Metabolic flexibility is getting some interest in the blogs recently. (Because of my own interest in walking up mountains, I found JS's account of his fasted walk up Mount Whitney both inspiring and educational). The idea is that a healthy body has the capacity to switch between fuel sources - fat or sugar - as necessary according to the circumstances.
If you can put up with the photos and adverts, Mike has a piece on metabolic flexibility on T nation, presumably culled from his PhD
Anyway, it is an interesting topic for you to go and google if you want to find more. What prompted this post was this abstract that popped up which indicates that basic inactivity itself is a key cause of metabolic inflexibility and its associated problems.
Physical inactivity as the culprit of metabolic inflexibility: evidence from bed-rest studies