Body Composition Techniques 2
Doubly Indirect Methods for the Estimation of % Body Fat
General Research Approachfor Doubly Indirect Methodologies
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
d
Y
X
% 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 from Indirect Method
New Predictor Variable Measures
Doubly Indirect Methodsfor Estimating % Body Fat
Skinfold (Anthropometric) predictions Ultrasound Radiography Bioelectrical Impedance Analysis (BIA) Near-infrared Spectrophotometry (NIR) DEXA (Dual Energy X-Ray Absorptiometry)
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
Anthropometric (skinfolds)prediction of % Fat
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.
Adipose Tissue
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
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)
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 cross-sectional area
Typical BIA 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
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
caffeine, dehydration, exercise, edema, fed/fasted
Sensitive to body temperature– Avoid exercise
Sensitive to placement of electrodes– conductor length vs. height
Near Infra-Red Spectrophotometry (NIR)FUTREX
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, DXADual Energy X-ray Absorptiometry
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
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 = 23.7%Fat = 48.1%
BMI = 18.1%Fat = 23.1%
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