Creatin.

[RoadRaven]

Creatine occurs naturally in meat (including fish) and milk (I think maybe in eggs too?).

In humans, over 95% of the total creatine is located in skeletal muscle.

Creatine is biosynthesized in liver, kidney and pancreas... the daily use of creatine for a 70kg person is about 2g. It plays an important role in the regulation and balance of skeletal muscle energy metabolism and is essential in the provision and transfer of energy.

The demand for creatine is bigger when muscles are working at high levels... like intense training or a cycle race when we push ourselves. I understand it is thought by researchers/scientists that having creatine available has an impact on the ammount of force your muscles are able to make available.

My understanding is that atheletes use up creatine (or creatin) more than less atheletic people (like most things our bodys need to make them meatbolise/work at a higher level) so it is useful if you are making demands of your muscles at high levels/high intensities to be aware of this. Extra creatine can be useful to your body, or your muscles may feel tired or not function as well for 1 or a few days after a big event.

I replace creatine in my body naturally... that is, after time trial training, or significant hill work, or a longer than usual ride at reasonable intensity, I eat steak, or a meal like mince and pasta.
Red meat is an excellent, easily metabolised source of creatine. Vegetarians and vegans may have to resort to supplements, but many of us shouldn't need to.