Amino acids play many roles in the body. They are the building blocks of protein and are needed to build the various proteins used in the growth, repair, and maintenance of body tissues. Amino acids provide the body with nitrogen to perform these essential functions. Aside from these general functions, individual amino acids also have specific functions in many aspects of human physiology and biochemistry. Amino acids serve as precursors for many nitrogenous substances. These include heme, purines, pyrimidines, hormones, and neuro-transmitters, including biologically active peptides. In addition, many proteins contain amino acids that have been modified for a specific function, e. g. , calcium binding or collagen cross-linking. Creatine is naturally synthesized in the human body from arginine, methionine and glycine primarily in the kidney and liver and transported in the blood for use by the muscles. Creatine is converted into phosphocreatine, which is used to produce ATP, the source of energy for muscle contraction and many other functions in the body. It can also play important roles in supporting healthy energy production and muscle structure and performance.