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