book For this activity, you should practice all of the skinfold sites, as demonstrated in lab. You may work at a facility that uses these sites for the estimation of body fat, so you should be familiar with all skinfold sites (described on page of ACSM Guidelines book). 2. With a partner, measure body weight in kg, height in cm, and bioelectrical impedance using the Tanita body composition scale or the handheld BIA. 3. Measure waist and hip circumference of your partner. Calculate Waist-to-hip ratio (WHR). a. Waist: Narrowest part of the torso (above the umbilicus and below xiphoid process) b. Hip: At the maximal girth of the hips or buttocks region above the gluteal fold 4. Utilize girth measurements to estimate body composition. Each measurement should be taken in duplicate and an average measurement value should be calculated for each site. Utilize the tables in Appendix (provided) to determine the constants for each site. a. Pull the Gulick tape to proper tension (so it is snug) without pinching the skin b. Check that tape is neither indenting the skin nor loose enough to leave gaps between the tape and skin c. Retest if the measurements do not fall within 1 cm d. Record the measurements e. For Young Men (18-26 years): Percent Fat = Constant A + Constant B Constant C 10.2 A=upper arm, B=abdomen, C=forearm For Older Men (27-50 years): Percent Fat = Constant A = Constant B Constant C 15.0 A=buttocks, B=abdomen, C=forearm f. For Young Women (18-26 years): Percent Fat = Constant A + Constant B Constant C 19.6 A=abdomen, B=thigh, C=forearm For Older Women (27-50 years): Percent Fat = Constant A + Constant B Constant C 19.6 A=abdomen, B=thigh, C=calf Again, you should practice and be familiar with all girth measurements, as demonstrated in lab and described on pages in ACSM Guidelines book.

4 Write Up: After each of you have practiced, conduct a body composition assessment on your partner using the above methods in the form of a lab report. Include the following sections: introduction, results, discussion/implications. 1. Utilize the skinfolds, bioelectrical impedance, and girth measurements above to calculate % body fat and lean body mass. Do these numbers agree with one another? If not, can you suggest some reasons why? 2. Utilize measurements of body weight and height to calculate body mass index (weight in kg/(height in meters) 2 ). 3. Identify the Disease Risk Classification for BMI and WHR from measurements (ACSM, p. 58). Compare the results of your subject to norms (if norms are provided). 4. Indicate the results of the Goal Body Fat Percentage and Target Weight for your subject. 5. Identify the positive and negative components of the various body composition methods in relation to cost, time, validity, ease of test, and overall practicality.

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