Phosphocreatine is a substance that, in its chemical partnership with adenosine triphosphate (ATP), is fundamental to the ability of the body to produce muscular energy. Phosphocreatine, which is also known as creatine phosphate is a compound constructed of carbon, hydrogen, nitrogen, oxygen, and phosphorus, in the molecular structure C4H10 N3O5P.

Phosphocreatine is formed naturally within the body, with over 95% of the compound stored within the muscle cells. Approximately 5 oz (120 g) of phosphocreatine is present in the body of a healthy adult; the levels of the compound do not fluctuate to a significant degree. When phosphocreatine stores become reduced, the body replenishes its supply from one of two sources. The first source is amino acids, the muscle- and tissue-building blocks present in all proteins. The liver produces phosphocreatine from amino acids. The body also receives dietary creatine primarily through the consumption of meat.

The role of phosphocreatine in the production of the energy required to produce muscular contractions must be understood in the context of the three pathways or systems through which the body produces energy, and the circumstances that dictate which pathway will be relied on at any given time. The aerobic system is the primary means by which muscular energy is produced, where the activity involving muscle movement lasts longer than approximately 90 seconds. The anaerobic lactic system responds to demands of between approximately 10 seconds and 90 seconds. The anaerobic alactic is the system employed where the energy need is short and intense, up to 10 seconds in duration.

In each of these systems, the cells engaged in energy production will utilize ATP, itself the ultimate product of glucose stored in the body. ATP is essential to the life of the cell. Phosphocreatine is not an energy source itself, like ATP, but it is crucial to the cyclical chemical reaction that is repeated in the mitochondria of each cell to continue the availability of ATP. When the need for energy is immediate and of short duration, as with weightlifting or a short sprint, ATP will provide the energy; phosphocreatine is available in the cell to be immediately broken down into its phosphate component, to provide further materials for the recycling of greater amounts of ATP. This recharging process can occur with tremendous speed during the 10-second period that the body utilizes the anaerobic alactic system, creating an indefinite cycle of energy generation and replenishing ATP, through the agency of phosphocreatine. The rate at which phosphocreatine is broken down depends almost entirely on the intensity of the muscle contraction required. Once other sources of fuel, through the aerobic system, are made available, the phosphocreatine stores will be restored.

The amount of phosphocreatine available to restore ATP through periods of intense muscle exercise are small. It is for this reason that muscle fatigue will be noticeable to the athlete through this process, even when the activity is of short duration.

The essential role of creatine phosphate in the ATP generation and restoration process spurred significant interest in the use of creatine as an athletic supplement. By virtue of the energy system that is primarily influenced by phosphocreatine, most interest in this compound as a training aid has come from athletes in sports where explosive power is of critical importance, including weightlifters, velodrome sprint cyclists, and race sprinters.

Creatine supplementation has been proven to assist athletes in extending their maximum ability to work. As with most minerals otherwise available in through a balanced diet, such supplementation is not required to address a deficiency, but to seek optimal performance. The body has difficulty absorbing phosphocreatine, and it is for this reason that the common creatine supplement is in the form of creatine monohydrate. The body can readily produce the necessary phosphocreatine once the creatine has been ingested; phosphocreatine is manufactured in the liver, pancreas, and kidneys. Studies also demonstrated that when creatine supplements were combined with a carbohydrate component, the ability of the body to retain phosphocreatine was significantly increased. As the period within which the cell is working and will require phosphocreatine to assist in the ATP cycle, the greater the amount of creatine phosphate to be retained, the greater the likelihood that the ATP cycle can be extended.

When phosphocreatine in the muscle breaks down, it is not reprocessed into a working form. Phosphocreatine metabolizes into a substance known as creatinine, which is excreted through the kidneys and passed as urine. The level of creatinine in the blood is a useful indicator in the determination of kidney function; high levels of creatinine are a symptom of an inability of the kidney to filter the creatinine wastes.

SEE ALSO Creatine supplementation; Glycogen depletion; Phosphate.