ATPAdenosine triphosphate

ATP, adenosine triphosphate, is the molecule every cell actually spends. Contracting a muscle fibre, pumping ions across a membrane, building a protein: all of it is paid for by breaking a phosphate off ATP and releasing the energy stored in that bond.

The striking fact about it is how little you hold. The body contains only a few grams at any moment, enough for a couple of seconds of maximal effort, yet an adult turns over something close to their own bodyweight in ATP across a day. It is not a fuel tank. It is a currency that is constantly spent and constantly regenerated.

Three systems do the regenerating, and they overlap rather than take turns. The phosphocreatine system refills ATP almost instantly for the first several seconds of a maximal effort. Glycolysis breaks down carbohydrate quickly without oxygen and can sustain hard work for a minute or two. Aerobic metabolism in the mitochondria is slower to ramp up, far more efficient, and effectively unlimited while fuel and oxygen last.

That structure explains the shape of fatigue. A heavy set of five is limited by the phosphocreatine system, a 400 metre run is limited by glycolysis and what accumulates alongside it, and a marathon is limited by fuel supply to the aerobic system. Different limits, different training.

It is also why creatine works. Supplementing raises muscle phosphocreatine stores, which extends the immediate system slightly and lets you do a little more high intensity work, which over months becomes more training and therefore more muscle and strength.

Products marketed as containing ATP or boosting cellular energy generally miss the point: swallowed ATP is digested rather than absorbed intact, and the body's constraint is the machinery that regenerates it, not the molecule itself.

In practice, train the system your event uses, and note that our creatine calculator covers the one supplement that acts directly on this pathway.