Creatinine excretion and N-methyl-histidine excretion
In the muscle cell, creatine phosphate, necessary for the energy metabolism of the cell, degenerates to creatinine at a constant daily rate of about 2%. It is assumed that 1 g of excreted creatinine is equivalent to 18–22 kg of muscle mass. As the cell cannot recycle creatinine, the kidneys excrete it. Since metabolized creatine phosphate is not the only source of urinary creatinine (e.g., creatinine in ingested meat is also excreted immediately), the validity of the method is dubious. A day-to-day coefficient of variation in the excretion of creatinine of almost 20% is reported, when the subject is “free living” and the urine is sampled over constant periods of 24 hours. The high variation is due to the ingestion of creatinine with nonvegetarian meals, differences in physical activity levels, and variation in creatinine excretion within the phase of the menstrual cycle. After careful standard-ization, which includes a controlled diet, the day-to-day variability in excretion can be decreased to about 5%. To obtain a reliable assessment of the creatinine excretion, sampling of urine over multiple 24 hour periods is necessary.
The excretion of 3-methylhistidine has also been proposed as a measure for muscle mass. FFM deter mined by densitometry correlates well with excreted 3-methylhistidine. The chemical determination of 3-methylhistidine is, however, more complicated than that of creatinine. A unique feature of 3-methylhistidine is that it gives a measure of muscle protein breakdown. Given the greater expense of measuring 3-methylhistidine and the limited benefit for muscle mass estimates, it is probably best to use it primarily for turnover studies.
The main disadvantages of creatinine and 3-methylhistidine excretion as measures for body com-position are the large variability in excretion, the necessity to follow a controlled (meat-free) diet for several days before and during the urine collections, and the difficulties associated with collecting 24 hour urine samples.