Making good choices is critical to the success of your event. The pre-event meal can supply your body with significant amounts of energy. You should eat the right kinds of food for several days before the event to charge up your muscles with glycogen (your racing fuel!). Although the pre-event meal won't cause large increases in muscle glycogen, it will help avoid hunger during the event, stabilize blood-sugar levels and add some food energy to complement existing energy stores of muscle glycogen, hydrate the body (by supplying water to the body's cells), provide a relatively empty stomach at event time and prevent gastrointestinal upset or other adverse reactions to food.
Based upon your weight, effort, economy, etc, you will burn between 600-1200 calories per hour. Your body has two primary sources for this energy: fat and carbohydrate. At about 3500 calories per pound, even the thinnest triathlete has more than enough fat to carry him through a 12-16 hour Ironman.
Your body must burn carbohydrate in order to burn fat. "Fat burns in the fire of carbohydrate." Your body's primary sources for carbs are glycogen stored in the muscles and liver (about 2000 caloriess), and food ingested during exercise.
During low intensity exercise, say 600 calories per hour, your body gets most of its energy from fat. So let's say that of this 600 calories, 450 is from fat and 150 is from carbohydrate. However as intensity increases, the percentage of fat burned remains relatively constant, and the difference is made up by carbohydrate. For example, as intensity increases, you are now burning 900-1000 calories per hour. Only about 500 will be in the form of fat, the remaining 500 will be carbohydrate.
Here are some basic tips:
Five sample pre-race meals:
Cereal (avoid highly sweetened cereals)
Chicken noodle soup
Pancakes (limit butter and syrup)
Turkey sandwich on whole-wheat bread and lettuce
Breakfast should be a decent sized meal consisting mostly of complex carbohydrates-- energy bars or bagels are good sources--although you should include some protein to help with satiation. Many energy bars have a good ratio between carbohydrates and protein, so carefully read the labels and choose your energy bar accordingly. Your breakfast should be consumed 3-4 hours before your race start time; aim for 900-1500 calories, depending on your body size and the race distance. To top off your energy stores, it's important to have a light snack 70-90 minutes before the race start. For your snack, try to eat between 300-600 calories. An example: an energy bar, 16-20 oz of a sports drink, and 16-20 oz of water.