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Body Fat Measurements
Measurement techniques
A person's exact body fat percentage generally cannot be determined, but there are several techniques which can be used
to estimate it:
Skinfold Methods
The skinfold estimation methods are based on a skinfold test, whereby a pinch of skin is precisely measured by calipers
at several standardized points on the body to determine the subcutaneous fat layer thickness. These measurements are
converted to an estimated body fat percentage by an equation. Some formulas require as few as three measurements, others
as many as seven. The accuracy of these estimates is more dependent on a person's unique body fat distribution than on the
number of sites measured. As well, it is of utmost importance to test in a precise location with a fixed pressure. Although
it may not give an accurate reading of real body fat percentage, it is a reliable measure of body composition change over
a period of time, provided the test is carried out by the same person with the same technique.
Skinfold-based body fat estimation is sensitive to the type of caliper used, and technique. This method also only measures
one type of fat: subcutaneous adipose tissue (fat under the skin). Two individuals might have nearly identical measurements
at all of the skin fold sites, yet differ greatly in their body fat levels due to differences in other body fat deposits
such as visceral adipose tissue: fat in the abdominal cavity. Some models partially address this problem by including
age as a variable in the statistics and the resulting formula. Older individuals are found to have a lower body density
for the same skinfold measurements, which is assumed to signify a higher body fat percentage. However, older, highly
athletic individuals might not fit this assumption, causing the formulas to underestimate their body density.
Body Average Density Measurement
1. The most accurate method of estimating body fat percentage was to measure that person's
average density (total mass divided by total volume) and apply a formula to convert that to body fat percentage.
Since fat tissue has a lower density than muscles and bones, it is possible to estimate the fat content. This estimate
is distorted by the fact that muscles and bones have different densities: for a person with a more-than-average amount
of bone tissue, the estimate will be too low. However, this method gives highly reproducible results for individual
persons (tolerance 1%), unlike the methods discussed below, which can have an uncertainty up to 10%. The body fat percentage
is commonly calculated from one of two formulas:
Brozek formula: BF = [(4.57/d- 4.142) x 100] %
Siri formula is: BF = [(4.95/d- 4.50) x 100] %
In these formulas, d is the body density in kg/L. For a more accurate measurement, the amount of bone tissue must be
estimated with a separate procedure. In either case, the body density must be measured with a high accuracy. An error of
just 0.2% (e.g. 150 mL of trapped air in the lungs) would make 1% difference in the body fat percentage.
2. Another way to determine body density is by hydrostatic weighting, which refers to measuring the apparent weight of a
subject under water, with all air expelled from the lungs. Since approximately 1.2 L of the air in the lungs cannot be
expelled, the formulas require an adjustment described below. This procedure is normally carried out in laboratories with
special equipment.
The weight that is thus found will be equivalent to the body's weight in air, minus the weight of the volume of water
which that object displaces. The following formula can be used to compute the relative density of a body: its density
relative to the liquid in which it is immersed, based on its weight in that liquid:
dr=W/(W-Wi)
where dr is relative density, W is the weight of the body, and Wi is the apparent immersed weight of the body. Absolute
density is determined from the relative density, and the density of the liquid. Because the density of water is very close
to 1 , when density is computed relative to water, for many purposes it may be treated as absolute density.
It is unnecessary to actually weigh a body under water in order to determine its volume, density or, for that matter,
its weight under water. Volume can be easily determined by measuring how much water is displaced by submerging that body.
For a human body, a vertical tank which has a uniform cross-section-area, such as a cylinder or prism, can be used. As the
subject submerges and expels air from the lungs, the rise in the water level is measured. The water level rise, together
with the interior dimensions of the tank, determine the displaced volume. An adjustment for air remaining in the lungs
(about 1.2 liters after full exhalation) is requried.
3. It is possible to obtain an estimate of body density without directly measuring weight underwater, and without directly
measuring water displacement. What is required is a swimming pool or other tank where the subject can be fully immersed.
The idea is to balance the body with a buoyant floatation device of a suitable mass and volume, such that the body plus
floatation device neither sink nor float. The viability of this method rests in choosing a floatation device which has some
convenient attribute that makes it possible to determine its volume easily: it is small, regularly shaped, and perhaps
manufactured to a specific volume. From the volume and mass of the balancing floatation device, the mass of the body, and
a 1.2 liter adjustment for air still in the lungs after full exhaling, the volume and density of the body can be determined.
A person who neither floats nor sinks with empty lungs in water would have a density of approximately 1 kg/L
(the density of water) and an estimated body fat percentage of 43% (Brozek) or 45% (Siri), which would be extremely obese.
Persons with a lower body fat percentage would need to hold some kind of floatation device, such as an empty bottle,
in order to keep from sinking. If the floatation device has mass m and volume v, and the person has a mass M, then his
or her density is
d = dw/[1+(m/M)-(dw x v/M)]
where dw is the density of water [0.99780 kg/L at 22 deg. C (72 deg.F)]. For example, a person weighing 80 kg who needs to
hold a floater with a volume of 4.5 L and a mass of 0.5 kg, and has 1.2 liters of air remaining in the lungs (1.2 kg water
displaced), has a density of 1.07 kg/L and hence a body fat percentage of 13%. Note that both the Brozek and Siri formulas
are claimed to be systematically too high by about 10% (e.g., a 20% body reading might be 20% minus (20% x 10%) = 18% body
fat).
A simpler version of the above formula can be derived by making two assumptions, and one small algebraic change. Firstly,
the density of water can be taken to be 1 kg/L, which is very accurate. Secondly, the mass of light floatation device such
as an empty plastic bottle is very small and so the m / M term is negligible: if this assumption is invalid, it can easily
be compensated for, as described below. Thirdly, the numerator and denominator can be multiplied by M, finally yielding
d=M/(M-v)
Note the similarity of this formula to that given earlier for relative density, except that masses are substituted for
weights. The v term represents weight of the water that was displaced by the floatation device and by the air remaining
in the lungs. That to use v, dw was taken to be 1. The air remaining in the lungs is about 1.2 liters (1.2 kg of displaced
water) after full and forceful exhalation.
A light plastic bottle filled with air makes a convenient floater, since the amount of air in it can be adjusted and
accurately measured. The plastic occupies only a small volume is about the same density as water so that there is very
little error in not correcting for the plastic bottle. The measurement begins with the bottle completely empty.
The subject is asked to expel as much as as possible from the lungs and use the bottle as a completely submerged floater.
Water is allowed to enter the submerged bottle until the person sinks beneath the surface without touching the bottom.
The liters of air in the container is equal to the kg of water displaced. v is equal to this kg plus the estimated 1.2 kg
of water displaced by the air remaining in the lungs after full exhalation.
Bioelectrical Impedance Analysis
The Bioelectrical impedance analysis (BIA) method is a more affordable but less accurate way to estimate body fat percentage.
The general principle behind BIA: two conductors are attached to a person's body and a small electrical current is sent
through the body. The resistance between the conductors will provide a measure of body fat, since the resistance to
electricity varies between adipose, muscular and skeletal tissue. Fat-free mass (muscles) is a good conductor as it contains
a large amount of water (approximately 73%) and electrolytes, while fat is anhydrous and a poor conductor of electrical
current. Factors that affect the accuracy and precision of this method include instrumentation, subject factors, technician
skill, and the prediction equation formulated to estimate the Fat Free Mass. Criticism of this methodology is based on where
the conductors are placed on the body; typically they are placed on the feet, with the current sent up one leg, across the
abdomen and down the other leg. As technician error is minor, factors such as eating, drinking and exercising must be
controlled since hydration level is an important source of error in determining the flow of the electrical current to
estimate body fat. As men and women store fat differently around the abdomen and thigh region, the results can be less
accurate as a measure of total body fat percentage. Another variable that can affect the amount of body fat this test
measures is the amount of liquid an individual has consumed before the test. As electricity travels more easily through
water, a person who has consumed a large amount of water before the test will measure as a lower body fat percentage. Less
water will increase the percentage of body fat. Bioelectrical impedance analysis is available in a laboratory, or for home
use in the form of body fat scales and hand held body fat analyzers.
Height and Circumference Methods
There also exist formulas for estimating body fat percentage from an individual's weight and girth measurements. For example,
the U.S. Navy Circumference method compares abdomen or waist and hips measurements to neck measurement and height, and
other sites claim to estimate one's body fat percentage by a conversion from the body mass index. In the Navy the method is
known as the "rope and choke." It is not uncommon for Chiefs to take advantage of flaws in the procedure to pass those who
are otherwise out of standards.
The U.S. Marine Corps and U.S. Army also rely on the Height and Circumference method. For males, they measure the neck and
waist just above the navel. Females are measured around the hips, waist, and neck. These measurements are compared to a
height/weight chart with age factored in as well. This method is used because it is a cheap and convenient way to implement
a body fat test throughout the entire Department of Defense. This method poses a particular threat of inaccuracy because one
can hold one's stomach in more if needed to pass the requirements, and/or flare the neck out and make it bigger resulting in
a lower body fat percentage.
Due to different body compositions, those with larger necks have an advantage over those with smaller necks.
Another well-known method using height and circumference is the YMCA formula. It uses only body weight in pounds, and waist
in inches (at navel), to calculate body fat percentage using the formulas,
For Men, BF =[(-98.42 + 4.15 x waist - 0.082 x weight)/weight] x 100 %
For Women, BF =[(-76.76 + 4.15 x waist - 0.082 x weight)/weight] x 100 %
Near-Infrared Interactance
A beam of infra-red light is transmitted through the skin of the biceps. The light is reflected from the underlying muscle,
and absorbed by the fat. This method shows good correlation with DXA measurements.
Dual energy X-ray absorptiometry
Dual energy X-ray absorptiometry, or DXA (formerly DEXA), is a good method for estimating body fat percentage.
Two different types of X-ray scans the body, one that detects all tissues and another that doesn't detect fat. A computer
can subtract the second picture from the first one, giving only fat detection. The mass of this can be estimated by the
grade of exposure.
Ultrasonic waves
A device called the USBOX (for UltraSonic Box) sends ultrasonic waves and analyses the data gathered. As the different types
of cells (muscular, osseous, etc) have different values of reflexion, one can easily tell apart the volumes occupied by
each kind of cell, and deduce the mass of body fat.
