For more than fifty years, athletes in sports requiring endurance have used a training technique called carbohydrate loading. A recent study from South Africa shows that this technique slows sprint performance of cyclists ( Journal of Applied Physiology , January 2006).

Competitive bicycle racers ate a high fat or high-carbohydrate diet for six days followed by a high-carbohydrate diet for one day and completed time trials on their bikes. Then they ate the opposite diet for six days followed by a high carbohydrate diet for one day and repeated their time trial. Diets did not affect their times or power output for 100 kilometers (62 miles), but the high fat diet slowed their sprint performance over one kilometer (0.6 miles.)

Muscles get their energy from sugar and fat stored in muscles and coming into muscles from the bloodstream. The limiting factor in how fast an endurance athlete can exercise is the time it takes to transport oxygen from the blood in the lungs to the muscle. Muscles require far more oxygen to burn fat than to burn sugar for energy. So when a muscle runs out of its stored sugar, called glycogen, it becomes less efficient, hurts, is difficult to co-ordinate and slows you down. Many previous studies show that it doesnt make any difference what an trained endurance athlete eats on the week before competition because the muscles of trained athletes store the most glycogen when they reduce training for several days, regardless of what they eat. Any difference in the muscle and fat concentration inside muscles becomes unimportant during endurance competition.

This study shows that a high-fat diet before sprint competition hurts performance. A high fat diet causes muscles to burn a higher percentage of fat. Using fat for energy requires more oxygen than carbohydrates do, and how fast you can sprint 0.6 miles on a bicycle is limited by how rapidly you can deliver oxygen to muscles.