Picture this. You just finished a long, hard day at work and you want to unwind with a five mile run. As you leave the office, the pit in your stomach reminds you that you haven't eaten in four hours. You feel you don't have time to snack, so you opt to run on an empty stomach. One hour later, you struggle through your run feeling lethargic, light-headed and ravenously hungry.

Many runners experience low blood sugar levels when they do not properly fuel their bodies prior to exercise. When low blood sugar occurs, less glucose fuels the muscles and the brain, which makes running difficult. If you experience hypoglycemia (low blood sugar), you may feel dizzy, shaky, irritable and just plain hungry. How can you prevent low blood sugar levels from slowing you down?

Unfortunately, no "perfect snack" exists but you can experiment to determine which energizer works best for you. For most runners, a carbohydrate-rich meal consumed two hours prior to exercise allows enough time for digestion and provides plenty of energy for your body. Carbohydrates are either broken down into glucose for immediate energy, stored as glycogen, an energy reserve, or stored as fat. Snacking during the day on low fat, high carbohydrate foods such as bagels, bananas, dried fruit and fig bars will keep you from getting hungry and give you a burst of energy needed for an upcoming workout.

For some runners, consuming simple sugars such as candy or soda 30 to 60 minutes before exercise may actually hinder endurance capacity due to a sudden release of insulin followed by a quick drop in blood sugar levels. The insulin response inhibits the mobilization of fat for fuel and low blood sugar drains your energy levels. Fruits (or fruit juices) and vegetables may serve as a better choice because they contain fructose which is absorbed more slowly and causes only a minimal insulin response. For some runners, fibrous and acidic fruit may cause gastrointestinal stress. Morning runners who don't have time for breakfast but need an energy spurt may want to drink a low acidic fruit juice such as apple or grape.

Keep in mind that the intensity, duration and type of exercise may also affect the way your body utilizes energy. On slower, long distance runs, the body relies on fat and carbohydrate stores (glycogen) for fuel. If glycogen stores are fully depleted, fatigue occurs. Runners who perform high intensity training such as intervals, hills and sprints depend more on glycogen and less on fat as an energy source. Therefore, high intensity runners must make an effort to consume at least 65% of calories from carbohydrates whereas casual runners can eat a well balanced diet (at least 55% calories from carbohydrates, no more than 30% calories from fat, and 15% of calories from protein) without worrying about carbohydrate intake.

The timing and amount of food consumed at a meal or snack also plays a role in balancing adequate blood sugar levels without hindering performance. In fact, eating several small, regular and well-balanced meals may help regulate blood sugar levels and allow time for digestion more effectively than eating three large meals. Foods high in fat take longer to digest. For instance, do not schedule your interval training on the same day that you go on a fat- filled "power lunch" with your boss. Your gastrointestinal system will rebel. After a rich meal, you may want to engage in low impact exercise such as walking or light cycling to avoid upsetting your stomach.

Some runners prefer supplementing their diets with commercial sports bars and drinks to maintain energy levels. Sports bars vary in nutritional content depending on their purpose, so you may want to read the label and find out which products meet your nutritional needs. (For the most part, sports bars and drinks pack a lot of nutrition in a very convenient snack. Other runners may prefer snacking on less expensive, more conventional foods that add taste and variety to their diets. Sports supplements are just as effective as conventional foods in maintaining adequate blood sugar levels.)

So, if you lack energy while running, consider snacking during the day. The optimal snack depends on the intensity, duration and type of exercise you perform along with the timing and amount of food consumed. Experiment with different snack strategies until you find the one that meets your energy needs.

Rose Kehoe holds a M.S. in Nutrition and Exercise Physiology and is a nutrition educator at the Egg Nutrition Center in Washington, D.C.