Changes

Running requires energy, and that energy comes from a mixture of carbohydrate, fat and protein[[Protein]]. The supply of fat is practically unlimited, but there is a limited supply of carbohydrate. This carbohydrate is stored as [[Glycogen]] in the liver and muscles. The liver glycogen [[Glycogen]] can travel via the blood to any of the muscles as well as the brain, but muscle glycogen [[Glycogen]] cannot leave the muscle fibers. Typically, there is sufficient glycogen [[Glycogen]] for about 20 miles of running at marathon pace, making the final stages of the marathon tough. Carbohydrate loading increases the glycogen [[Glycogen]] stores beyond their normal levels.

For runners who run for vast majority of the marathon distance and are pushing themselves for fast time, carbohydrate loading is critical. This is because the marathon is raced at a relatively high intensity, which burns [[Glycogen]] quickly and the standard supply will not be sufficient. I believe this to be the case for runners finishing in 2 hours or 5 hours. Taking walking breaks [[Walking Breaks]] can reduce the intensity sufficiently that [[Glycogen]] may not be a limiting factor. Likewise, those who run the marathon at an easier pace than they are capable of can complete the distance without [[Glycogen]] being a factor.

The original work on carbohydrate loading in the 1960s used a bout of prolonged exercise to deplete the glycogen [[Glycogen]] stores. This was then either followed by three days of low carbohydrate intake and three days of high carbohydrate intake, or just the final three days of high carbohydrate intake. This depletion of the glycogen [[Glycogen]] stores has been shown not to be necessary, but it does seem to improve the level of glycogen [[Glycogen]] storage. However, the 3 days of low carbohydrate intake after the glycogen [[Glycogen]] depletion exercise tend to be unpleasant both physically and mentally, with depression, irritability and reduced mental capacity. This process also tends to interfere with [[Practical Tapering|Tapering]]. This approach should be given careful consideration, but I would not generally recommend it.

By following a low carbohydrate, high fat diet for a few days or weeks, it is possible to improve the body's ability to burn fat and spare glycogen[[Glycogen]]. This has obvious benefits akin to carbohydrate loading itself. The fat adaptation phase is less drastic than the glycogen [[Glycogen]] depletion approach, and with the diminished training load of [[Practical Tapering|Tapering]] can have minimal impact. Personally, I've found it easier to avoid gaining weight while tapering by following a low carbohydrate, high fat diet during the taper period. Following a single day carbohydrate loading approach as described above does not negate the fat adaptation.

A 2002 study used 150 seconds of high intensity exercise (130% [[VO2max|V̇O₂max]]) followed by an all-out 30 second sprint before carbohydrate loading. They found that doing that level of exercise followed by 10 grams of carbohydrate per Kg body weight in a day increased glycogen [[Glycogen]] storage by 82%. Sadly, this study did not have a control to see if the exercise actually helped. In fact, the results are slightly worse than taking 10g/Kg carbohydrate without the exercise! In addition, the exercise is likely to leave you sore for the race. This is not a recommended approach

Gluconeogenesis is the creation of glucose from non-carbohydrate sources. While Gluconeogenesis can turn trace amounts of fat into glucose, this is insignificant. The primary mechanism of Gluconeogenesis is the creation of glucose from protein[[Protein]]. A high protein [[Protein]] diet may produce some [[Glycogen]] replenishment, carbohydrate should be used for pre-race loading.