Carbohydrates are the main source of fuel for the muscles and the brain. For the best, quickest sources of energy, Goglia tells WebMD, choose single-ingredient carbohydrates such as vegetables, fruits, or oatmeal over multi-ingredient items like breads or muffins. Simple carbohydrates, like fruit, are a quicker energy source than complex carbohydrates like a yam, but both are better choices than a blueberry muffin.
Fats provide the body with a slower release of energy than carbohydrates because it has a higher caloric content. Protein mainly works to build and repair muscles and is only used as energy if inadequate carbohydrates are available.
"Proteins do not give you energy," says Goglia, "proteins repair muscle tissue. Carbs move the muscles."
Before a run, then, says Goglia, "have a piece of fruit and peanut butter or oatmeal. Eat whole foods and let your body digest them. That's what it wants to do.
"Then, while running, if you feel as though you've depleted your carbohydrates, then use a supplement."
If you're a body builder, or a training athlete, says Goglia, and you're using a supplement as an aid to your balanced diet for convenience, that's OK, he says.
"But absolutely don't depend on those things within any given regular civilian day."
Besides replacing depleted carbohydrates or balancing the diet with a vitamin and mineral fortified bar, a sports drink or gel might be a good choice when an athlete cannot digest whole foods, says Cooper, right before or during performance.
Stout works with athletes, strategizing what they eat before and after exercise in order to maximize training. One hour before exercise, says Stout, "a bagel is just as effective as anything on the market. It's a complex carb, so it breaks down, but not as fast."
Not all carbohydrates are created equal, however. Some enter the bloodstream more quickly than others and they are considered to have a high glycemic index. Baked potatoes and raisins are examples. These are best eaten right before, during or right after exercise, whereas moderate (orange juice or a sweet potato) and low (apple or pear) glycemic index carbohydrates enter the bloodstream more slowly and are best consumed in the hours before a workout.