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 MASS0.9 gm/ml LEAN BODY MASS 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 V water displaced = Measured Volume of water displaced by the Body RV = Residual Volume C = Estimate of volume of entrapped intestinal gas

Archimedes ( 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 DENSITOMETRY Underwater Weighing Small Tank or Open Swimming Pool

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

Doubly Indirect Methods for the Estimation of % Body Fat

General Research Approach for 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 % 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 Methods for 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 = (Sum 6 SF) Female:% Fat = (Sum 6 SF) Canadian University Students Can never give a negative answer. What if weight alone changes or is different?

Durnin & Womersley Density = a (log 10 Sum 4 SF) + c Overpredicts by 3 - 5% Fat British (left side) Age and gender specific equations Upper body sites Electronic Skinfold Caliper

Dual-Energy X-ray Absorptiometry

DEXA, DXA Dual 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

BMI = 12.6 %Fat = 3.2% BMI = 23.7 %Fat = 48.1% BMI = 18.1 %Fat = 23.1%

What DEXA Measures v Fat and fat-free mass (based upon the standards) v Bone Mineral Mass v Regional results for the above

DEXA Cannot Measure... v Protein Mass v 3-D Fat Distribution v Hydration Status v 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