Assessment of Body Composition

David L. Gee, PhD

FCSN 442 - Nutrition Assessment Laboratory

Body Composition Analysis vs. Body Weight Assessment

Advantages “Direct” assessment of body fatness

– Overweight Overmuscled or overfat

– Athletes– Assessing need for weight loss

– inadequate stores in patientsMonitor changes

– weight loss quality– effect of medical therapy

Body Composition Analysis vs. Body Weight Assessment

Disadvantagesrelatively limited databaseall field methods are estimations

– false assumptions in all field methods

– errors by technicianslimited understanding by clients

Nutrition and Athletic PerformanceACSM/ADA 2000 Position Paper

“Body fat assessment techniques have inherent variability, thus limiting the precision with which they can be interpreted.”

“With carefully applied skin-fold or BIA,…– relative body fat % error of 3% - 4%

15% (12-18%)– estimate fat-free mass within 2.5-3.5 kg

50kg (47.5-52.5kg) Would you buy a bathroom scale with this type

of accuracy?– 110 pounds + 7 pounds

Models of body composition

2 compartment models Fat mass and Fat-free mass

– Fat mass and Lean body mass LBM includes cell membranes, TG in

cells

assessment methods using this model– skinfold thickness– hydrodensitometry– bioelectric impedance

Models of body composition

4 compartment models water, protein, fat , minerals Assessment methods using this model

– isotope dilution– dual emmision x-ray absorptiometry

(DEXA)– computed tomography (CT, CAT)

Research techniques – Not covered in this course

Skinfold Thicknessmeasures double thickness of skin

and subcutaneous fatAdvantages:

– inexpensive– fast– portable– large database

Skinfold ThicknessAssumptions: predicts non-subcutaneous fat

– >50% of fat is subcutaneous sites selected represent average thickness

of all subcutaneous fat compressibility of fat similar between

subjects thickness of skin negligible

Skinfold ThicknessLimitationsTechnician error Skinfold thickness affected by factors

other than amount of fat– exercise increases skin thickness– dehydration reduces skin thickness– edema increases skin thickness– dermatitis increases skin thickness

Poorly predicts visceral fat

Single Site Measurements

Tricep skinfold thickness Subscapular skinfold thickness not for estimating body fat determination for comparing against other reference data

– NHANES II (1097-1980)

– appendix O (p530-532) (TSF)– appendix P (p533-535) (SSF)

Two site measurements

Tricep SF and Subscapular SFcorrelated with body fatness in

children– fig. 6-32 (p192)

Tricep SF and calf SF– fig. 6-33 (p 192)

Multiple Site Measurementsmany sitesmany equations table 6-9 (p193) Jackson & Pollock table 6-10 (p193) Durnin & Womersley density and %body fat

– Siri % BF = (495/BD) – 450

– Brozek % BF = (457/BD) - 414

Circumference MeasurementsKatch and McArdle

Principle: – measure two “fat” sites – measure one “muscle site” – estimate fat and lean body mass.

Very limited databaseEasy to do

Hydrodensitometry

HydrodensitometryPrinciple:two compartment modeldensity related to relative amounts of

two compartments– D(fat) = 0.90 g/ml

– D(lbm) = 1.10 g/ml

– D(water) = 1.00 g/ml

HydrodensitometryDensity = Body weight/Body volumeHow does one estimate body volume?Archimedes principles:

– volume of submerged object = volume of water displaced

– weight in air - weight underwater = weight of water displaced

Hydrodensitometry wt of water displaced = vol of water displaced Wt of water displaced = vol of body (BV) Since weight of water displaced = weight in air - weight underwater

– BV = BW-UBW To calculate body density

– BD = BW / BV calculate %BF from BD

Hydrodensitometry Calculations

DATA– BW(air) = 180 lbs = 81.6 kg– BW(water) = UWW = 3.6 kg– RV = 1.30 L, est GI gas vol = 0.1 L– Density of water @ 77 deg = 0.997 kg/L

CALCULATIONS– BV = (BW-UWW)/.997 – (RV +0.1)– BV = (81.6-3.6)/.997 – (1.3+0.1)– BV = 78.23 – 1.4 = 76.83 L

Hydrodensitometry Calculations

BV = 76.83 LBD = BW / BV = 81.6/76.83 = 1.062 kg/L% BF = (495/BD)- 450 = (495/1.062)-450

– %BF = 466.09-450 = 16.09% = 16% Fat mass = 16% x 81.6kg = 13.1 kgLean mass = 81.6-13.1 = 68.5 kg

Hydrodensitometry:Assumption

Density of fat and lean are constant– bone density

– muscle density

– hydration status GI gas volume is constant

Hydrodensitometry:Limitations

Measurement of residual lung volume

Precision of underwater weightCostNon-portableLimited types of subjects

Whole Body Pethysmography

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– results in bigger increase in pressure with

injection of known volume of air

Whole Body Pethysmography

Advantages over hydrodensitometry– subject acceptability

– precision

– residual lung volume not factorLimitations

– costs: $25-30K

– still assumes constant density of lean and fat

Bioelectrical Impedance Analysis

1994 NIH Technology Assessment Conference

“BIA provides a reliable estimate of total body water under most conditions.”

“It can be a useful technique for body composition assessment in healthy individuals”

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

BIA: basic theory Volume is equal to length of the

cylinder times its area

Therefore, knowing the resistance and the length, one can calculate volume.

Assuming that the current flows thru the path of least resistance (water) , then the volume determined is that of body water.

BIA: basic theory

Assume fat free mass has a constant proportion of water (about 73%)– Then calculate fat free mass from body water

Assume BW = FFM + FM– Then calculate fat mass and %body fat

NHANES IIIBIA 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 CalculationsDATA

– R = 520 ohms– BW = 170 lbs = 77.3 kg– H = 70” = 178 cm

CALCULATIONS– FFM = -10.68+(0.65H2/R)+0.26W+0.02R– FFM = -10.68+(0.65x1782/520)+0.26(77.3)+0.02(520)

– FFM = -10.6 + 39.6 + 20.1 + 10.4 = 59.5 kg– FM = W – FFM = 77.3 – 59.5 = 17.8 kg– %BF = (17.8/77.3)x100 = 23%

BIA: Advantages and Limitations

Advantages– costs ($500-$2000)– portable– non-invasive– fast

Limitations– accuracy and precision– no better/worse than hydrodensitometry

Major types of BIA analyzers

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

What is a ‘normal’ % body fat?Classification Males Females

Unhealthy range (too low)

< 5% < 8%

Acceptable range (lower end)

6-15% 9-23%

Acceptable range (higher end)

16-24% 24-31%

Unhealthy (too high)

> 25% > 32%

Nieman, 1999 (p195)

Body Composition DataNHANES III – 1988-1994

All adults > 19 yrs Mean % Body Fat

– Men: 21.9% + 11.6% (SD)– Women: 32.4% + 17.8%

Mean BMI– Men: 26.5 + 7.8– Women: 26.4 + 11.7

Mean waist circumference– Men: 95.1 + 18.6 cm (cutpoint > 101.6 cm)– Women: 88.6 + 30.2 cm (> 89 cm)

Body Composition DataNHANES III – 1988-1994

Adults with BMI = 18.5-25 Mean % Body Fat

– Men: 17.6% + 7.8% (SD)– Women: 26.7% + 8.9%

Mean BMI– Men: 22.7 + 3.2– Women: 22.0 + 2.2

Mean waist circumference– Men: 84.7 + 8.9 cm (cutpoint > 101.6 cm)– Women: 78.0 + 13.4 cm (> 89 cm)

Dual-Energy X-ray Absorptiometry

DEXA, DXA

Two different energy level X-rays Lean, fat, and bone mass each reduce (attenuate)

the X-ray signal in unique ways Computer analyzes scan point by point to

determine body composition Method

– Low dose radiation

– 20-30 minutes

– Applicable to young and old