Creatine in muscles is converted to creatine phosphate (CP - also known as phosphocreatine), involving the enzyme creatine kinase, which bonds creatine to a high-energy phosphate group. Creatine is permanently stored in muscle cells as CP until it is required to replenish phosphate.
I'll now attempt to briefly explain some of the science of energy production, to give you insight into the background as to how creatine helps. Adenosine triphosphate (ATP) is the molecule used to provide energy in all cells. It is the key molecule formed from the Krebs cycle, glycolysis and lipolysis, three of the key metabolic pathways that show the chain of reactions whereby nutrients are converted to energy or stored. ATP carries three phosphate atoms, and when each bond, which holds a phosphate group to the adenosine molecule, is broken, a 'unit' of energy is released. By this process our muscles have access to energy enabling them to contract and our bodies to function. Each molecule of ATP can release two 'units' of energy by being broken down firstly into adenosine diphosphate (ADP - with two phosphate atoms), and then into adenosine monophosphate (AMP - adenosine plus one phosphate group).
What happens when all the ATP in cells has been used up? Where do we get our energy from then? Well, this is where CP comes in, and creatine replenishes AMP to ATP, by transferring the phosphate in creatine phosphate back to the adenosine in AMP and ADP. At this point creatine becomes creatinine, which is removed by the blood and excreted via the kidneys. In the clinical setting, creatinine levels are measured to assess physiological parameters such as kidney function. Creatine supplementation raises creatinine levels, although it is completely non-toxic to the kidneys (Robinson, et al 2000).
Numerous studies have demonstrated that the more creatine that is present in muscle cells, up to a maximum storage level, the more efficient ATP can be replenished, and, hence more ATP is available for energy. Typically, the average person metabolises about two grams of creatine per day, which is roughly the same amount as can be synthesised by the body. The richest food source of creatine is meat and fish, but it has been found that muscles can store far more CP than is possible to obtain from food (Hultman, et al 1996), so by supplementing with creatine monohydrate you can maximise these stores. You would have to consume over 10lbs of raw steak a day during the creatine-loading phase to optimise stores!
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