Enzyme

Enzymes are catalysts, almost always proteins, that lower the energy needed for a reaction to proceed. Each is highly specific to its substrate, and their activity is sensitive to temperature, pH and the presence of cofactors — often vitamins and minerals.

It matters because essentially every process discussed in nutrition and training is enzyme-mediated: digesting food, releasing fat from storage, building muscle protein, regenerating ATP. Training adaptations frequently amount to producing more of the relevant enzymes — endurance training increases oxidative enzyme activity substantially.

In practice enzymes also appear as blood markers. ALT and alkaline phosphatase are enzymes measured to assess liver and bone activity, and creatine kinase leaking from muscle is used as a rough index of muscle damage.