From Spark People:
Your Pre-Exercise Meal or Snack
News flash: Most of the fuel you use during exercise doesn’t come from the food you’ve recently eaten! It actually comes from the carbohydrates (called “glycogen”) and fat that’s stored in your muscles, liver, and fat cells. That’s enough to fuel 1-2 hours of very intense exercise or 3-4 hours of moderate intensity exercise.
This means that if your overall diet is adequate to keep your fuel tanks topped off, you may not need to eat anything before you work out. So, if eating before exercise upsets your stomach or you like to exercise first thing in the morning or at a time when eating first isn’t convenient, don’t feel like eating first is a must.
Some people do have a hard time exercising without eating first, especially if it’s been a long time since their last meal or snack. These individuals often are more sensitive to changes in their blood sugar levels, which fall during the first 15-20 minutes of workout. That drop in blood sugar can cause tiredness, mild dizziness, or even faintness—especially if your blood sugar was already low, but eating something beforehand can help prevent this. If you have health issues like diabetes or hypoglycemia that can cause low blood sugar, you’ll probably want to eat before your workout. If you get very hungry during a workout (and it interferes with your energy levels or focus), or become so ravenous after an exercise session that you end up overeating, try eating before you hit the gym to avoid these problems.
What to Eat Before You Work Out
Eating Before Exercise for Maximum Results
-- By Dean Anderson, Fitness Expert
Ask the Experts
If you are a moderate exerciser who tends to perform better with a pre-exercise snack, there are two ways to handle your needs:
1. Eat a small (100- to 200-calorie) snack about 30 minutes before you work out.
2. Eat a nutritionally balanced meal 1-2 hours before your exercise. This is the best option for many people.