AMPKAMP-activated protein kinase

AMPK is an enzyme that detects a falling energy state inside the cell — specifically a rising ratio of AMP to ATP. When activated it switches the cell towards producing energy: increasing glucose uptake, fat oxidation and mitochondrial biogenesis, while turning down energy-expensive building processes.

It matters because it is the counterweight to mTOR, the growth pathway. Endurance exercise and energy restriction activate AMPK; resistance training and protein feeding activate mTOR. The metformin used in diabetes works partly through AMPK, which is also why it appears in longevity discussions.

In practice the supposed 'interference effect' between endurance and strength training was built on this opposition. Real-world evidence suggests it is much smaller than the molecular story implied — for most people, training both qualities works perfectly well with sensible scheduling.