An article from Australia shows that novice exercisers who train after skipping breakfast have higher muscle levels of glycogen (stored sugar) than those who train after eating breakfast (Journal of Science and Medicine in Sport, May 2010). When you run out of stored muscle sugar, you have to slow down, so having more sugar stored in a muscle should help you exercise longer. The faster you exercise, the greater the percentage of sugar that you use for energy. However, starting workouts with depleted stores of glycogen will not benefit competitive athletes who train for many hours each day, because restricting carbohydrates will cause them to tire earlier and thus do less work.
In another study, researchers asked competitive athletes to train either on a high or low-carbohydrate diet (Journal of Applied Physiology, November 2008). Those training on the low carbohydrate diet had much greater gains in stored muscle sugar and ability to use fat for energy during cycling, although they couldn't train as intensely as the high-carbohydrate group in the first few weeks. However, during the last week there were no differences in training. Both groups improved their one-hour time-trial performances by about 12 percent.
More recent data show that taking sugar during training sessions increases the amount of training an athlete can do without interfering with racing times (Journal of Applied Physiology, February 2009). At this time we do not have enough data to recommend restricting carbohydrates during training, or that it will increase endurance during competition.
CAVEAT! Eating foods or drinks that cause a high rise in blood sugar within an hour before a race will cause you to tire earlier. A high rise in blood sugar causes your pancreas to release huge amounts of insulin which causes you to use up your stored muscle sugar at a much faster rate. When you run out of stored muscle sugar, you have to slow down because it forces you to burn more fat which requires more oxygen. Getting oxygen into muscles is the limiting factor in how fast you can race. Researchers at the University of Hull in the United Kingdom showed that bicycle racers rode much faster 40 kilometer time trials 45 minutes after eating a low glycemic index (GI) pre-race meal than a high glycemic one (Journal of Science and Medicine in Sport/Sports Medicine Australia, January 2010). The low GI meal led to an increase in the availability of carbohydrates and a greater carbohydrate oxidation throughout the time trial. More references