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Assessment of Body Composition
David L. Gee, PhD
FCSN 442 - Nutrition Assessment Laboratory
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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 patientsMonitor changes
– weight loss quality– effect of medical therapy
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Body Composition Analysis vs. Body Weight Assessment
Disadvantagesrelatively limited databaseall field methods are estimations
– false assumptions in all field methods
– errors by technicianslimited understanding by clients
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Nutrition and Athletic PerformanceACSM/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 2.5-3.5 kg
50kg (47.5-52.5kg) Would you buy a bathroom scale with this type
of accuracy?– 110 pounds + 7 pounds
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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
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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
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Skinfold Thicknessmeasures double thickness of skin
and subcutaneous fatAdvantages:
– inexpensive– fast– portable– large database
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Skinfold ThicknessAssumptions: 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
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Skinfold ThicknessLimitationsTechnician 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
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Single Site Measurements
Tricep skinfold thickness Subscapular skinfold thickness not for estimating body fat determination for comparing against other reference data
– NHANES II (1097-1980)
– appendix O (p530-532) (TSF)– appendix P (p533-535) (SSF)
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Two site measurements
Tricep SF and Subscapular SFcorrelated with body fatness in
children– fig. 6-32 (p192)
Tricep SF and calf SF– fig. 6-33 (p 192)
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Multiple Site Measurementsmany sitesmany 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
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Circumference MeasurementsKatch and McArdle
Principle: – measure two “fat” sites – measure one “muscle site” – estimate fat and lean body mass.
Very limited databaseEasy to do
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Hydrodensitometry
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HydrodensitometryPrinciple:two compartment modeldensity 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
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HydrodensitometryDensity = Body weight/Body volumeHow does one estimate body volume?Archimedes principles:
– volume of submerged object = volume of water displaced
– weight in air - weight underwater = weight of water displaced
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Hydrodensitometry 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
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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 water @ 77 deg = 0.997 kg/L
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
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Hydrodensitometry Calculations
BV = 76.83 LBD = BW / BV = 81.6/76.83 = 1.062 kg/L% BF = (495/BD)- 450 = (495/1.062)-450
– %BF = 466.09-450 = 16.09% = 16% Fat mass = 16% x 81.6kg = 13.1 kgLean mass = 81.6-13.1 = 68.5 kg
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Hydrodensitometry:Assumption
Density of fat and lean are constant– bone density
– muscle density
– hydration status GI gas volume is constant
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Hydrodensitometry:Limitations
Measurement of residual lung volume
Precision of underwater weightCostNon-portableLimited types of subjects
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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
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Whole Body Pethysmography
Advantages over hydrodensitometry– subject acceptability
– precision
– residual lung volume not factorLimitations
– costs: $25-30K
– still assumes constant density of lean and fat
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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”
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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)
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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
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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.
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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
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NHANES IIIBIA Equations
Males– FFM = -10.68 + 0.65H2/R + 0.26W + 0.02R
Females– FFM = -9.53 + 0.69H2/R + 0.17W + 0.02R
Where – FFM = fat free mass (kg)– H = height (cm)– W = body weight (kg)– R – resistance (ohms)
% BF = 100 x (BW-FFM)/BW
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BIA CalculationsDATA
– R = 520 ohms– BW = 170 lbs = 77.3 kg– H = 70” = 178 cm
CALCULATIONS– FFM = -10.68+(0.65H2/R)+0.26W+0.02R– FFM = -10.68+(0.65x1782/520)+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%
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BIA: Advantages and Limitations
Advantages– costs ($500-$2000)– portable– non-invasive– fast
Limitations– accuracy and precision– no better/worse than hydrodensitometry
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Major types of BIA analyzers
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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
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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, 1999 (p195)
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Body Composition DataNHANES III – 1988-1994
All adults > 19 yrs Mean % Body Fat
– Men: 21.9% + 11.6% (SD)– Women: 32.4% + 17.8%
Mean BMI– Men: 26.5 + 7.8– Women: 26.4 + 11.7
Mean waist circumference– Men: 95.1 + 18.6 cm (cutpoint > 101.6 cm)– Women: 88.6 + 30.2 cm (> 89 cm)
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Body Composition DataNHANES III – 1988-1994
Adults with BMI = 18.5-25 Mean % Body Fat
– Men: 17.6% + 7.8% (SD)– Women: 26.7% + 8.9%
Mean BMI– Men: 22.7 + 3.2– Women: 22.0 + 2.2
Mean waist circumference– Men: 84.7 + 8.9 cm (cutpoint > 101.6 cm)– Women: 78.0 + 13.4 cm (> 89 cm)
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Dual-Energy X-ray Absorptiometry
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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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