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”

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

DENSITOMETRYVolumetry by Water

Determine body volume by displacing water and directly measuring the change in water volume

DENSITOMETRYVolumetry by Water

 

Where:Wa = Body Weight in AirVwater displaced = Measured Volume of water displaced by the BodyRV = Residual VolumeC = 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

use Archimedes’ principle to determine body volume by calculating weight of water displaced

DENSITOMETRYUnderwater Weighing

Small Tank or Open Swimming Pool

use Archimedes’ principle to determine body volume by calculating weight of water displaced

DENSITOMETRYUnderwater Weighing

 

Where:Wa = Body Weight in AirWw= Body Weight freely submerged in waterDw = Density of water RV = Residual VolumeC = Estimate of volume of entrapped intestinal gas

DENSITOMETRYHelium 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

DENSITOMETRYBODPOD - 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

DENSITOMETRYBODPOD - Whole Body Plethysmography

DENSITOMETRYBODPOD - 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 Methodsfor Estimating % Body Fat

Skinfold predictions Ultrasound Radiography Bioelectrical Impedance Analysis (BIA) Near-infrared Spectrophotometry (NIR) DEXA

General Research Approachfor 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 Equationsto Predict % Body Fat

d

Y

X

Y = mX + c

Y = % Body Fat

X = Anthropometric measure (Skinfolds etc)

Correlation Coefficient (r)

Standard Error of Estimate (SEE)

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 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