Meat is the only type of food that contains a significant amount of creatine, in particular beef, tuna and salmon. Other types of seafood also contain high amounts of creatine.
Aside from being obtained from food, creatine is also produced by the body in small amounts. It is converted to phosphocreatine in the body so that it can be stored in muscles. Muscles use adenosine tri-phosphate (ATP) for energy, which turns into adenosine di-phosphate (ADP) once used. Using the extra phosphate group from stored phosphocreatine is the fastest way to replenish ATP stores. However, muscles can only store a small amount of phosphocreatine before they become saturated.