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Assessment of Body Composition David L. Gee, PhD FCSN 442 - Nutrition Assessment Laboratory.

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Presentation on theme: "Assessment of Body Composition David L. Gee, PhD FCSN 442 - Nutrition Assessment Laboratory."— Presentation transcript:

1 Assessment of Body Composition David L. Gee, PhD FCSN 442 - Nutrition Assessment Laboratory

2 Body Composition Analysis vs. Body Weight Assessment H Advantages H “Direct” assessment of body fatness –Overweight u Overmuscled or overfat –Athletes –Assessing need for weight loss –inadequate stores in patients H Monitor changes –weight loss quality –effect of medical therapy

3 Body Composition Analysis vs. Body Weight Assessment H Disadvantages H relatively limited database H all field methods are estimations –false assumptions in all field methods –errors by technicians H limited understanding by clients

4 Nutrition and Athletic Performance ACSM/ADA 2000 Position Paper H “Body fat assessment techniques have inherent variability, thus limiting the precision with which they can be interpreted.” H “With carefully applied skin-fold or BIA,… –relative body fat % error of 3% - 4% u 15% (12-18%) –estimate fat-free mass within 2.5-3.5 kg u 50kg (47.5-52.5kg) H Would you buy a bathroom scale with this type of accuracy? –110 pounds + 7 pounds

5 Models of body composition H 2 compartment models H Fat mass and Fat-free mass –Fat mass and Lean body mass u LBM includes cell membranes, TG in cells H assessment methods using this model –skinfold thickness –hydrodensitometry –bioelectric impedance

6 Models of body composition H 4 compartment models H water, protein, fat, minerals H Assessment methods using this model –isotope dilution –dual emmision x-ray absorptiometry (DEXA) –computed tomography (CT, CAT) H Research techniques –Not covered in this course

7 Skinfold Thickness H measures double thickness of skin and subcutaneous fat H Advantages: –inexpensive –fast –portable –large database

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9 Skinfold Thickness H Assumptions: H predicts non-subcutaneous fat –>50% of fat is subcutaneous H sites selected represent average thickness of all subcutaneous fat H compressibility of fat similar between subjects H thickness of skin negligible

10 Skinfold Thickness H Limitations H Technician error H Skinfold thickness affected by factors other than amount of fat –exercise increases skin thickness –dehydration reduces skin thickness –edema increases skin thickness –dermatitis increases skin thickness H Poorly predicts visceral fat

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12 Single Site Measurements H Tricep skinfold thickness H Subscapular skinfold thickness H not for estimating body fat determination H for comparing against other reference data –NHANES II (1097-1980) –appendix O (p530-532) (TSF) –appendix P (p533-535) (SSF)

13 Two site measurements H Tricep SF and Subscapular SF H correlated with body fatness in children –fig. 6-32 (p192) H Tricep SF and calf SF –fig. 6-33 (p 192)

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15 Multiple Site Measurements H many sites H many equations H table 6-9 (p193) Jackson & Pollock H table 6-10 (p193) Durnin & Womersley H density and %body fat –Siri u % BF = (495/BD) – 450 –Brozek u % BF = (457/BD) - 414

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18 Circumference Measurements Katch and McArdle H Principle: –measure two “fat” sites –measure one “muscle site” –estimate fat and lean body mass. H Very limited database H Easy to do

19 Hydrodensitometry

20 H Principle: H two compartment model H density related to relative amounts of two compartments –D(fat) = 0.90 g/ml –D(lbm) = 1.10 g/ml –D(water) = 1.00 g/ml

21 Hydrodensitometry H Density = Body weight/Body volume H How does one estimate body volume? H Archimedes principles: –volume of submerged object = volume of water displaced –weight in air - weight underwater = weight of water displaced

22 Hydrodensitometry H wt of water displaced = vol of water displaced H Wt of water displaced = vol of body (BV) H Since weight of water displaced = weight in air - weight underwater –BV = BW-UBW H To calculate body density – BD = BW / BV H calculate %BF from BD

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24 Hydrodensitometry Calculations H DATA –BW(air) = 180 lbs = 81.6 kg –BW(water) = UWW = 3.6 kg –RV = 1.30 L, est GI gas vol = 0.1 L –Density of water @ 77 deg = 0.997 kg/L H CALCULATIONS –BV = (BW-UWW)/.997 – (RV +0.1) –BV = (81.6-3.6)/.997 – (1.3+0.1) –BV = 78.23 – 1.4 = 76.83 L

25 Hydrodensitometry Calculations H BV = 76.83 L H BD = BW / BV = 81.6/76.83 = 1.062 kg/L H % BF = (495/BD)- 450 = (495/1.062)-450 –%BF = 466.09-450 = 16.09% = 16% H Fat mass = 16% x 81.6kg = 13.1 kg H Lean mass = 81.6-13.1 = 68.5 kg

26 Hydrodensitometry: Assumption H Density of fat and lean are constant –bone density –muscle density –hydration status H GI gas volume is constant

27 Hydrodensitometry: Limitations H Measurement of residual lung volume H Precision of underwater weight H Cost H Non-portable H Limited types of subjects

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29 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

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31 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

32 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”

33 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)

34 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

35 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.

36 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

37 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

38 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%

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

40 Major types of BIA analyzers

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42 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

43 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)

44 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)

45 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)

46 Dual-Energy X-ray Absorptiometry

47 DEXA, DXA H Two different energy level X-rays H Lean, fat, and bone mass each reduce (attenuate) the X-ray signal in unique ways H Computer analyzes scan point by point to determine body composition H Method –Low dose radiation –20-30 minutes –Applicable to young and old

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