A study from The University of Bern in Switzerland shows 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 (European Journal of Applied Physiology, November, 2006). 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.
The data on fat storage may have no practical value for endurance athletes because the authors were not able to show that the extra fat stored in muscles increased endurance. This is probably because there is almost an unlimited amount of energy available from a person’s own body fat. Changing the percentage of fat use from body fat to muscle fat would not increase energy sources and therefore would not increase endurance.
Carbohydrates are another story. Normally there is only a small amount of carbohydrates stored in the muscles, liver and bloodstream. Storing extra carbohydrates in muscles is beneficial because when a person runs out of stored muscle sugar, his muscles hurt and are more difficult to control. In the 1940s, Per Olaf Ostrand showed that a high carbohydrate diet for several days before athletic competitions helps a person store more sugar in muscles, which does increase endurance. Since then athletes have eaten high-carbohydrate diets before competition and often have pre-race pasta parties. Subsequent studies showed that highly-conditioned endurance-trained athletes can maximally fill their muscles with sugar just by eating their usual meals and cutting back on their heavy workloads for a few days before competition.