L3 Body Composition Outline

Laboratory Instrumentation and Test Interpretation 420:260 Theoretical Basis of Body Composition

I. Underwater Weighing: A. Theory: 1. Archimedes Principle:

2. Two compartment model:

3. Differences in density between the two compartments: a. Fat density:

b. Fat-free density:

B. What’s Being Measured: Variables of Interest: 1. Body Density (Db): a. What is it?:

b. Unit of measurement:

2. Weight in Air (Wa): a. What is it?:

3. Tare Weight: a. What is it?:

4. Underwater Weight (Ww): a. What is it?:

5. Water Density (Dw): a. What is it: i. The density value of water changes with changes in temperature. See Table for corrected values. b. Unit of measurement:

6. Residual Lung Volume (RV): a. What is it?:

C. Sources of Error: 1. Variation in the estimation of residual volume.

2. Determination of underwater weight: a. Studies have indicated that 9-10 trials are needed in order to get a representative underwater weight, due the learning curve involved with immersion and the evacuation of air from the lungs. b. Which of the trials should be chosen as the underwater weight? i. Guideline #1: The average of the last 3 trials. ii. Guideline #2:  Choose the highest weight, if it is observed more than once.  Choose the second highest weight, if it is observed more than once and the first criterion is not met.  Choose the third highest weight, if it is observed more than once and the first two criteria are not met. iii. It is important to choose your selection procedure and stick with it. c. Conversion of Db to body fat %: i. It is assumed that the fat and fat-free compartments have a constant density for all individuals. Deviation from this assumption results in error. ii. When formulating the regression equations only a few cadavers were used. The cadavers did not represent a distribution of the normal population. D. Procedures: 1. Preparation of the Environment: a. Wash the tank. b. Tank filled with warm water (32-35 C). i. Bleach c. Spirometer: i. Paper loaded on drum. ii. Unused disposable tubes for measurement of VC. d. Weight scale and stadiometer. 2. Calibration: a. Spirometer: b. Weight scale:

c. Load cell or autopsy scale:

3. Preparation of the Subject: a. The validity of the underwater weighing measurement is affected by factors that influence the hydration status of the subject. The following recommendations should be made to the subject prior to testing: i. Recommend a normal diet, fluid intake and exercise pattern the day before testing. ii. Foods that cause excessive intestinal gas should be avoided. iii. Subjects should abstain from eating or smoking for a minimum of 2 hours prior to testing. iv. Measurement in the morning is ideal. v. Females should not be measured during menstruation or 3 days before or after. b. Subjects should wear minimal clothing due to the trapping of air by the fabric. c. Before entering the tank, subjects should void the bladder and bowels (if necessary). 4. Test Administration: a. Weight and height. b. Vital capacity for residual volume estimation. i. Perform at least 3 trials and use the best trial. ii. NOTE: Must correct VC for body temperature, pressure and water vapor (BTPS): Use the following table for the appropriate conversion factor. T (ºC) BTPS T (ºC) BTPS 20 1.102 29 1.051 21 1.096 30 1.045 22 1.091 31 1.039 23 1.085 32 1.032 24 1.080 33 1.026 25 1.075 34 1.020 26 1.068 35 1.014 27 1.063 36 1.007 28 1.057 37 1.000

c. Enter tank: i. Subject should rub skin and scalp vigorously once in the water to release air trapped against skin and hair. d. Sit in chair and put on weight belt (if necessary). e. Upon command subject blows out air from lungs and slowly lowers trunk until the head is completely submerged under the water and then continues to expel air until no more air can be expelled. f. Upon command, the subject brings head out of the water. g. Repeat until all trials are complete. 5. Calculations: See handout a. Residual Volume i. VC technique:  Men: RV = 0.24*VC  Women: RV = 0.28*VC ii. Stature technique:  Men: RV = 0.017 (age in years) + 0.06858 (height in inches) – 3.477  Women: RV = 0.009 (age in years) + 0.08128 (height in inches) – 3.900 b. Density of water (Dw):

Celcius Dw Celcius Dw 23 .9975690 31 .9953720 24 .9973270 32 .9950570 25 .9970750 33 .9947340 26 .9968140 34 .9944030 27 .9965440 35 .9940630 28 .9962640 36 .9937160 29 .9959670 37 .9933600 30 .9956780

c. Body Volume (BV): i. BV = [Wa – Ww / Dw] – (RV + 0.1)

d. Body Density (Db): i. Db = BW/BV e. Body fat percentage (BF%): i. Several equations:

 Brozek: BF% = [(4.57/Db) – 4.142]*100

 Siri: BF% = [(4.95/Db) – 4.5]*100 6. Interpretation: a. Check textbooks for normative data.