“Up to 70% of total energy used in a marathon will come from carbohydrates compared to 30% from fats.”
This is of huge importance when training for a marathon and needs careful consideration. Many people believe that fat is the most efficient energy source. It is true that the body prefers to burn fat since it releases 9 calories per gram as opposed to 4 for carbohydrates, but unlike carbohydrates it is not an efficient fuel for vigorous exercise.
During exercise carbohydrates are broken down into simple sugars like glucose. These sugars are used by the body to:
1. Circulate in the blood to keep us exercising and to feed the brain and prevent mental fatigue.
2. To be stored as glycogen in the liver where it can be quickly reconverted into glucose to meet the energy needs of the body.
3. Converted into glycogen and stored in our working muscles.
Point 3 is the key to why carbohydrates are so much more important to the marathon runner than fats. Muscle glycogen stored in muscle fiber is directly available only to that fiber. Fats on the other hand are stored in large complex molecules all over the body, and therefore, it takes much longer for fat to break down and make its way to our working muscles to supply energy.
Oxygen consumption is also of importance. At low levels of exercise intensity oxygen is readily available to burn fat for fuel. At higher levels, however, such as those used in a marathon, oxygen is not as readily available. It takes approximately three times as much oxygen to burn a gram of fat as it does a gram of carbohydrate. In addition, as mentioned before, carbohydrates can be delivered much more quickly than fats.
The body does need other food sources as well and it is considered that a healthy person should be consuming 50% carbohydrates, 20 to 30% fats and 10 to 15% protein.
