Carbs and Fat

[Owlie]

First, how's your protein intake?
biochemistry nerd:
Carbs (especially simple ones) are rapidly burned off; there are a lot more steps needed to convert fat into usable fuel. There's a reason all those gels and things are essentially sugar and flavoring. If you don't have enough when you need them, you start converting amino acids (from muscle) to glucose--muscle and many other tissues can use fat and its breakdown products for fuel, but your brain and red blood cells are pretty much dependent on glucose.

/biochem nerd