Most people now understand that to have a healthier body means to have a leaner body. These people are ready for Body Composition Analysis (BCA.) This article will attempt to acquaint you with the most popular methods of BCA used in gyms, homes, and professional practices.
Weighing (Underwater Weighing) - This method measures whole body density
by determining body volume. There is a variety of equipment available
to do underwater weighing ranging in sophistication from the standard
stainless steel tank with a chair or cot mounted on underwater scales,
to a chair and scale suspended
from a diving board over a pool or hot tub.
This technique first requires weighing a person outside the tank, then
immersing them totally in water and weighing them again. The densities
of bone and muscles are higher than water, and fat is less dense than
water. So a person with more bone and muscle will weight more in water
than a person with less bone and muscle, meaning they have a higher body
density and lower percentage of body fat. The volume of the body is calculated
and the individual's body density is determined by using standard formulas.
Then body fat percentage is calculated from body density using standard
equations (either Siri or Brozek).
The underlying assumption with this method is that densities of fat mass
and fat-free mass are constant. However, underwater weighing may not be
the appropriate gold standard for everyone. For example, athletes tend
to have denser bones and muscles than non-athletes, which may lead to
an underestimation of body fat percentage. While the body fat of elderly
patients suffering from osteoporosis may be overestimated. To date, specific
equations have not been developed to accommodate these different population
An important consideration in this method is the amount of air left in
a person's lungs after breathing out. This residual lung volume can be
estimated or measured, but it is established that a direct measure is
desirable and it should be taken in the tank whenever possible. Another
consideration is that the water in the tank must be completely still;
there can be no wind or movement.
Although this method has long been considered the laboratory "gold
standard", many people find it difficult, cumbersome, and uncomfortable,
and others are afraid of total submersion or cannot expel all the air
in their lungs. Clinical studies often require subjects to be measured
three to five times and an average taken of the results.
(Anthropometry- Skinfold Measurements) - Using hand-held calipers
that exert a standard pressure, the skinfold thickness is measured at
various body locations (3-7 test sites are common). Then a calculation
is used to derive a body fat percentage based on the sum of the measurements.
Different prediction equations are needed for children and specific ethnic
groups (over 3,500 equations have been validated). This approach usually
uses underwater weighing as a reference method. The caliper method is
based upon the assumption that the thickness of the subcutaneous fat (found
just under the skin) reflects a constant proportion of the total body
fat (contained in the body cavities), and that the sites selected for
measurement represent the average thickness of the subcutaneous fat.
Skinfold measurements are made by grasping the skin and underlying tissue,
shaking it to exclude any muscle and pinching it between the jaws of the
caliper. Duplicate readings are often made at each site to improve the
accuracy and reproducibility of the measurements. Often to save time in
large population studies, a single skinfold site measurement is made to
reduce the time involved. Such a test should be used only for a rough
estimate of obesity.
Generally speaking, skinfold measurements are easy to do, inexpensive
and the method is portable. Overall, results, can be very subjective as
precision ultimately depends on the skill of the technician and the site
measured. The quality of the calipers is also a factor; they should be
accurately calibrated and have a constant specified pressure. Inexpensive
models sold for home use are usually less accurate than those used by
an accredited caliper technician. The more obese the subject, the more
difficult to "pinch" the skinfold correctly, requiring even
more skill to obtain an accurate measurement.
(Dual Energy X-ray Absorptiometry) - A relatively new technology that
is very accurate and precise, DEXA is based on a three-compartment model
that divides the body into total body mineral, fat-free soft (lean) mass,
and fat tissue mass. This technique is based on the assumption that bone
mineral content is directly proportional to the amount of photon energy
absorbed by the bone being studied.
DEXA uses a whole body scanner that has two low dose x-rays at different
sources that read bone and soft tissue mass simultaneously. The sources
are mounted beneath a table with a detector overhead. The scanner passes
across a person's reclining body with data collected at 0.5 cm intervals.
A scan takes between 10-20 minutes. It is safe and noninvasive with little
burden to the individual, although a person must lie still throughout
DEXA is fast becoming the new "gold standard" because it provides
a higher degree of precision in only one measurement and has the ability
to show exactly where fat is distributed throughout the body. It is very
reliable and its results extremely repeatable; in addition, the method
is safe and presents little burden to the subject. Although this method
is not as accurate in measuring the extremely obese and the cost of equipment
is high, DEXA is quickly moving from the laboratory setting into clinical
NIR (Near Infrared Interactance) - A fiber optic probe is connected
to a digital analyzer that indirectly measures the tissue composition
(fat and water) at various sites on the body. This method is based on
studies that show optical densities are linearly related to subcutaneous
and total body fat. The biceps is the most often used single site for
estimating body fat using the NIR method. The NIR light penetrates the
tissues and is reflected off the bone back to the detector. The NIR data
is entered into a prediction equation with the person's height, weight,
frame size, and level of activity to estimate the percent body fat.
This method has become popular outside of the laboratory because it is
simple, fast, noninvasive, and the equipment is relatively inexpensive.
However, the amount of pressure applied to the fiber optic probe during
measurement may affect the values of optical densities, and skin color
and hydration level may be potential sources of error. To date, studies
conducted with this method have produced mixed results; a high degree
of error has occurred with very lean and very obese people; and the validity
of a single-site measurement at the biceps is questionable. Numerous sources
report that more research is needed to substantiate the validity, accuracy
and applicability of this method.
Magnetic Resonance Imaging (MRI) - An x-ray based method in which
a magnetic field "excites" water and fat molecules in the body,
producing a measurable signal. A person lies within the magnet as a computer
scans the body. High-quality images show the amount of fat and where it
is distributed. MRI takes about 30 minutes and is very safe as it uses
no ionizing radiation, but use is limited due to the high cost of equipment
Total Body Electrical Conductivity (TOBEC) - This method is based
on lean tissue being a better conductor of electricity than fat. A person
lies in a cylinder that generates a very weak electromagnetic field. The
strength of the field depends on the electrolytes found in the person's
body water. In about 10 seconds, TOBEC makes 10 conductivity readings
that estimate lean body mass. Although very accurate, its use is limited
due to the high cost of the equipment.
Computed Tomography (CT) - CT produces cross-sectional scans of the
body. An x-ray tube sends a beam of photons toward a detector. As the
beam rotates around a person, data is collected, stored, and applied to
complex algorithms to build images that determine body composition. CT
is particularly useful in giving a ratio of intra-abdominal fat to extra-abdominal
fat. It is noninvasive, but potential is limited by exposure to radiation
and high equipment cost.
BOD POD (Air Displacement) - Based on the same principle as underwater
weighing, the BOD POD used computerized sensors to measure how much air
is displaced while a person sits for 20 seconds in a capsule. It uses
a calculation to determine body density, then estimated body fat. The
equipment is very expensive and limited in availability.
BIA (Bioelectrical Impedance)
- The only method that is based on measuring something, not estimating
anything, is Bio-Impedance measurement. Bio-Impedance is a means of measuring
electrical signals as they pass through the fat, lean mass, and water
in the body. Through laboratory research we know the actual impedance
or conductivity of various tissues in the body, and we know that by measuring
current between two electrodes and applying this information to complex
proven scientific formulas accurate body composition can be determined.
The fact that the measurement is based on a reading of lean mass and not
an estimate of fat mass, lends to a much more comprehensive testing method