Body Composition
Techniques 2
Doubly Indirect Methods for
the Estimation of % Body Fat
General Research Approach
for Doubly Indirect Methodologies
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Selected subject sample based upon required criteria.
Equations produced will be highly sample specific.
Determine body density or % fat of each subject using
an accepted indirect methodology; often underwater
weighing
Measure subjects with new predictor measure
Produce regression equations to best predict density or
% fat from new predictor measure
Equations often also include height, weight and activity
level and are age and sex specific
Regression Equations to Predict % Body Fat
% Body Fat from Indirect Method = m(New Predictor Variable Measures) + c
Regression Analysis produces: m = slope, c= intercept
Correlation Coefficient (r)
Standard Error of Estimate (SEE in units of Y)
% Body
Fat fromY
Indirect
Method
d
New Predictor Variable
Measures
X
Doubly Indirect Methods
for Estimating % Body Fat
There are no constants that can be applied to these
predictor variables that will give a prediction of % Fat.
They are all doubly indirect methodologies
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Skinfold (Anthropometric) predictions
Ultrasound
Radiography
Bioelectrical Impedance Analysis (BIA)
Near-infrared Spectrophotometry (NIR)
DEXA (Dual Energy X-Ray Absorptiometry)
Anthropometric (skinfolds)
prediction of % Fat
Adipose
Tissue
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Adipose Tissue not Fat
Equations predict % Fat (Lipid)
Over 100 equations available for the prediction of percentage body
fat or body density
All are sample specific
Specific for age, gender, activity level, nutrition etc.
Assumptions inherent in
prediction of % Fat from Skinfolds
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Based upon
densitometry
“Which is better UW Weighing or Skinfold predictions?”
%fat from skinfolds is predicted using equations developed from UW
Weighing of subjects.
UW Weighing: S.E.E. = 2.77% Fat
Skinfolds: S.E.E. = 3.7% Fat
Assumptions inherent in prediction
of % Fat from Skinfolds
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Constant Skinfold Patterning
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Constant Skinfold Compressibility
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Constant Tissue Densities
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Constant Ratio of external/internal adipose tissue
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Constant Fat (lipid) content of adipose tissue
YUHASZ
Male:
% Fat = 0.1051(Sum 6 SF) + 2.585
Female:
% Fat = 0.1548(Sum 6 SF) + 3.580
Canadian University Students
Can never give a negative answer.
What if weight alone changes or is different?
Durnin & Womersley
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Density = a (log10Sum 4 SF) + c
Overpredicts by 3 - 5% Fat
British (left side)
Age and gender specific equations
Upper body sites
Electronic Skinfold Caliper
Ultrasound
High Frequency Sound (6
MHz)
Some sound reflected at
tissue interfaces
Time taken for return of
sound used to estimate
distance based upon assumed
speed of sound in that tissue
% Fat prediction from Ultrasound
Regression equations predicting
densitometrically determined % Fat
 S.E.E.’s comparable to skinfold
predictions
 Beware of “predict anything from
anything” once it is in a computer
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RADIOGRAPHY
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Measurements from radiographs
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uncompressed tissue thicknesses
Regression equations predicting
densitometrically determined % Fat
BIOELECTRICAL IMPEDANCE
ANALYSIS (BIA)
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BIA measured by passing a
microcurrent through the
body
% Fat predicted from sex,
age, height, weight & activity
level + BIA
Influenced by hydration level
Claims that you can guess %
fat more accurately
Bioelectrical Impedance Analysis
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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)
BIA: basic theory
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The body can be considered to
be a series of cylinders.
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Resistance is proportional to the
length of the cylinder
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Resistance is inversely
proportional to the crosssectional area
Typical BIA Equations
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Males
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Females
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FFM = -9.53 + 0.69H2/R + 0.17W + 0.02R
Where
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FFM = -10.68 + 0.65H2/R + 0.26W + 0.02R
FFM = fat free mass (kg)
H = height (cm)
W = body weight (kg)
R – resistance (ohms)
% BF = 100 x (BW-FFM)/BW
BIA: Advantages and Limitations
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Advantages
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costs ($500-$2000)
portable
non-invasive
fast
Limitations
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accuracy and precision
no better, usually worse than hydrodensitometry
Major types of BIA analyzers
Client Friendly
Site
Specific?
BIA Protocol
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Very sensitive to changes in body water
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normal hydration
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Sensitive to body temperature
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caffeine, dehydration, exercise, edema, fed/fasted
Avoid exercise
Sensitive to placement of electrodes
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conductor length vs. height
Near Infra-Red Spectrophotometry (NIR)
FUTREX
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Near Infra-Red light emitted
from probe
Reflected light monitored
Changes due to differing
optical densities
Influenced by hydration
Relative Fat/Water Index may
be useful
Dual-Energy X-ray Absorptiometry
DEXA, DXA
Dual Energy X-ray Absorptiometry
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Two different energy level X-rays
Lean, fat, and bone mass each reduce
(attenuate) the X-ray signal in unique ways
Whole body
Regional
Osteoporosis
X-Ray Measurement System
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Dual energy attenuation values are measured
for each point in the image
Calibration standards (acrylic, aluminum,
delrin) are measured
The fat and lean mass of each point in the
image is calculated by direct comparison to the
standards
BMI = 12.6
%Fat = 3.2%
BMI = 18.1
%Fat = 23.1%
BMI = 23.7
%Fat = 48.1%
What DEXA Measures
 Fat
and fat-free mass (based upon the
standards)
 Bone Mineral Mass
 Regional results for the above
DEXA Cannot Measure...
 Protein
Mass
 3-D Fat Distribution
 Hydration Status
 Tissue inside bone (brain, marrow,
blood)
Next generation of Body
Composition Models
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Two compartment plus
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Water
Bone mineral
Protein
3 or 4 compartment models now regarded as
the reference standard rather than underwater
weighing