Body Composition
Fact sheet 1
Body composition
What is the body
The body composition of a hypothetical, normal weight adult is shown in Figure
made of ?
1.0. It can be seen that the major component of the human body is water. The
protein and fat component are relatively small, with the remainder being
primarily bone and minerals. When we measure body composition, nutritionist
use terms to describe compartments of the body.
Water
42kg
Glycogen
Phosphorous
Other
Mg Cl
Calcium
Fe Zn
Potassium
Cu
Sodium
Fat 12kg
Protein 12kg
Figure 1.0 Body composition: example of normal weight male
The non-fat component of body composition is termed fat free mass (FFM)
and exists primarily as the chief structural and functional component of the
human body. The FFM compartment consists in proportions of water (72%),
protein (21%) and bone minerals (7%).
Bone minerals can be measured by dual-energy x-ray absorbitanetry
(DEXA) scan See figure 2.0 for a picture of a subject undergoing a dexa
scan. Typically, an adult has around 2-4 kg of body weight only, from
bone.
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Figure 2.0 Subject undergoing a DEXA scan to measure bone mass
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The fat compartment of the body is termed fat mass (FM) and will vary
considerably between individuals in terms of absolute amount. Fat mass
consists of 20% water and 80% adipose tissue and can, in obese persons
be the largest component of the body. Figure 2.0 shows the composition
be the largest component of the body
of an obese male. He has twice the amount of adipose tissue on his body
of an obese male. He has twice the amount of adipose tissue on h
,
is body
compared to the lean man.
Figure 3.0
Lean man
Obese man
70 kg
100 kg
Water
60%
47%
Protein
17%
13%
Fat
17%
35%
Remainder
6%
5%
There is a small amount of body protein available for energy, in the labile
There is a small amount of body protein available for energy
amino acid pool and muscle proteins during catabolism, (when the body
is starving).
Carbohydrate is stored in the body typically as glycogen in the liver and
in muscle and can vary between individuals ranging from approximately
500g in normal individuals to over a kilogram in trained athletes. Values
s. V
also vary depending on body size and previous carbohydrate ingestion.
Body composition
When people gain or lose weight they will add or reduce the amount of fat
and obesity
mass and to a much lesser degree, fat-free mass. We can measure all the
mass and to a much lesser degree, fat-free mass. W
compartments in the body using the following techniques (Figure 4.0)
BONE
by DEXA (dual-energy x-ray absorptiometry) scan
by DEXA
FA
F T
A
by bod pod (densitometry)
PROTEIN
by bod pod (densitometry)
WA
W TER
A
by isotopic dilution
125
100
Protein
75
Fat
Bone
50
Water
LCD study:Mass (kg)
25
0
Pre-weight Post-weight
loss
loss
Figure 4.0. Changes in body composition with weight loss
Figure 4.0 shows some data on weight loss in obese men. The men underwent
Figure 4.0 shows some data on weight loss in obese men.
a dietary weight loss regime at the Rowett Research Institute’s Human Nutrition
a dietary weight loss regime at the Rowett Research Institute’
Unit. They lost, on average 12 kg in 6 weeks.
Unit.
Around 80% of the weight loss
They lost, on average 12 kg in 6 weeks.
was due to fat loss. Bone mass remained unchanged, with a small loss in
protein and water mass. The reduction in body fat led to an improved quality
protein and water mass.
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and fitter
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Is there an ideal
There is no ‘ideal body weight’, instead health professionals look at a range
body weight ?
of healthy weight which reflects the lowest risk of ill health. Other factors such
as age, smoking status, fitness and family history will determine the overall
health of an individual. Body mass index (BMI) is a simple ratio of weight-for-
height that is commonly used to classify overweight and obesity in adults. It
is calculated as the weight in kilograms divided by the square of the height in
metres (kg/m2).
For example, an adult who weighs 70kg and whose height is 1.75m, will have
a BMI of 22.9 (normal weight for height)
wt (kg)
70
BMI =
=
= 22.9
Height (m2)
1.752
BMI is age-independent and the same for both sexes. However this calculation
does not correspond to the same degree of fatness in different populations.
For example a body builder will have a lot of muscle and therefor weigh more
than a non-active individual. His BMI may be higher, but his body fat will be
low. Therefore, the BMI calculation is only a useful index of your body fatness
and should be used in conjunction with other measures of fat distribution e.g.
waist circumference.
This calculation is not suitable for children, because they are still growing and
not reached their full, adult height. Growth charts should be consulted for
height for age in children.
BMI less than 18.4 is classified as underweight for height
Being underweight also introduces some health risks, such as infertility in
women, increased risk of infections and osteoperosis (thin bones). Ensure
your diet is healthy and avoid further weight loss.
BMI 18.5 – 24.9 is classified as normal weight for height
This is the range that adults should aim to be for optimal health. Individuals
should aim for body weight maintenance. Consider other lifestyle factors
e.g. smoking, non-healthy diet, physical inactivity as these may be a risk
factor for less optimal health.
BMI 25.0 – 29.9 is classified as overweight
Body fat will be elevated increasing the risk of coronary heart disease,
diabetes and high blood pressure. Aim to reduce body weight slowly, with
a weight loss of 5-10kg over 12 weeks. Seek medical advice before initiating
a dieting regime.
BMI 30.0 – 39.9 is classified as obese
Weight loss is required to reduce heath risks. Set a goal of 5-10kg weight
loss. Consider a lower-calorie diet and drug therapy if diet, exercise and
lifestyle programme is unsuccessful after 12 weeks.
BMI more than 40 is classified as severely obese
Immediate weight loss is required, consult your GP to be referred to
specialist management for of surgical or drug intervention.
Aim for a 20-30% weight reduction.
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Body Composition Techniques
Circumference:
The circumference technique measures body shape, using a tape measure.
waist and hip
It is important that the same investigator takes repeated measurements to
reduce inter-individual differences due to positioning of the tape. Subjects are
required to wear underwear during the measurement, so that the thickness of
clothing does not influence the result. The measurement is typically conducted
as a morning measurement, before eating and after emptying bladder. Subjects
are measured in the standing position. Subjects should be asked to breathe
normally and at the time of the measurements, and asked to breathe out gently.
This prevents the subject from contracting their muscles or from holding their
breath. Measurements are made in duplicate (twice or thrice). See photo of
waist circumference measurement.
Waist circumference greater than or equal to 94 cm action level 1 (BMI>25);
greater or equal to 102 cm action level 2 (BMI > 30)
Health risks
Waist/hip ratio - divide the waist circumference by the hip circumference.
Above 0.95 for men (or 0.8 in women) indictive of a health risk.
Equipment /
A plastic tape measure i.e. non elastic or metal.
Preparation:
Marker pen (felt tip)
A. Waist
Measurements should be taken midway
between the inferior margin of the last
rib and the crest of the ilium, in a
horizontal plane. Each landmark is
palpated and marked and the midpoint
determined by tape measure. The
observer needs to sit in front of the subject
and fit the tape round the site, not
compressing the soft tissue. The duplicate
measurement is measured to the nearest
0.1 cm at the end of a normal (gentle)
expiration. Ask the subject not to tuck
their stomach in (see adjacent photo).
B. Hip
Subject needs to be wearing non-
restrictive underwear standing with arm
by the side and feet together. The
observer needs to sit in front of the subject
and fit the tape round the widest part of
the trochanters (buttocks). This is
measured in duplicate to the nearest 0.1
cm (see adjacent photo).
C. Mid upper
The subject should be standing with the
arm (MUAC)
arm hanging loosely by the side palms
towards thighs. The circumference is
measured in the midpoint of the arm
(between the tip of alecranon, elbow bent
at 90 degrees). Duplicate circumferences
are measured to the nearest 0.1 cm. See
diagram of arm (see adjacent photo).
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D. Thigh
Measured in duplicate for both legs 2
cm below the gluteal fold, with the weight
on the non measured leg. ie. the leg
being measured is relaxed. Measured
in duplicate for both legs (see adjacent
photo).
E. Calf
Subject needs to be standing with the
feet about 20 cm apart and weight
equally distributed between both feet.
Position the tape measure horizontally
so that it is in complete contact with the
skin, but not indenting. Record the
measurement by moving up and down
the calf to find the widest part. Measured
in duplicate for both legs (see adjacent
photo).
Bod Pod (body density)
Principles and
The Bod Pod Body Composition System (Life measurements Instruments,
assumptions
Concord, USA) is based on air displacement plethysmography. It is based on
a two-compartment model of body composition (fat mass and fat-free mass),
and uses the inverse relationship between pressure and volume (Boyle’s law)
to derive body volume (l) for a subject.
Once body volume is determined, the principles of densitometry are used to
determine body composition from body density. If volume and body weight are
both known then density can be calculated from the following equation:
Body Density = mass/volume
where mass= body weight (kg)
volume = litres (l)
Thus, the change in pressure and volume between the empty chamber and
the subject present can be used to calculate the subjects volume, since one
side of the equation is known.
Boyles law: P1 x V1 = P2 x V2
where P = pressure
V = volume
The relationship between pressure and volume is calculated using Poissions
Law and Boyles law (above). The calculation of body volume is then corrected
for body surface area by a constant (Dubois and Dubois 1916) and thoracic
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Window
Reference
chamber
Measurement
chamber
Diaphragm
system
Electronics
Lung volume
measurement
Computer
Scale
Figure 5.0 Diagrammatic representation of the Bod Pod
Description of
Prior measurement the volume of the chamber is calibrated with a known
method
standard (49.550 l) and the weighing scales are also calibrated against a
known weight (20kg). Subjects are asked to wear minimal clothing such as a
swimsuit as it is thought that hair and clothing show apparent negative volume
effects altering body surface area calculations. The Bod Pod is housed in a
room with a constant temperature (24-26 OC), with barometric pressure and
relative humidity recorded on the day of measurement. Before entering the
Bod Pod an accurate measurement of body weight is taken, using the scales
attached to the Pod Bod system. The subject then enters the Bod Pod chamber
and asked to sit in a standardised way, with back straight and not touching the
back wall of the machine, feet slightly apart and hands placed in a relaxed
manner on their lap. The subject can viewed by the investigator though the
clear perspex window during the measurement. The subject’s age, height and
sex are also entered in the computer and 4 estimates of body volume are then
obtained using predicted lung volume. Percentage body fat is then estimated
using the Siri formula (1961). See photo of bod pod system below.
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Skinfold thickness (SFT)
A skinfold thickness (SFT) is defined as a measure of the double thickness of
the epidermis, underlying fascia and subcutaneous adipose tissue. See
photographs below. There are two main assumptions in determining total body
fat from skinfolds:
(i) that there is a constant relationship between total body fat and
subcutaneous fat at the sites measured. The equation of Siri (1961) uses
a two-compartment model, such that the human body consists of fat mass
(FM) and fat-free mass (FFM) and assumes that the density of the two
compartments is constant between individuals at 0.90 g/cm3 for FM and
1.10 g/cm3 for FFM.
(ii) that the density of FFM is constant.
Skinfold measurements also assume that subcutaneous fat is a reliable indicator
of total body fat and that skinfold compressibility remains constant. Durnin and
Womersley (1974) validated the sum of four SFT (biceps, triceps, subscapular
and suprailiac) against densitometry and devised sex and age dependent
population-based linear regression equations to estimate total body density.
Collection of
All SFT measurements should be taken by the same trained member of staff
skinfold
from identical positions on each subject, following the World Health Organisation
thickness
(WHO 1987) anthropometic guidelines. Holtain skinfold calipers (Holtain Ltd.,
measurements
Dyfed, Wales) can be used, with the subject in a standing position. A tape
measure and finger and thumb should be used when the calipers are not large
enough. See photo of measurements.
The four sites were as follows (Durnin and Womersley 1974):
1. Triceps: A mark is made at the mid-upper
arm, midline of the posterior aspect of the
arm over the triceps muscle, measured with
the elbow bent at 90o, used for identifying
the biceps and triceps SFT. During the
measurement, the arm should be hanging
freely by the side, palms inwards towards
the thighs.
2.
Biceps: Measured midline of the anterior
aspect of the arm, over the biceps muscle,
mid-point on the arm as above.
3.
Subscapular: Found just below and lateral
to the bottom tip of the scapula, measured
in a 45o angle. Subjects stand with their arm
relaxed by their side. The scapula was
palpated with the fingertips to find the bottom
of the bone and the SFT is then measured
in the natural crease. Subject’s shoulders
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are relaxed.
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4. Suprailliac (waist): Found 1 cm above the
anterior superior iliac spine (top of the hip
bone) in the mid-axillary line (waistline).
Measured horizontally with the subject
breathing gently.
To take the measurement, the skin is gripped about 1cm above the selected
site and the calipers applied below this site, the grip is removed and the
measurement noted to the nearest 0.2mm. The calipers are then removed.
This is repeated for 3 successive measurements, with the mean value calculated.
Body density and percentage body fat is calculated using the equations of
Durnin and Womersley (1974), for each side of the body, using the following
equations:
Density (g/cm3) = c – m (log ΣS)
Where:
D = Density
c and m = standard age and sex-specific coefficients
ΣS = Sum of all four skinfold measurements (mm)
Once density is calculated, the Siri (1961) equation is used to estimate
percentage body fat:
Fat (%) = [(4.95 / D) – 4.5] x 100
Where:
D = Density
4.95 and 4.5 are the constants calculated by Siri (1961) using the
assumptions on the density of FM and FFM
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