You are probably familiar with carbo-loading or consuming extra carbohydrate in the days leading up to a race. This "topping off the tank" attempts to maximize energy stores. Some athletes are now adjusting their diets to consume mainly fat in a period of a few days to a week prior to carbo-loading. This is in an effort to get the body to utilize more fat as a fuel source and spare precious glycogen stores.

Glycogen, the stored formed of carbohydrate is a valuable commodity for an athlete. We have only enough glycogen stored in the body to perform about two to four hours worth of exercise depending on intensity. This only amounts to roughly around 2000kcals. Once this is used up we rely solely on fat and protein as an energy source and endurance and intensity are severely compromised. This is called "hitting the wall." However, we have enough fat in our bodies to complete virtually any event with energy to spare. Better utilization of this stored energy for long endurance events could give an athlete a key advantage.

Does it work?

It appears that consuming a high fat diet followed by a high carbohydrate does increase fat metabolism. In one study, a group of cyclist following this protocol did increase fat burning- and lowered carbohydrate oxidation. Although this process raises the level of an enzyme that is important in fat metabolism, it also LOWERS levels of one responsible for carbohydrate breakdown.

The conclusion; don't change your diet yet. The access and utilization of carbohydrate is very important for athletic performance and decreasing the ability to utilize carbohydrate adversely affects performance. There may be some benefit to ultra-endurance athletes but it has yet to be proven.

References
Peak Performance Cycling; number 227: 1-4