Body Composition Laboratory
Body composition studies play a key role in understanding
childhood growth and development. Questions such as
whether body weight or lean body mass is the best indicator
of when premature babies are healthy enough to go home,
or whether very chubby babies are more prone to develop
cardiovascular problems as adults, can only be answered
with research using precise body composition measurements
and appropriate standard reference values.
The CNRC Body Composition Laboratory is the only
laboratory of its type in the nation that can provide
a complete complement of body composition measurements
in all populations ranging from low-birth-weight infants
to adults. These high-precision measurements are associated
with body water, mineral, protein and fat content.
These measurements are used in a wide variety of CNRC
research studies and in collaborative projects with
other organizations. The laboratory also develops and
validates new methods for assessing body composition
in children. Data from the body composition laboratory
research is also being used to develop age, gender
and ethnic-specific body composition standard references,
such those for bone mineral content and bone mineral
density.
The Body Composition Laboratory uses prompt
gamma activation analysis and total
body potassium counting to measure whole body muscle
mass and lean tissue quality and quantity. Additional
methods include stable isotope dilution for measuring
the body's water compartments; dual-energy
x-ray absorptiometry (DXA) for measuring bone,
fat and lean-body mass; Air displacement
plethysmography (BodPod) for determining fat-free
mass and body fat; Bioelectrical impedance
analysis (BIA) for water and fat ratios; Computed
tomography (CT) for measuring fat distribution
and bone density; portable Lipometer for
measuring subcutaneous fat; and portable Ultrasound for
localized bone density measurements.
Prompt Gamma Activation Analysis
 Prompt
Gamma Activation Analysis is the gold-standard method
for measuring the amount of nitrogen in the body, which
is a direct indicator of total body protein (TBP).
Substantial losses of total body protein can occur
in chronic diseases and in aging. Such losses impact
negatively on immunity and quality of life,and on growth
rates in children. Direct measurements of total body
nitrogen (TBN) monitor the integrated changes in TBP
over time and allow comparison with normal subjects.
When this measurement is combined with measurements
from the Total Body Potassium Counter
(40K), scientists can determine total
organ and muscle mass.
Total Body Potassium
Counting
 This
extremely sensitive device measures the gamma rays
emitted from an isotope of potassium known as 40K,
which exists naturally in the human body at a known
natural abundance (0.012%). This knowledge, plus
the fact that potassium is only found inside body cells
and is not present in stored triglycerides, makes 40K
data an accurate index of the body's total cell mass
(the active growing tissues in the body), which in
turn can be used to estimate fat-free mass.*
 DXA
was originally developed to determine bone mineral
density and to aid in the treatment of osteoporosis.
More recently, the technique has been expanded to include
the analysis of fat mass and lean body mass in addition
to bone mass.
The basic principle of DXA data acquisition is based
on the differences between bone and soft tissue attenuation
at high and low x-ray levels. As an x-ray beam passes
through the subject, detectors register the varying
levels of x-rays that are absorbed by the anatomical
structures of the subject. The raw scan data, which
includes values of tissue and bone, are captured and
sent to a computer. The computer generates an image
of the body in pinpoint pixels, which can be 'counted'
to assess bone status and fat distribution. The radiation
exposure during DXA scanning is very low.
Air Displacement Plethysmograph (BodPod)
 The
BodPod uses a technique called Air Displacement Plethysmography
(ADP) to measure body volume , which in turn can be
used to estimate percent body fat.
This technique relies on the physics of Boyle's
Law, which states that pressure and volume vary inversely
with one another. In other words, as pressure goes
up, volume goes down, and vice versa . Monitoring pressure
changes in a closed chamber allows one to calculate
volume.
Bioelectrical impedance analysis
(BIA)
 The
resistance to an applied electric current flowing through
the body is related to the volumes of conductive tissues
that the current passes through. This measurement can
estimate water and fat ratios.
Bioelectrical impedance analysis is
based on the conductive and non-conductive properties
of various biological tissues. Most of the body's fat-free
mass is composed of conductive tissues such as muscle,
while fat is part of the non-conductive tissue mass.
The volume of these tissues can be estimated from the
measurement of the resistance to an applied
electric current flowing through the body.
Water is a constant fraction of fat-free
mass, usually about 73 percent. The water measurement
can therefore be used to estimate levels of fat-free
mass. Water ratios, however, can change with the onset
of certain illnesses (mostly in the extra-cellular
compartment). Accurate body water measurements using
BIA can be important in studying disease.
Computed Tomography (CT) Scan
Computed tomography produce cross-sectional scans
of the body, which is particularly useful in determining
ratios of visceral (intra-abdominal) fat versus subcutaneous
fat. CT is also used to obtain true three-dimensional
bone mineral density (BMD in g/cm 3 ), usually of the
lumbar spine. The BMD measurement looks specifically
at trabecular bone - the softer, spongy bone within
a vertebra that is most closely associated with bone
loss in osteoporosis and other bone diseases.
Lipometer
This small, portable, optical device
enables researchers to measure the thickness of subcutaneous
adipose tissue at any location in the body in a rapid,
safe and non-invasive manner.
Ultrasound
Ultrasound measurements operate by
emitting high frequency sound waves that penetrate
the skin surface and pass through the fat layer, bounce
off the muscle or bone, and return to the ultrasound
unit. The time for the transmission of the ultrasound
wave is recorded and converted to a body composition
measurement based on comparative data. This portable
technology can be used to estimate the bone density
of individuals using localized measurements, such as
of the tibia and radius, as well as to estimate subcutaneous
fat. |
