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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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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Body density

Under water weighing or air displacement (BodPod) skin fold measurements


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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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)


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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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

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Body Composition