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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?
Click
the Image to Enlarge
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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
Click
the Image to Enlarge
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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