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II.
The Importance of Maintaining Lean Body Mass
In
order to better understand the impact of erosion of lean
mass and the normal or abnormal utilization of protein
and fat for fuel, a general understanding on normal body
composition is required.
The
body composition can be divided into fat and fat-free
components. Body protein is present in the fat free or
lean body mass (LBM) compartment. Fat mass is usually
about 20-30% of total.
The
Lean Body Mass is highly active metabolically and
physiologically and the size is genetically defined and
maintained. The LBM is 75% of body weight and contains
all the body protein. There is no real protein store as
every protein molecule has a role in maintaining
homeostasis. Loss of any body protein is deleterious.
The majority of the protein in the lean body mass is in
the skeletal muscle mass. Lean body mass is 50-60%
muscle mass by weight, the rest is bone and tendon.
Protein makes up the critical cell structure in muscle,
viscera, red cells and connective tissue. Enzymes which
direct metabolism and antibodies which maintain immune
functions are also proteins.
It is the
loss of body protein which accompanies the injury, not
fat loss, that produces the complications of
malnutrition.
The
majority of fat mass is stored and biologically
inactive. Its only role is a reservoir for calories.
What
is Lean Mass and Its Importance?
| Complications
Relative to Loss of Lean Body Mass * |
|
Lean
Body Mass
(% loss of total)
|
Complications
(Related to lost lean mass)
|
Associated
Mortality (%)
|
| 10 |
Impaired
immunity,
..increased
infection |
10 |
| 20 |
Decreased
healing,
..weakness,
infection
|
30 |
| 30 |
Too
weak to sit, pressure
..sores,
pneumonia, no healing |
50 |
| 40 |
Death,
usually from
..pneumonia |
100 |
| |
|
|
*
Assuming no preexisting loss.
METHODS
ROUTINELY AVAILABLE TO ASSESS LEAN BODY MASS
| Method |
Description |
Advantage |
Disadvantage |
Precision |
| Measurement
of skin fold thickness |
Thickness
of subcutaneous adipose tissue is measured
as an indication of fat and fat-free mass |
Easily
performed with portable equipment |
Possibility
of error & inter-observer variability
in measurement |
5-10 |
| Bioimpedance
analysis |
Low-Level
current is introduced, and measurements of
impedance are used to calculate fat and
fat-free mass |
Easily
performed with portable equipment, used to
calculate body cell mass |
Results
will be affected by hydration |
<
5 |
| Dual
X-ray absorption metry |
Soft-tissue
attenuation of low-level radiation source
is used to determine bone, fat, and lean
body mass |
Non-invasive,
takes 20-30 min to complete, negligible
amount of radiation |
Results
may be affected by hydration; regional
measurements not standardized |
3 |
| Nitrogen
Balance |
Measurement
of nitrogen intake minus urinary nitrogen
loss to determine net nitrogen gain or
loss |
Easy
to perform |
Not
totally reliable as there are other
nitrogen losses besides urine |
10-15 |
PRIORITY
FOR PROTEIN INTAKE VS.
% LOSS OF LEAN TISSUE
As
lean mass decreases, more consumed protein is used to
restore LBM with less being available to the wound.
Wound healing rate decreases until lean mass is
restored.
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