Inc. has it pretty much right:
The body is able to replace only about one-third of what it loses during exercise; this is true for fluids, calories, and electrolytes - so in part we train to cope with this, i.e. 'cycling on fat'.
You cannot (and probably shouldn't) replace everything you use, but you can top up with what your body can handle - a very personal thing, so experiment and work out (and thoroughly test) what works for you.
To my mind most of us, if fit and able to handle 300-400's comfortably within time limits (70-90%), then its all down to if you know what you body likes, can cope with, and how it handle lack of sleep - another benefit of completing at east one 600. Get this sussed and you should be able to go day after day without too much trouble.