2-(methylguanidino) ethanoic acid a.k.a. "creatine"
is synthesized from L-arginine, glycine, and L-methionineby,
by the liver, kidneys, & pancreas;
it chemically delivers energy in vertebrates to the skeletal-muscles, brain, & heart.
ADP/adenosine 5'-(trihydrogen diphosphate)
Creatine-supplements are sometimes used to gain muscle-mass,
but they have many negative-side-effects:
allergies, asthma, diabetes/hypoglycemia, kidney/liver damage, ...