Endogenous creatinine derives from its precursor, creatine, synthesized in the liver and kidney, and 98% localized in skeletal muscles in the form of creatine phosphate (CF or PC).
Creatinine is formed by the non-enzymatic hydrolysis of free creatine during dephosphorylation of creatine phosphate.
PC + ADP = C + ATP
where is it:
PC = CREATINE PHOSPHATE synthesized at rest in skeletal muscle by associating an inorganic phosphate molecule with a creatine molecule
There is a direct proportionality between the creatine present in the body and the production of creatinine, so that the excretion of urinary creatinine can be used to estimate the lean mass and muscle mass of the subject.
Some researchers have proposed a constant relationship between creatinine excretion and muscle mass:
1 g of creatinine excreted in 24 hours corresponds to 17.9 kg of muscle mass
For other authors it is about 20 kg.
There are therefore equations from which it is possible to trace muscle mass values starting from the excretion of creatinine.
LIMITING FACTORS:
1) great intra-individual variability in daily excretion, on which the diet also acts. Diets poor in meat for long periods cause poor production of CU.
2) accuracy of urine collection time.
Forbes 1976: small time difference of 15 minutes compared to 24 hours leads to a 1% error in the determination of daily excretion. It is advisable to take 3 consecutive urine samples in 24h.
Error related to lean mass is similar to densitometry.
In 1981 Schutte et al proposed the use of total plasma creatinine as an index of skeletal muscle mass. Indeed, there is a strong relationship (r = 0.82, p <0.001) between total plasma creatinine (plasma volume x [creatinine plasma]) and urinary creatinine excretion over 24 hours.
1 mg of CPT (total plasma creatine) corresponds to 0.9-1.0 kg of skeletal mass
This study reported a 4% error between the predicted lean mass value and the measured value, an acceptable error after all.
