Originally posted by Estyn:

Totally based on food. But as you progress, the food number stats grow too, of course.

Aww. That's a bit disappointing, honestly.
I saw how skills leveled up, so I thought maybe spending stamina and taking damage might level those as well :(

In one of the videos I saw, I watched them get a level up for jumping. Does increasing the level of a skill that uses stamina decrease the stamina usage?