Creatine is a compound ingested in red meats or that the kidneys and liver synthesize. It is used to replenish adenosine triphosphate, ATP, the main energy carrier of cells. As such, it is found in the highest concentrations in muscle and nerve tissues. It is broken down into creatinine, which the kidneys then secrete. Urea is another compound the kidneys secrete, but it is a breakdown product of protein metabolism. It is important to consult with a physician for the diagnosis and treatment of health conditions.
Creatine
Creatine is found in the diet, particularly in red meat. It is also available as a supplement in health food stores. The liver and kidneys synthesize creatine as well. It travels through the blood to the muscle and nerve cells. Once it enters the cells, a phosphate is added, converting it to creatine phosphate. When the primary energy carrier of the cell, ATP, is used, such as for muscle contraction, it loses a phosphate to become adenosine diphosphate, ADP. Creatine phosphate donates its phosphate to ADP, rapidly replenishing ATP. Creatine is broken down into the chemical creatinine, which the kidneys filter and excrete in the urine.
Urea
The digestive system breaks dietary protein down into its constituent amino acids. Amino acids not used for the protein synthesis are broken down as an alternative source of energy in the liver, yielding urea and carbon dioxide. Urea, also known as blood urea nitrogen, BUN, is largely a waste chemical, although it can help the kidneys reabsorb water. The kidneys filter BUN and excrete it in the urine.
Diagnostic Value
Frequently, blood tests are run for both creatinine and BUN but generally not for creatinine alone. Creatinine is a sensitive indicator of kidney function. Increasing levels indicate decreased filtration of the blood from impaired kidney function. BUN also rises with worsening kidney function but many other conditions can cause this as well, such as dehydration or gastrointestinal bleeds. In a gastrointestinal bleed, the intestines absorb proteins from blood and the liver synthesizes more urea. Low creatine and BUN levels usually are of no significance.
Treatment
Creatinine and BUN are not known to be toxic; however, they can help guide treatment. For example, kidney filtration can decrease due to inadequate blood volume as in dehydration. In that case, the BUN will rise relatively more than the creatinine, indicating the need for hydration therapy. As another example, an elevated BUN and normal creatinine could indicate a gastrointestinal bleed. The treatment of high creatinine and BUN aims to improve the underlying condition.
References
- "Henry's Clinical Diagnosis and Management by Laboratory Methods"; Richard A. McPherson, et al.; 21st Ed.; 2006
- "Robbins and Cotran Pathologic Basis of Disease"; Vinay Kumar, et al.; 8th Ed.; 2009
- "Physiology"; Linda S.Costanzo; 4th Ed.; 2008
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