Carbohydrates, fats and protein are our fuel.
Before they can be used to fuel activity, their energy must be converted into the high energy compound adenosine triphosphate or ATP. ATP is the immediate source of energy for all the body’s functions.
In a resting muscle, there is enough stored ATP to sustain activity for a few seconds. As the ATP in the muscle is used, enzymes break down another high energy compound called creatine phosphate (CP) to replenish the ATP supply.
During the first 10-15 seconds of exercise, the muscles use the energy from the ATP and CP that is stored there, but activity of longer duration requires the body to replenish ATP from the metabolism of energy-yielding nutrients.
Carbohydrates, fats and protein are all used to replenish ATP when oxygen is available.
When no oxygen is available, only carbohydrates can be used; although carbohydrates produce ATP rapidly, it is not used efficiently. The availability of oxygen in muscle cells is determined by how quickly the heart can pump oxygen via the blood to the working muscles.
Producing energy from carbohydrates
The carbohydrate fuel used for exercise is glucose. Glucose is found in the bloodstream and is stored as glycogen in the muscles and the liver. The transformation of glucose into energy occurs in one of two ways: with oxygen (aerobically) or without oxygen (anaerobically). Glucose burned with oxygen produces carbon dioxide (a waste product), water, ATP and heat. Glucose burned without oxygen produces lactic acid, ATP and heat.
Glucose is converted to energy in muscle cells which have mitochondria (tiny bodies found inside most cells) so they can process glucose with oxygen. Being able to turn glucose into energy without oxygen (i.e. when exercising strenuously) is one of the advantages of using carbohydrates as a fuel. The disadvantage is that the lactic acid by-product can lead to muscle soreness.
Producing energy from fats
Fats can only be used as an energy source when oxygen is present (when working aerobically). They provide a lightweight, energy-dense fuel supply. The process of burning fat for energy is called lypolysis. Fatty acids are released into the blood and then taken up by the muscle cells. Inside the muscle cells, fatty acids are transported to the mitochondria to produce ATP.
Producing energy from proteins
Proteins are broken down into amino acids, which can also be used to produce ATP when oxygen is available. For most exercise, protein contributes only a small percentage of the energy is used. It becomes an important source of energy only when exercise continues for many hours.
Which fuels are used?..and when?
As a general rule, the body burns either fat or carbohydrates depending on the intensity of the activity.
However, the body does not completely stop using one fuel source to use another. The intensity and duration of exercise and the conditioning of the exerciser can affect the contribution that carbohydrate and fat make as fuels for energy production.
Fat utilisation, in turn, affects how long exercise can continue before fatigue sets in. In general, the more intense the exercise, the more the muscles rely on glucose to provide energy.