It is common Biochemistry knowledge that fat cells provide the most bang for their buck, giving 9kcal/g. Carbohydrates and proteins only offer 4kcal/g. For those of you good at math this is over double the BANG. The only problem is that our body normally uses this as a secondary energy source to carbohydrates. I recently read a paper by L. Burke et al that looked at the body’s ability to oxidize fat as an energy source and a lot of other interesting things about fat. They concluded that, “5 days of a high fat diet enhanced rates of fat oxidation during sub-maximal exercise despite increased CHO availability before and during exercise”. Basically the way the body works is that it uses minimal amounts of fat along with glycogen (stored sugar) as energy. It isn't until most of the glycogen stores are gone that the body reverts into extensively burning fat as an energy source. By changing your diet to be high in fat and low in carbohydrates you are forcing the body to become better at using fat as an energy source by increasing the number of substrates necessary to do so. This concept was the basis for the famous Atkin’s Diet that was popular a few years back. Burke discovered that after 5 days of the high fat diet, when they introduced carbohydrates back into the system the body still had significantly increased fat oxidation during exercise. This information could be extremely helpful in ultra-endurance activities because it would allow the body to preserve glycogen stores longer, using higher amounts of fat as it drains these stores. It would also improve the body’s ability to oxidize the fat once the body has depleted much of its stored glycogen and is relying more heavily on fat.
L Burke et al. Adaptation to short-term high-fat diet persist during exercise despite high carbohydrate availability. Official Journal of American College of Sports Medicine. 2002