Although exercising on an empty stomach can give you a higher amount of fat burn, it also can cause your body to rely on your muscles for fuel. This in turn means you are losing muscle mass.
The body needs some amount of carbs in order to burn calories and fat more efficiently without looking to the muscles for the fuel it needs. How much you need to eat depends on the type of workout you have planned, the duration and the time of day. 
For instance, if you are an early morning exerciser, then eating before working out can be a light meal (200 to 300 calories) of carbs and protein. Your foods for exercise do not have to be an elaborate meal. Just make it simple but be sure to include carbohydrates and protein.
Additionally, be sure to hydrate sufficiently as you have gone all night without water. Drink 2 glasses of water upon waking and then have your meal. Wait for about ½ to one hour before working out.
Another helpful tip to prevent stomach cramps is to make your meal liquid like a protein shake. Since you are only waiting less than hour for your workout this can help prevent stomach pains while exercising. Eating before exercise will provide your body with the needed fuel so you can workout longer and burn more calories.
If your workout is during the day or evening and after larger meals, you should wait longer before exercising. You should allow 3 to 4 hours after eating before exercise for big meals and 2 to 3 hours after smaller meals.
If you are planning a workout for less than an hour, you will want to choose carbs that can be easily digested like crackers, bagels and breads. Be sure to make these low fat choices.
However, if you are planning a workout for more than an hour than you want to select carbohydrates that take longer to digest. Foods like bananas or yogurts are carbs that take longer to digest. Additionally, you want to be well hydrated before, during and after your workout, especially during for the longer exercise routines.
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