depends on what you want to do..
the liver stores 300-400 cals of glycogen that is quickly and readily available for energy use during exertion, the muscle also stores some, but it's hard to say..
this is of coarse unless you are in a fasted state, meaning it's been so long since you last ate (over night) that your body has very little glucose to use for energy (high intense cardio can cause a sugar crash in this case)..
it takes about 1.5 hours for the body to process food in the digestive system, during this 1.5 hours your body is sending large quantities of blood to the digestive system to facilitate digestion, water too.. if you exercise in this condition your body is fighting back and forth, your muscles are screaming for blood that has oxygen, then your digestive system is screaming for blood to carry off nutrients..
so again it depends on what you are trying to do/accomplish..
stored fat burning, is fasted cardio... but long duration and moderate effort, if too much effort is used the body will convert proteins (muscle) for energy .. example... moderate for me is 110-125 bpm .. .during high intensity lifting, 164 is the max bp and the body/muscle can only use the energy stored in the muscle during contraction.. sooooo never do heavy lifting in a fasted state..
so a pre workout with low or rather fast digesting foods is ok, and preferred.. 45minutes to 1.5 hours pre workout..
post workout same thing, fast digesting proteins... i do not advocate dextrose and simple sugars, (some do, it's to create an insulin spike to effect the "dumping" of nutrients into the muscles but it just makes me fat) but then i don't allow for a routine that lasts longer than 45 minutes for resistance training... i can train right up to the point of low blood sugar levels, and knowing how your body performs is what really matters..