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Assessment of Body Composition

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

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

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

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

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

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

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


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

10 Skinfold Thickness Limitations Technician error
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 Poorly predicts visceral fat


12 Single Site Measurements
Tricep skinfold thickness Subscapular skinfold thickness not for estimating body fat determination for comparing against other reference data NHANES II ( ) appendix O (p ) (TSF) appendix P (p ) (SSF)

13 Two site measurements Tricep SF and Subscapular SF
correlated with body fatness in children fig (p192) Tricep SF and calf SF fig (p 192)


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



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

19 Hydrodensitometry

20 Hydrodensitometry Principle: two compartment model
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 Density = Body weight/Body volume
How does one estimate body volume? Archimedes principles: volume of submerged object = volume of water displaced weight in air - weight underwater = weight of water displaced

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


24 Hydrodensitometry Calculations
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 77 deg = kg/L CALCULATIONS BV = (BW-UWW)/.997 – (RV +0.1) BV = ( )/.997 – ( ) BV = – 1.4 = L

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

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

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


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


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

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

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

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

35 BIA: basic theory Volume is equal to length of the cylinder times its area Therefore, knowing the resistance and the length, one can calculate volume. 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 Assume fat free mass has a constant proportion of water (about 73%) Then calculate fat free mass from body water Assume BW = FFM + FM Then calculate fat mass and %body fat

37 NHANES III BIA Equations
Males FFM = H2/R W R Females FFM = H2/R W R Where FFM = fat free mass (kg) H = height (cm) W = body weight (kg) R – resistance (ohms) % BF = 100 x (BW-FFM)/BW

38 BIA Calculations DATA CALCULATIONS R = 520 ohms BW = 170 lbs = 77.3 kg
H = 70” = 178 cm CALCULATIONS FFM = (0.65H2/R)+0.26W+0.02R FFM = (0.65x1782/520)+0.26(77.3)+0.02(520) FFM = = 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
costs ($500-$2000) portable non-invasive fast Limitations accuracy and precision no better/worse than hydrodensitometry

40 Major types of BIA analyzers


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

43 What is a ‘normal’ % body fat?
Classification Males Females 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, (p195)

44 Body Composition Data NHANES III – 1988-1994
All adults > 19 yrs Mean % Body Fat Men: 21.9% % (SD) Women: 32.4% % Mean BMI Men: Women: Mean waist circumference Men: cm (cutpoint > cm) Women: cm (> 89 cm)

45 Body Composition Data NHANES III – 1988-1994
Adults with BMI = Mean % Body Fat Men: 17.6% + 7.8% (SD) Women: 26.7% + 8.9% Mean BMI Men: Women: Mean waist circumference Men: cm (cutpoint > cm) Women: cm (> 89 cm)

46 Dual-Energy X-ray Absorptiometry

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


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