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 patients

Monitor changes
– weight loss quality
– effect of medical therapy
Body Composition Analysis vs.
Body Weight Assessment

Disadvantages

relatively limited database
all field methods are estimations

– false assumptions in all field methods
– errors by technicians

limited understanding by clients
Nutrition and Athletic Performance
ACSM/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 Thickness

measures double thickness of skin
and subcutaneous fat
 Advantages:
– inexpensive
– fast
– portable
– large database
Skinfold Thickness

Assumptions:

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 Thickness

Limitations

Technician 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 SF
 correlated with body fatness in
children

– fig. 6-32 (p192)

Tricep SF and calf SF
– fig. 6-33 (p 192)
Multiple Site Measurements





many sites
many 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 Measurements
Katch and McArdle

Principle:
– measure two “fat” sites
– measure one “muscle site”
– estimate fat and lean body mass.
Very limited database
 Easy to do

Hydrodensitometry
Hydrodensitometry
 Principle:


two compartment model
density 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
Hydrodensitometry



Density = Body weight/Body volume
How 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 L
BD = 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 kg
Lean 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 weight
 Cost
 Non-portable
 Limited 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 factor

Limitations
– 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 III
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 Calculations

DATA
– 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)
Acceptable range
(higher end)
6-15%
9-23%
16-24%
24-31%
> 25%
> 32%
Unhealthy (too
high)
Nieman, 1999 (p195)
Body Composition Data
NHANES 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 Data
NHANES 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