2. Whole Body Pethysmography H Measures body volume by air displacement –actually measures pressure changes with injection of known volume of air into closed chamber H Large body volume displaces air volume in chamber –results in bigger increase in pressure with injection of known volume of air

4. Whole Body Pethysmography H Advantages over hydrodensitometry –subject acceptability –precision –residual lung volume not factor H Limitations –costs: $25-30K –still assumes constant density of lean and fat

5. Bioelectrical Impedance Analysis H 1994 NIH Technology Assessment Conference H “BIA provides a reliable estimate of total body water under most conditions.” H “It can be a useful technique for body composition assessment in healthy individuals”

6. Bioelectrical Impedance Analysis H BIA measures impedance by body tissues to the flow of a small (<1mA) alternating electrical current (50kHz) H Impedance is a function of: –electrical resistance of tissue –electrical capacitance (storage) of tissue (reactance)

7. BIA: basic theory H The body can be considered to be a series of cylinders. H Resistance is proportional to the length of the cylinder H Resistance is inversely proportional to the cross-sectional area

8. BIA: basic theory H Volume is equal to length of the cylinder times its area H Therefore, knowing the resistance and the length, one can calculate volume. H Assuming that the current flows thru the path of least resistance (water), then the volume determined is that of body water.

9. BIA: basic theory H Assume fat free mass has a constant proportion of water (about 73%) –Then calculate fat free mass from body water H Assume BW = FFM + FM –Then calculate fat mass and %body fat

10. NHANES III BIA Equations H Males –FFM = -10.68 + 0.65H 2 /R + 0.26W + 0.02R H Females –FFM = -9.53 + 0.69H 2 /R + 0.17W + 0.02R H Where –FFM = fat free mass (kg) –H = height (cm) –W = body weight (kg) –R – resistance (ohms) H % BF = 100 x (BW-FFM)/BW

11. BIA Calculations H DATA –R = 520 ohms –BW = 170 lbs = 77.3 kg –H = 70” = 178 cm H CALCULATIONS –FFM = -10.68+(0.65H 2 /R)+0.26W+0.02R –FFM = -10.68+(0.65x178 2 /520)+0.26(77.3)+0.02(520) –FFM = -10.6 + 39.6 + 20.1 + 10.4 = 59.5 kg –FM = W – FFM = 77.3 – 59.5 = 17.8 kg –%BF = (17.8/77.3)x100 = 23%

12. BIA: Advantages and Limitations H Advantages –costs ($500-$2000) –portable –non-invasive –fast H Limitations –accuracy and precision –no better/worse than hydrodensitometry

13. Major types of BIA analyzers

15. BIA Protocol H Very sensitive to changes in body water –normal hydration u caffeine, dehydration, exercise, edema, fed/fasted H Sensitive to body temperature –Avoid exercise H Sensitive to placement of electrodes –conductor length vs. height

16. What is a ‘normal’ % body fat? ClassificationMalesFemales Unhealthy range (too low) < 5%< 8% Acceptable range (lower end) 6-15%9-23% Acceptable range (higher end) 16-24%24-31% Unhealthy (too high) > 25%> 32% Nieman, 1999 (p195)

17. Body Composition Data NHANES III – 1988-1994 H All adults > 19 yrs H Mean % Body Fat –Men: 21.9% + 11.6% (SD) –Women: 32.4% + 17.8% H Mean BMI –Men: 26.5 + 7.8 –Women: 26.4 + 11.7 H Mean waist circumference –Men: 95.1 + 18.6 cm (cutpoint > 101.6 cm) –Women: 88.6 + 30.2 cm (> 89 cm)

18. Body Composition Data NHANES III – 1988-1994 H Adults with BMI = 18.5-25 H Mean % Body Fat –Men: 17.6% + 7.8% (SD) –Women: 26.7% + 8.9% H Mean BMI –Men: 22.7 + 3.2 –Women: 22.0 + 2.2 H Mean waist circumference –Men: 84.7 + 8.9 cm (cutpoint > 101.6 cm) –Women: 78.0 + 13.4 cm (> 89 cm)