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Section II:
NORMAL BODY COMPOSITION
The body
can be divided into two compartments. Lean body mass (LBM)
makes up about 75% of body weight and contains three
quarters water and all the protein and mineral in the body.
The fat mass makes up about 25% of body weight and contains
all the fat (water free).
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Figure 1:
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Legend: The contents of the
lean body mass and fat mass compartments are shown. |
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Lean Body
Mass |
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The lean body mass (LBM) contains all of the
protein in the body. The LBM is highly metabolically active and
essential for survival. Protein makes up all the muscle (50-60% of
the LBM). The critical cell structure in all organs and connective
tissue, in addition to enzymes, growth factors and antibodies
(Figure 1). Any significant loss of protein is deleterious.
The LBM is maintained
by an intense genetic drive for preservation and action of the
anabolic hormones, growth hormone, testosterone and insulin.
Resistance exercise also maintains the anabolic activity (protein
synthesis) needed to maintain LBM. Adequate protein intake is of
course required for sufficient substrate needed by the protein
synthesis pathway. |
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Muscle is a very dynamic tissue, with
about 50-100 grams of muscle continually broken down and
re-synthesized each day. In healthy man, the rate of loss and
synthesis is equal, with a net zero nitrogen (N) balance. With
increased anabolism, (increased protein synthesis) muscle gain, is
increased with a nitrogen gain. With burn injury, there is a net
loss. Muscle is 75% water or (1 gram of muscle N=25-30 grams
muscle). Multiplying protein by 4 is a good estimate of muscle
(gms). N (grams) x 6.25 = protein (gms) x 4 = muscle (gms). Intake
of adequate calories, protein, vitamins and trace elements, are
required to maintain normal body composition. |
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Fat Mass |
| Fat mass
contains mostly fat stored as an energy reserve. Some fats
essential for survival are also present. The fat mass typically has
over 100,000 potential calories assuming the metabolic environment
is in the fat burning mode as seen with pure starvation.
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