Low Carb Diet Does Not Increase Endurance or Speed

Some athletes and exercisers believe that a low carbohydrate diet will increase their endurance and speed, but there is no evidence that it will. Runners get fuel for their muscles from fat and sugar in muscles, fat and sugar in the bloodstream and, to a lesser degree, from protein. The key to increasing endurance for racing is to store as much sugar in muscles before you race and keep it there as long as possible. Muscle sugar gives you the most energy for the least amount of oxygen.

Restricting carbohydrates does not stimulate muscles to store more sugar (Sports Medicine, April-May 2007). A low carbohydrate diet may impair performance if carried out for extended periods because a runner cannot train on a low- carbohydrate diet. If there are benefits from depleting muscles of their stored sugar supply, they probably come from the high- volume depletion workouts, not from the diet. To maximize stored muscle sugar, a runner should reduce his workouts for two to three days before a race. He or she should not restrict carbohydrates.

Another study from The University of Bern in Switzerland showed that a high carbohydrate, high-fat diet for three days before competition can help athletes store more fat in their muscles and use much more muscle fat for energy during exercise. Endurance-trained athletes exercised for three hours to empty sugar and fat reserves from their muscles. Then they ate a high-carbohydrate, low-fat diet for 2.5 days or the same diet with lots of added fat for the last 1.5 days. Athletes who ate the high-carbohydrate, high-fat diet stored 55 percent more fat in their muscles and used more than three times as much of that fat during exercise. More