"To give me energy." This is the usual response when I ask why someone has eaten an energy bar or other food in the hour or so before starting exercise.

In fact, the opposite is true. Eating an hour or even half an hour before exercise is likely to make you feel tired and sluggish.

Twenty-five years ago, if a food or drink label proclaimed "high energy," it would have sounded a death knell for that product. In contemporary times though, people interpret that to mean that it will give them high energy.

Red Bull drinks proudly display that they are high-energy drinks. Sports bars are high-energy foods. Consequently, people eat or drink them so they have high energy during exercise.

In the nutrition world, "high energy" is synonymous with "high calorie." The way that the body deals with high calorie, particularly high sugar, is by releasing insulin. Insulin is a storage hormone. When you eat a high-calorie, high-sugar food an hour before exercise, you will start exercise with high insulin levels.

This has two important results. First, it will change how you perceive exercise. You feel sluggish and it feels hard. This is called the rating of perceived exertion, or RPE. Your RPE is higher when you start exercise with high insulin levels.

Second, you will burn a lower amount of fat for fuel. Insulin lowers fat removal from the fat deposits and reduces fat metabolism inside the muscle. Consequently, you rely more on carbohydrate for energy.

Obviously, if you are trying to increase fat metabolism, it is counterproductive to eat an hour or half an hour before exercise.

In general, during the day, it is better to time your last meal or snack to be two hours before the start of exercise. If you wait too long, however -- say, three or four hours after eating -- you'll spend the whole exercise session hungry and fantasizing about food.

In fact, the best time to exercise is after an overnight fast. At his time, blood levels of fat, growth hormone and testosterone are all high. Under these conditions, fat metabolism will be at its highest. Even better if you've had some coffee beforehand.

Those who have low fasting blood sugar in the morning may want to eat a banana, a sports gel or some juice within five minutes of starting exercise. You know who you are: you feel dizzy and shaky in the morning until you've had something to eat. If you eat a small amount a few minutes before starting, you won't get a substantive insulin response.

Instead of eating before exercise, eat within 30 minutes of finishing your exercise session. It is common for people to make the mistake of not eating after exercise, either because they think the increased metabolic rate will help them lose weight or just because they're not hungry. In reality, during exercise you are breaking down protein from the muscles, liver and kidney. This catabolic state persists after exercise.

The more intense the exercise -- such as a heavy strength training session at the gym or an interval workout at the track -- or the longer the exercise lasted, the more muscle breakdown will occur.

This when insulin is your friend. Insulin is a growth hormone and stops muscle breakdown. This is the best time to get larger doses of carbohydrate. Your glycogen storage tanks will be low and the combination of insulin and carbohydrate will refill them to be ready for the next exercise session and will help your muscles grow.

Deborah Shulman, Ph.D. is a physiologist specializing in nutrition, health and sports performance. For more information on her and programs, visit www.BodyScience.us or e-mail dshulman@frii.com.