There are three food sources that are delivered to the muscle cell energy systems:
The various digestible carbohydrates are converted into glucose and used in glycolysis. During recovery, glucose is stored as glycogen in muscles. During exercise glycogen is readily broken down back into glucose for use in glycolysis. Glycogen is also stored in the liver and during exercise it is broken down so that glucose can be delivered to the working muscle cells. This, in addition to any glucose digested during exercise, is delivered to the glycolysis energy system.
Once glucose is converted into pyruvate by glycolysis it can be further processed in the mitochondria. Therefore, carbohydrate can be used both directly in glycolysis and indirectly in oxidative phosphorylation to produce ATP.
As glucose is used in glycolysis it is associated with high power levels (large amount of ATP generated per second). However the storage of glucose is limited and can run out during a session. This means glucose ingestion can be very beneficially during exercise.
Fats result in the most energy of the food sources by mass. They are used in the mitochondria to generate ATP. Although fat utilisation is the most efficient in terms of energy/mass, the delivery of lipids to the cells takes time to ramp up in response to exercise and the limits of mitochondria respiration mean lipid based production of ATP is a low power process. However, fat reserves are very high (several days) compared to carbohydrate stores so fat based ATP production has a very large capacity. If the exercise is of a low enough intensity then energy production does not become a limiting factor as lipids can supply all that is needed. Furthermore, as the brain exclusively uses glucose for energy production, fat utilisation is favoured in muscles when the intensity of exercise is low enough to allow it.
Some lipids are stored in muscle cells in the form of triglyceride.
Protein (amino acids)
During digestion protein is broken down into amino acids. As well as being the building blocks for the proteins in our bodies, amino acids can also be used by the mitochondria. Their contribution is much lower than carbohydrate and fat sources but can become significant. As they are processed by the mitochondria they are a low power source.
Some amino acids and lipids can be converted into glucose (gluconeogenesis). Some lipids can be formed from amino acid and carbohydrates. The degree and flow of conversion is determined by the presence of surpluses and the relative amounts of the various components.