Body Composition
Techniques
DIRECT ASSESSMENT


The only direct methods for body composition assessment
are dissection or chemical analysis
Brussels Cadavre Study
– 13 female and 12 male cadavers, age range 55–94 years, 12
embalmed and 13 unembalmed
– After comprehensive anthropometry, each cadaver was
dissected into skin, adipose tissue, muscle, bones, organs and
viscera.
– Volumes and densities of all tissues were determined by
weighing the tissues underwater.
– A complete dissection lasted from 10 to 15 h and required a
team of about 12 people.
Indirect or Doubly Indirect
estimation of % Body Fat

All the techniques used routinely for % Body
Fat estimation are either:
–
Indirect
%
body fat is estimated using one or more
assumptions e.g. Underwater Weighing
–
Doubly Indirect
%
body fat is estimated by predicting the
results of an Indirect methodology from a
related measure by regression analysis
e.g. Skinfold prediction equations
Indirect Methods for the
Estimation of % Body Fat
DENSITOMETRY
BODY DENSITY = MASS / VOLUME
Units: gm/ml
Any method that determines the volume of the
body is a densitometric method
“GOLD STANDARD”


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Densitometry via underwater weighing was the
“gold standard” for determination of % body fat
since the 1940’s. Since the late 1990’s a 4
compartment method is regarded as the best
reference method.
Body Density can be determined accurately
Unfortunately, % Body Fat can not because of
assumptions made in transforming density to
% Fat
DENSITOMETRY
Volumetry by Water
Underwater (Hydrostatic) Weighing
Helium Dilution
BodPod – Whole Body Plethysmography
Predicting % Fat from Density
ASSUMPTIONS
Body can be divided into two components:
Fat & Non-Fat (Fat Free) Masses
Each has different, known and constant densities
Fat is not Adipose Tissue

FAT is ether extractable lipid molecules

ADIPOSE TISSUE is a tissue designed to
store FAT (lipid) in adipocytes. Contains all the
components of a tissue: cellular structures,
extracellular matrix, water etc. as well as FAT
(lipid) in the adipocytes. Adipose tissue is
found subcutaneously and internally
SIRI EQUATION
Assumed Densities:
FAT MASS
0.9 gm/ml
NON-FAT (FAT FREE) MASS
1.1 gm/ml
Equation:
% Body Fat = (4.95/Density) - 4.5) x 100
Siri Equation:
% Fat = (4.95/Density)-4.5) x 100
BROZEK EQUATION
Assumptions:
FAT MASS
0.9 gm/ml
LEAN BODY MASS
1.095 gm/ml
(some essential lipids in Lean Body Mass)
Equation:
% Fat = (4.57/Density)-4.142) x 100
DENSITOMETRY
Volumetry by Water
Determine body volume
by displacing water and
directly measuring the
change in water volume
DENSITOMETRY
Volumetry by Water
Where:
Wa = Body Weight in Air
Vwater displaced = Measured Volume of water displaced by the Body
RV = Residual Volume
C = Estimate of volume of entrapped intestinal gas
Archimedes (287-212 BC)
King Heiro of Syracuse summoned him to test the
composition of a supposedly gold wreath
If assumed to be an alloy of only Gold and Silver
he could use the laws of bouyancy to determine
the fractional composition
Pure Gold and Silver have constant and different
densities
DENSITOMETRY
Underwater Weighing
use Archimedes’ principle to determine body volume by
calculating weight of water displaced
Small Tank
or
Open Swimming Pool
DENSITOMETRY
Underwater Weighing
use Archimedes’ principle to determine body volume by
calculating weight of water displaced
Where:
Wa = Body Weight in Air
Ww= Body Weight freely submerged in water
Dw = Density of water
RV = Residual Volume
C = Estimate of volume of entrapped intestinal gas
DENSITOMETRY
Helium Dilution


Densitometry
Volume determined using a sealed chamber into which
a known volume of Helium is introduced.
–
–
–
–



Volume of air in chamber determined from dilution of Helium.
Volume without subject determined (V1)
Volume with subject determined (V2)
Body Volume of Subject = V1 – V2
Density = Mass / Body Volume
%Fat from Siri or Brozek equation
Does not require Residual Volume calculation
DENSITOMETRY
BODPOD - Whole Body Plethysmography
DENSITOMETRY
BODPOD - Whole Body Plethysmography



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 which results in bigger
increase in pressure with injection of known volume of air
Advantages over hydrodensitometry
– subject acceptability
– precision (reliability not accuracy)
Limitations
– costs: $25-30K
– still assumes constant density of FFM and fat for prediction
of % Body Fat from whole body density
DENSITOMETRY
BODPOD - Whole Body Plethysmography
TOTAL BODY WATER
(isotope dilution)

Determined by introducing a marker fluid that moves
freely in body water and is not metabolized.
–
–
–
–

Isotopes of water - Deuterium Oxide, tritiated water
Marker introduced.
Following equilibriation period (eg 2 hrs) sample body fluid
apply conversion formulae to estimate TBW,
% FAT predicted from TBW
–
–
Assume a constant for the fraction of water in the Fat Free
Mass or at least FFM (73.8%, 72,3% etc.)
Even if no technical error in Body Water, there would still be
S.E.E. = 3.6% Body Fat associated with biological variability
K40 - Whole Body Counting



K40 emits gamma radiation
Using whole body counters the amount of
radiation emitted can be determined
Fat Free Mass (Non-fat Mass) estimated
Assumptions:
– Constant fraction of K40 in potassium
– Constant fraction of potassium in non-fat
mass
Doubly Indirect Methods for
the Estimation of % Body Fat
Doubly Indirect Methods
for Estimating % Body Fat

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Skinfold predictions
Ultrasound
Radiography
Bioelectrical Impedance Analysis (BIA)
Near-infrared Spectrophotometry (NIR)
DEXA
General Research Approach
for Doubly Indirect Methodologies




Selected subject sample
Determine body density or % fat using an
accepted methodology; often underwater
weighing
Measure subjects with other technique
Produce regression equations to best predict
density or % fat from new technique
Regression Equations
to Predict % Body Fat
Y = mX + c
Y = % Body Fat
Y
X = Anthropometric
measure (Skinfolds
etc)
d
Correlation Coefficient
(r)
X
Standard Error of
Estimate (SEE)
Anthropometric (skinfolds)
prediction of % Fat
Adipose
Tissue





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

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

Constant Skinfold Patterning

Constant Skinfold Compressibility

Constant Tissue Densities

Constant Ratio of external/internal adipose tissue

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

RADIOGRAPHY

Measurements from radiographs
–

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


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

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 crosssectional area
Typical BIA Equations

Males
–

Females
–

FFM = -9.53 + 0.69H2/R + 0.17W + 0.02R
Where
–
–
–
–

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

Advantages
–
–
–
–

costs ($500-$2000)
portable
non-invasive
fast
Limitations
–
–
accuracy and precision
no better, usually worse than hydrodensitometry
Major types of BIA analyzers
Client Friendly
Site
Specific?
BIA Protocol

Very sensitive to changes in body water
–
normal hydration


Sensitive to body temperature
–

caffeine, dehydration, exercise, edema, fed/fasted
Avoid exercise
Sensitive to placement of electrodes
–
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 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

Two compartment plus
–
–
–

Water
Bone mineral
Protein
3 or 4 compartment models now regarded as
the reference standard rather than underwater
weighing