It's a good idea to eat something before a long run, especially on runs longer than two hours. Just like logging miles is part of the conditioning process for a marathon, so is eating before and during the run so you’ll know what works best the day of the race.
Digestion can be difficult during runs because the body diverts blood flow away from internal organs and shunts it to the large muscles of the body in order to supply them with oxygen-enriched blood to meet the demands of running. This means areas like the gastrointestinal tract receive less blood flow during exercise.
The amount of blood flow that is diverted away from internal organs is usually correlated to the intensity of the exercise. In addition, the longer the run, the more time these organs receive less blood flow, which can contribute to GI issues on long mileage days.
Eating before the run provides two very important functions. One, it gives you some blood sugar, which is much needed after a night of sleeping and fasting. Think of it as filling up the gas tank of your car - it's not completely empty, but you want to top it off.
The other important function eating beforehand serves is that it helps keep the GI system in working order during the run. Having food present in the stomach recruits blood flow for digestion. With additional blood flow present, the stomach and GI system will be able to retain blood flow when the shunting process from running occurs, keeping the GI system in better working order. When no food is on board, blood flow is immediately diverted away. The GI system shuts down, making eating on the run very difficult.
A pre-run breakfast should contain about 200 to 300 calories. Start with the lower end of this range and gradually increase it as needed. (Better to go minimal first than to overload the system.) Think small meals that contain primarily easily digestible carbohydrates with some protein; about a 3:1 ratio of carb to protein works best.
You should also get up early and eat about one to two hours before the run. Low-fat and low-fibre foods are the best choices for most runners, but experimenting with different foods is important so you can figure what works best for you. The goal is that by race day you will have your personal nutritional plan all figured out.
Start with foods that are familiar to you and that are easy to digest. Some examples are bananas, rice, apple sauce, dry toast or half a bagel. Adding a nut butter or houmous to toast or a banana adds some protein. Other options include cereal (with or without milk or low-fat yogurt), cooked quinoa, a sweet potato or an energy bar.