The body selects its fuel source based on how hard it is working and on what is available. Under most conditions, the body's preference is to use a mixture of carbohydrate and fat. In fact, with a normal mixed diet and at rest, the body typically uses about half carbohydrate and half fat as its fuel source. For all practical purposes, protein is little used when the other two food sources are available. The harder you exercise the more the body prefers carbohydrate, and when racing a relatively short race, like a 5,000 for example, chances are about 100 percent of the fuel being burned is carbohydrate.
Carbos are the more efficient fuel. The reason the body prefers carbohydrate when it is working hard is because a given amount of oxygen consumption will result in more energy being produced from carbohydrate than from fat. Clearly, the body has a far greater amount of fat available, and a gram of fat provides almost double the energy that a gram of carbohydrate produces, but the problem is that each runner can only consume a given amount of oxygen, which becomes the limiting factor in how many calories can be burned. For the oxygen available, carbos give us more energy, so hard work relies on carbohydrate metabolism.
Carbohydrate supply is rather limited. If the muscles and liver are fully stored with glycogen (the form of carbohydrate in which it is stored in the body), there is enough carbohydrate available to get a runner about 15 or 20 miles using carbos exclusively as fuel. When the carbs run out the runner doesn't feel good at all; in fact "hitting the wall" is the expression used to describe the feeling that is experienced when the body is forced to function on fat as the main source of fuel.
"R" tells the story. When carbohydrate is used as fuel, the ratio of carbon dioxide produced to oxygen consumed is 1.00 (each molecule of oxygen consumed is associated with a molecule of carbon dioxide production). However, when fat is the only fuel source, then the ratio of CO2 produced to O2 consumed is .700. This ratio of CO2 to O2 is referred to as "respiratory exchange ratio," or R. It is not difficult to measure the R value at rest or the R associated with any intensity of exercise. It just requires analyzing a sample of expired air under whatever conditions are of interest.
R at rest and during exercise. Since an R of .700 indicates 100 percent fat metabolism and an R of 1.00 is associated with 100 percent carbohydrate metabolism, R values between .70 and 1.00 can be used to determine what percentage of each type of fuel is being used at various exercise intensities. For example, at rest R is typically about .85, indicating about half carbo and half fat is the fuel source. Interestingly, when you start to run at a comfortable pace (even though a comfortable run is more demanding than is rest), R doesn't usually rise much at all, if any. Since you are not working that hard, when running "comfortably," the body accepts fat as a major fuel source, mainly in an effort to conserve carbohydrate for as long as possible (or for when needed in the event of a bout of harder work).
Avoiding the "wall." Remember that you have about 15-20 miles of energy available when running on carbohydrate exclusively. Well, the typical intensity of effort associated with a marathon run is easy enough that the body keeps the R down under .90, which means the carbohydrate supply will last almost twice as long, since it is not much more than half the fuel. Go harder than what is a comfortable pace and the R goes up and the carbo reserves are depleted more quickly. Of course, if you encounter a hill or a period of time going into the wind, then the work gets a little harder and carbos take on an increased share of the fuel supply and are more quickly depleted. What it boils down to is that it is easy to run faster than you should be going, early in a marathon (when you have plenty of stored glycogen), because the readily available carbs make the too-fast pace feel OK. Unfortunately, the wall is looming ahead and will be difficult to avoid if the intensity has been too great for too long.
Benefit of the long run. In addition to developing the ability to spend time on your feet and to learn to mentally press on for extended periods of time, long runs also teach your body to become better at metabolizing fat as fuel. The more you perform steady, prolonged runs, the more your body realizes it needs to conserve its carbohydrate supply for as long as possible so it becomes a better fat burner. On the other hand, it is not particularly desirable to try to voluntarily deplete your body of its carbohydrate stores (to better teach it to metabolize fat) because running on fat is not enjoyable and can lead to poor mechanics and greater chance of injury.
The good news. The good news is that running at the proper (comfortable) pace during a 26.2-mile run will not delete available glycogen stores, unless you go into this 26 miler already partially depleted. To avoid this situation, eat a good mixed diet, concentrating on complex carbohydrates during the four or five days leading up to the marathon. Actually, the diet doesn't have to be overloaded with carbos - just a good mixed diet will do - but it must be on a regular basis and especially during those final days leading up to the marathon. The fact that you are usually cutting back on your training mileage during those final days allows you to have more than usual amounts of carbos available for storage, since the training demands are less (even with no increase in food consumption). Three or four days of low mileage and a good mixed diet will bring you into the marathon in great shape for not having to experience the wall.
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