Creatine

Nitrogenous organic acid that occurs naturally in vertebrates and helps to supply energy to all cells in the body, primarily muscle. This is achieved by increasing the formation of adenosine triphosphate (ATP). Early analysis showed that human blood is approximately 1% creatine, and the highest concentrations are found in animal blood, brain (0.14%), muscle (0.50%), and testes (0.18%). The liver and kidney contain approximately 0.01% creatine.

see Creatine Kinase.