Creatine (They were only supplements, I swear!)
4th August 2006
Here’s one many will have heard of - creatine. Most people know it as a bodybuilding supplement; it is used to add lean mass (from working out harder, and it’s contended, some intracellular water retention), as well as allow harder anaerobic workouts before failure. It’s the latter we’re interested in.
Creatine actually occurs endogenously, and it is synthesized by the liver. An enzyme called creatine kinase exists in the body to move a phosphate from ATP to creatine, making N-phosphocreatine and ADP. This reaction is not very thermodynamically favorable - you will only make 1 molecule of phosphocreatine for every 1000 molecules of ATP or so. This is good, because your body needs the ATP for other things (you’ve probably heard of ATP as the “universal energy currency” of the body) as well. Phosphorylation of creatine only happens when your body has a large excess of ATP.
The reverse reaction takes place too, and, as you might guess, phosphocreatine still would prefer to transfer its phosphate to ADP, regenerating ATP. It’s favored in the low-ATP regime - such as during the last repetition of a weightlifting set. This latter reaction is shown below:
Phosphocreatine/creatine/creatine kinase/ATP/ADP is a system that allows your body to deliver energy very, very quickly. The slow system that generates most of this ATP is glycolysis, which breaks up sugar into useful carbon precursors for the body, as well as CO2 and substantial amounts of energy. It requires oxygen, creatine phosphate-delivered energy does not directly. It is from this that the approximate distinction between “aerobic” and “anaerobic” activity comes from - anaerobic activity, like weightlifting, gets energy delivered rapidly from ATP/Creatine phosphate and related systems . Aerobic activity uses energy sufficiently slowly that it can be carried on for longer periods of time, with glycolysis & related systems supplying a good chunk of the energy. Both systems are, of course, in effect constantly to one degree or another, whether you’re biking or benching - it’s not a black/white distinction.
Have a good weekend!