Body Composition
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Transcript of Body Composition
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Body Composition
Wendy OBrien
IFNHH
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Learning objectives
Outline the reasons for measuring body composition
Describe the body composition of a reference human being
Describe the different ways in which body composition can be measured
Outline the advantages and disadvantages of some of these methods
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Why measure body composition?
Athletes, health / disease, growth
Water - indicator for fluid balance & its regulation
Fat amount (& distribution) a risk indicator for obesity associated diseases / anorexia
Muscular mass indicator for physical activity & growth
Bone density
Measurement of current nutritional status & changes in nutritional status
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Body Composition
Diagrammatic representation of a normal adult male weighing 70kg
Contribution of the components to body weight are represented by the area in the diagram
Only fat, protein and glycogen contribute to the energy stores in the body
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Body composition
70kg male 12kg fat (17%) 58kg fat free mass (83%)
42kg water (72.5%) 28kg (2/3) water inside cells (intra cellular fluid) 14kg (1/3) water outside cells (extra cellular fluid)
Protein, bone mineral, glycogen, minor components (nucleic acids, electrolytes)
Healthy adult male 60% water, 17% protein, 17% fat
Usually - 90% of fat in layer under skin, also in abdominal cavity, small amount in fascial planes between muscles
Different parts of body - different chemical compositions Skin higher protein, lower water, little potassium Brain lower protein, higher water, high potassium
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Body Composition Over Time
0
10
20
30
40
50
60
70
80
90
fetus baby infant adult obese
water
fat
protein
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Body composition
Males - accumulate fat in abdomen, women on breast, hip & thigh
Throughout life - water content of body decreases; protein & fat increase through
gestation, infancy & into adolescence
Fat free mass remains relatively constant from 20 to 65, but then decreases
Fat mass tends to increase throughout adult life
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5 levels of body composition
5 Levels of body
composition N, Ca, P,
K, Cl, Na
atomic
level molecular
level
cellular
level
tissue
systems
total
body
H
C
O Protein
Water
Lipids
Glycogen
Minerals
Cells
extra-
cellular
solids bone
organs
muscle
arms fat
head
torso
legs
extra-
cellular
fluids
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Models of body composition
body weight
cell mass
bone
fat
extracell.
mass fat
protein water fat
fat free mass
I
II
III
IV
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Body Composition Assessment BMI, WHR
2 compartment models
Fat mass & fat free mass
Body density (Under water weighing, BodPod, skinfolds)
Bioelectrical impedance analysis
Total body potassium
Total body water
Infrared reactance
3 or 4 compartment models
Water, protein, mineral & fat
Computed tomography (CT)
Dual energy X-ray absorptiometry (DEXA)
Magnetic resonance imaging (MRI)
Neuron activation analysis
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Body Mass Index (BMI) (Quetlets Index)
Classification of body weight based on body weight and height
BMI = body weight in kg
(body height in m)2
Examples:
Weight: 54kg, Height: 1.75m, BMI = 54/1.752 , BMI = 17.6kg/m2
Weight: 85kg, Height: 1.88m, BMI = 85/1.882 , BMI = 24.0kg/m2
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Definition of overweight and obesity
Characterisation of overweight and obesity by body mass index grade BMI (kg/m2)
under weight < 18.5 normal weight 18.5-24.9 over weight > 25 - pre obesity 25-29.9 - obesity grade I 30-34.9 - obesity grade II 35-39.9 - obesity grade III > 40
WHO. Obesity A Major Global Public Health Problem. WHO Geneva 1998
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Proposed classification of weight by BMI on
different adult ethnic groups
NZ European Pacific Island &
Maori
Asian & Indian Risk of co-
morbidities
Underweight 25 Very high
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Limitations of BMI
Inappropriate for use in
Children / adolescents < 18 years
Very muscular people
Pregnant women
190cm
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Apple and Pear Body Shapes Compared
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Definition of overweight and obesity
Android (male) and gynoid (female) fat distribution waist-hip-Ratio > 0.85 (women) > 1.0 (men) Abdominal obesity is associated with a higher risk of metabolic disorders and cardio-vascular intricacies of overweight. Alternatively (better?): waist circumference
elavated distinctly risk elavated risk
men > 94 > 102 women > 80 > 88
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In-text Figure
Page 275
The wise consumer distinguishes between
loss of fat and loss of weight.
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Body Composition Techniques
Direct methods Carcass analysis In vivo neutron activation analysis
Indirect techniques (based on assumptions, sophisticated, expensive equipment) Densitometry Dilution techniques Total body potassium Dual-energy X-ray absorptiometry
Doubly indirect methods (require validation against Level II methods to determine %age body fat)
Anthropometry / skin fold thickness measurements
Near infrared reactance
Ultrasound measurements
Bioelectrical impedance
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2 compartment models
Divides body into Fat Mass (FM) & Fat Free Mass (FFM)
FM Density of 0.9g/cm3
Contains no potassium
FFM Density 1.1g/cm3
Water content of 72.5% (assume constant)
K 68.1 mmol.kg-1
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2 compartment models
Body density
Under water weighing or air displacement (BodPod) skin fold measurements
Total body potassium
Total body water
Bioelectrical impedance analysis
Near infrared reactance
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Underwater Weighing
Hydrodensitometry
Archimedes Principle
when a body is immersed in water it
is buoyed up by a
force which is
equivalent to the
weight of the volume
of water displaced
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Underwater Weighing
Hydrodensitometry
Density (g/cm3)
= Mass of body in air (g)_____
Mass in air (g) Mass in water (g) (volume)
Corrections
- Water density depends on temp
- Residual lung volume
- Measured with subject
maximally expired
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BD is then converted to % body fat by Siris formula
%BF= 495 450
BD
Underwater Weighing
Hydrodensitometry
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Underwater Weighing
Hydrodensitometry
Relatively inexpensive
Residual volume measured using N2 washout using O2 & CO2 analysers
Time effective compared with isotope dilution
Complete immersion can be difficult
BD can be measured with high precision (2-3% margin of error)
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The BODPOD
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Air displacement method (The BODPOD) mouth pressure
breathing pressure
closing valve
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The BODPOD
Air used instead of water measures how much air displaced once person enters chamber
Determines volume by measuring changes in pressure
Works on the gas law expansion of Boyles Law
Changes in pressure can be used to calculate volume & then density
Takes 5 mins
Can accommodate persons up to 500lb - sumo wrestlers
Swimming costume and cap
Error - similar to UWW
Accurate in Caucasians 1 study overestimated body fat in African American males
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Skin fold thickness measurements
Uses calipers to measure fold of skin
and underlying
subcutaneous fat
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Skin fold thickness measurements
Most often measured sites are biceps,
triceps, subscapular,
suprailiac
Equations used to predict % body fat
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Anthropometric estimations of body
fat skin folds Convenient, inexpensive, in the field
Difficult to do accurately cannot use in some groups (obese)
Based on equations (>400 available) derived by regressing anthropometric measures (skin folds) against under water weighing
Assumptions & errors at 3 stages Prediction of BD from anthropometric data
Measurement of BD using UWW
Transformation of BD to % fat scores
Total error 3.6% general popn or 2.6% homogenous popn (skilled technician)
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Assumptions (moving from
anthropometric equations to BD) Constant skin thickness & subcutaneous fat compressibility
Thickest skin subscap region 4mm; thinnest skin biceps
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Assumptions (moving from BD to %
fat)
Density of FM & FFM constant Considerable variation in FFM density
Variations in the proportions of muscle, bone, etc
Athletes have denser bone & muscle, results in under-estimation of % fat
Individual components of FM & FFM have constant densities
Proportional contributions of FFM (water, protein, bone mineral & non-bone mineral) components are invariant
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Bioelectrical Impedance Analysis
Body conducts electricity through water &
electrolytes
Resistance greatest in fat tissue
Pass weak current through body and
measure resistance to
current
Higher resistance more body fat
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Bioelectrical Impedance Analysis
Population specific need appropriate equations for group
Affected by water avoid eating and drinking prior (4 hours), avoid exercise 12 hours prior, no alcohol 48 hours prior, empty bladder
Electrode placement important
One prediction equation often used
Prediction error 3-4%
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Total body potassium
Whole body gamma counter (WBC) 40K
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Total body potassium (TBK)
All potassium (including human body) contains a
natural radioactive isotope
(40K)
Each g of K emits 3 gamma rays / sec
Can measure TBK, & assuming a constant K content
in FFM, can calculate FFM &
hence FM
Need enclosed room Not suitable for infants, kids Expensive
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Dilution techniques (eg. Doubly
labeled water)
Total Body Water (TBW) can be determined by isotope dilution
Subject given known dose of water labeled with deuterium (2H) (stable heavy isotope of hydrogen)
2H allowed to equilibriate with TBW (takes 3 hours)
Dilution of 2H measured (blood plasma)
TBW calculated
If assume fat free tissues contain 72.5% of water, can calculate FFM, and hence FM
Needs high precision isotope ratio mass spectrometer with competent operator
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Near Infrared Reactance
Probe / wand attached to biceps using Velcro strip
Infrared beam penetrates the arm and is reflected back into the probe
NIR analyser estimates a persons percentage body fat from optical density, or the amount of
light reflected by the underlying tissues,
measured only at this site
Margin of error 2-10%
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3 / 4 compartment model
Separates out fat mass, water, bone mineral mass and residual fat free mass
Based on measures of
Body density (fat & fat free mass)
Bone mass
Total body water
Dont need to assume FFM has density of 1.1 g/cm3 or a water content of 72.5%
Computed tomography (CT)
Dual energy X-ray absorptiometry (DEXA)
Magnetic resonance imaging (MRI)
Neuron activation analysis
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Enables visualisation of tissues in cross-sectional slices of the body
Shows distribution of fat, water, lean tissue and bone (all have different absorption characteristics)
Useful in showing size & position of abnormal tissue (eg. Tumor)
Involves high radiation dose
Expensive clinical settings
Computerised Assisted
Tomography (CT)
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Dual X-ray Absorptimetry (DEXA)
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DEXA
Simultaneous measurement of bone mineral, fat & non-bone tissue
Body scanned with X-rays of 2 distinct levels of energy - a 2-dimensional picture of the body
When beam passes through material of high opacity to X-rays (bone mineral), the emerging
energy is severely attenuated; when tissue being
irradiated is fat, little attenuation
Cannot distinguish between subcutaneous adipose tissue & discrete adipose tissue
Low radiation dose
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DEXA
DEXA
Quick (15-30 mins), easy to perform, few demands on
subject
More work needed on validation & popn specific
equations
High cost research Some subjects too large Can screen across area of
interest or whole body
Independent of operator bias Subject completely clothed No food or beverage
restrictions
Error 1-4%
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Magnetic resonance imaging (MRI)
Cross sectional slice
High quality image
Can accurately quantify both subcutaneous & visceral fat
Relatively rapid (~ 30 mins)
Some exclusions (eg. Pacemakers)
Expensive
Mid thigh of 30 year old male distance runner
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Neutron Activation Analysis
Humans are irradiated with a beam of fast neutrons
Some elements form very short-lived radioactive isotopes
Radiation from these isotopes can be detected body content of these elements can be calculated. Eg. Calcium
Extremely expensive
Significant radiation dose
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Summary
More accurate methods are often more complicated or expensive, or not suitable
for all the population
Choice depends on subjects, what is being investigated, time and the resources
available