WW has it right, just to elaborate (since she's in a hurry, lol), its so much easier for the body to convert proteins (ie muscle) into glucose (sugar) than it is to do the same for fats. Carbs are even easier to convert to glucose.
This is why pros generally suggest that you eat mostly carbs, followed by proteins (which can be converted easily into muscle if you're working out and are getting enough carbs), and then few fats. I don't know the exact proportions, I think its something like 50-40-10? 45-35-20?
Oh, something I should also mention, is that the unused glucose from foods will be converted into fat, of course, which is why eating too much makes you fat
If you look up the Glycogen Scale (if I spelled it right), it'll give you a giant list of foods and how quickly they're converted into glucose on a 100 point scale (100 being refined white sugar) - the lower the number, the slower the conversion, which generally means it gives your body time to metabolize it, and will also keep your energy levels even because you don't get sugar highs and then sugar crashes.