Creatine and its breakdown product, creatinine, play an important role in energy metabolism. The body synthesizes creatine and derives it from food sources. It enters cells to help provide energy before being broken down and excreted. The level of blood serum creatinine and creatine can give clues about certain disease states. It is important to consult a physician regarding elevated creatinine and the diagnosis and management of other health-related conditions.
Creatine Blood Levels
In blood, the breakdown product of creatine, creatinine, is measured by laboratory methods to help diagnose health conditions. Normal values are 0.6 to 1.0 mg/dL; however, it is possible to be slightly outside this range and still be healthy. The level of creatinine in the blood represents the balance between the production and use of creatine versus the excretion of creatinine.
Synthesis and Sources
Creatine is synthesized from the amino acids glycine, methionine and arginine in the kidney and liver. It travels in the blood, primarily to the skeletal muscles. It is also found in the diet, especially meats. Creatine supplements are also available.
When muscle contracts, it uses the energy-carrying molecule ATP to perform work. ATP carries three phosphate groups on the molecule. When muscle contracts, one phosphate is lost off of ATP, making ADP. Creatine carries phosphate inside the muscle, and can quickly donate the phosphate to ADP, replenishing ATP, which can then be used again for work.
Degradation and Excretion
Creatine is degraded into the structurally similar chemical creatinine. It is released from the muscle cells back into the blood. The kidneys then filter creatinine from the blood and excrete it into the urine.
There are a few diseases involving an inability of the body to synthesize creatine, which largely results in nerve damage. The most common reason to find elevated creatinine in the blood is decreased kidney function. Creatinine levels rise in proportion to the loss of filtration capacity of the kidneys.
Supplementation with oral creatine supplements is common among athletes. The effects on athletic performance are mixed in studies. A 2002 study published in the "Journal of Family Practice" concluded that creatine supplementation with resistance training increases the maximum weight lifted in young males under age 36. Furthermore, there is no evidence for enhanced performance in older persons or women or for other types of power or strength exercises. Finally, the safety of creatine remains unproven, and until these issues are addressed, its use cannot be universally recommended.