AMPK is an enzyme that acts as the cell's main energy gauge. It switches on when the ratio of AMP and ADP to ATP rises — that is, when the cell is running low on usable energy, as happens during fasting, caloric restriction, and exercise.
When AMPK is active it shifts the cell away from energy-consuming growth programs and toward energy-producing maintenance: it promotes fat and glucose oxidation, stimulates mitochondrial biogenesis, and helps trigger autophagy, the cell's recycling system. It also restrains mTOR, the growth-driving pathway — the two behave as opposing arms of a single metabolic switch.
This is why AMPK sits at the centre of the longevity interest in fasting and caloric restriction: interventions that raise AMPK activity tend to engage the same repair and stress-resistance programs linked to healthier aging in laboratory models. The diabetes drug metformin is thought to act in part through AMPK. Human longevity outcomes remain unproven, however — most of the lifespan evidence is mechanistic or from animal studies.