Metabolism is the set of physical and chemical processes that occur within a living cell. These processes are necessary for life, as they allow cells to grow, to reproduce, and to survive.
In 2153, aboard the Enterprise NX-01, Hoshi Sato brought some angel hair pasta to sickbay for Doctor Phlox. Phlox refused to eat, however, as it would accelerate his metabolism, which could cause the Borg nanoprobes infecting his body to spread further. (ENT: "Regeneration")
In 2267, Arlene Galway died of old age when her metabolism was accelerated by radiation. (TOS: "The Deadly Years")
In 2369, Doctor Julian Bashir cited the metabolism as one of the aspects which change when a Trill symbiont changed from one host to the next one. (DS9: "Dax")
In 2370, Voval claimed that when Iyaarans such as himself were injured, their metabolic processes slowed down in order to accelerate healing, making them appear dead for a time. (TNG: "Liaisons")
Voval's claim was used to deceive Captain Picard during secret study on love, so his statement regarding the Iyaarans' ability to slow down their metabolism may be false.