When energy is required, the body breaks up a substance called ATP (adenosine triphosphate), a high energy molecule consisting of three phosphates attached by energy bonds to adenosine. Energy is released by breaking off a phosphate from ATP to form ADP (adenosine diphosphate). This is a continuous cycle whereby ADP is converted back into ATP. So how is ATP made?
There are three systems in the body that create ATP energy. These systems work simultaneously but the contribution from each depends on the type of exercise, its intensity and duration.
1. ATP-CP running energy
The sprint system provides enough energy for a five to six second running sprint and doesn’t require oxygen (anaerobic). CP (creatine phosphate) is another high energy molecule where the phosphate can be broken off very quickly, so releasing energy and used to convert ADP back to ATP. The muscles don’t have a large store of CP so it’s used up fast, hence why some athletes use creatine supplements to maximise their muscle stores.
2. Anaerobic running energy
The high power system provides energy for a 90 second running power burst. This system is the fast anaerobic (without oxygen) breakdown of glucose for energy but only provides two molecules of ATP along with a waste product called lactic acid which can cause muscle fatigue.
3. Aerobic running energy
The endurance system is about how long you can keep going and depends on how fit you are. This system is the slow aerobic and uses oxygen. The breakdown of glucose for energy provides a massive 38 molecules of ATP — nearly 20 times more than the anaerobic system. The aerobic system can also use fat to produce ATP energy. Endurance training can make the muscles use fat more efficiently.
What are the food sources of energy?
The three main energy sources for exercise comes from carbohydrate, fat and protein. Each of these contains nutrients in differing amounts which in turn provide different quantities of energy which is measured as kilocalories (kcal) per gram (g).
Here are the energy values of each:
- Carbohydrate 3.75kcal/g
- Protein 4kcal/g
- Fat 9kcal/g
This means that 1g of fat produces more than twice as much energy as 1g of carbohydrate or protein. It is wrong to think though that this means you should just fuel you running by eating fat. The preferred energy fuel for the muscles is glucose.
Glucose is formed from the breakdown of carbohydrates in your diet and is stored as glycogen in the muscles and liver. However, there is a limit in the amount of glycogen the body can store, which is why marathon runners have to take on fuel during a race. A person weighing 70kg (154.3lb) will store approximately 450g or 1700kcal of glycogen.
Which running fuel?
The amount of each fuel — either carbohydrate, fat and protein — you use during running depends on various factors:
- Dietary intake
- Fitness level of your running
- Type of running exercise
- Training running intensity
- Length of run workouts
- Frequency of run training sessions
Anaerobic activities only use glucose, whereas aerobic activities use all three fuels, but protein is used to a lesser extent than glucose and fat.
During low-intensity exercise, which uses less than 300kcal each hour, you use a greater proportion of fat, a smaller proportion of glucose and fewer calories. As the exercise intensity increases, the body will gradually use less fat, more glucose and more calories. Therefore, most of the fuel during moderate and high intensity exercise (using more than 500kcal each hour) comes from glucose.
If you continue to exercise aerobically for a longer period, your body will gradually use more fat and less glucose in an attempt to conserve the limited glucose stores. The fitter you are, the more efficiently your muscles use fat and the longer you can work out. From all this, we can see that carbohydrate is the most important nutrient for exercise, because it’s the only energy source that can fuel intense exercise for prolonged periods. With the body's capability of storing glycogen being limited, it becomes apparent that if you don't restock glycogen stores sufficiently, you will run out of fuel while running.