Protein status is assessed by evaluating both somatic and visceral protein status. Somatic protein status is a measure of the protein in skeletal muscle while visceral protein status is a measure of all other proteins (organs, viscera, serum, blood cells, white blood cells).
Evaluation of somatic protein status can generally be performed using muscle circumference or mid-arm muscle area. However, because no single indicator is completely accurate biochemical measures can help better provide perspective for somatic protein status.
Creatinine serves as a useful measure because creatinine is produced in the skeletal muscle. The more skeletal muscle a person has, the more creatinine will be excreted. A 24-hour urinary creatinine excretion test is easily tested in the laboratory. The measure can then be compared to standards based on stature and body weight. The 24-hour urinary creatinine excretion can also be compared to reference values from the creatinine-height index (CHI). The CHI is a ratio of 24-hour urinary creatinine excretion and an expected amount depending on sex and stature. Creatinine measures have their limits samples have to be collected in exactly 24 hours and diet can compromise creatinine measurements and, thus, measures of excretion and CHI.
The amino acid, 3-methylhistidine, is another useful measure of muscle mass because it is found in the contractile proteins of muscle, actin and myosin. It is releasaed when the contractile proteins are catabolized and excreted in the urine. As long as protein synthesis and degradation is steady, the amount of 3-methylhistidine should paint a picture of muscle mass. However, just as 24-hour urinary creatinine excretion, the measure of 3-methylhistine is limited. The value can be affected by diet, age, sex, maturity, hormonal status, physical shape, any recent intense exercise, injury or disease. A significant pool of 3-mehtylhistidine also lies outside of skeletal muscle that also creates complication as an index of skeletal protein breakdown.