Exercise poses a major challenge to the various bioenergetic pathways in the working muscle. For example, during heavy exercise the body's total energy expenditure may increase 25 times above expenditure at rest. The body uses carbohydrate, fat and protein to provide necessary energy to sustain cellular activities both at rest and during exercise. The regulation of fuel selection during exercise is under complex control and depends on several factors, including diet as well as intensity and duration of exercise.

Carbohydrates

Carbohydrates or glucose, is a critical fuel source during any form of exercise, with one gram yielding approximately 4 kcal of energy. Carbohydrates are categorized as simple or complex. Fruit is a source of simple carbohydrate. Complex carbohydrates take a bit longer to be digested and provide energy at a slower rate than simple sugars. Examples of complex carbohydrates include breads, rice and pasta.

Once consumed, carbohydrates are broken down into smaller sugars for immediate energy or to be stored as glycogen in muscle and liver for future use. Muscle glycogen stores provide a direct source of carbohydrate for muscle energy metabolism, whereas liver glycogen stores serve as a means of replacing blood glucose.

Adequate carbohydrate intake is crucial for endurance athletes because glycogen stores can be depleted in a few hours. Without sufficient amounts of carbohydrate, the body has to turn to other sources of fuel.

Fats

Fat is the most energy dense nutrient utilized during exercise, with one gram providing 9 kcal of energy. Moreover, fats are the primary fuel source for muscle during low intensity exercise. Although most fat is stored in the form of triglycerides in fat cells, some is stored in muscle cells. Fatty acids are the primary type of fat used by muscle cells for energy. In terms of fat, plasma-free fatty acids and muscle triglycerides contribute equally to exercise metabolism. However, as the duration of exercise increases, there is a progressive rise in the role of plasma-free fatty acid as a fuel source.

Protein

While protein is a crucial party of every diet and for rebuilding muscle, protein is not a significant source of energy during exercise. Proteins are composed of many tiny subunits called amino acids. To be utilized as a fuel source, proteins must first be degraded into amino acids. The amino acid alanine can then be converted in the liver to glucose, which can then be used to synthesize glycogen. Additionally, many amino acids can be converted into metabolic intermediates in muscle cells and directly contribute as an energy source in the bioenergetic pathways. During prolonged periods of exercise, or after approximately three to five hours of work, the total contribution of protein to the fuel supply may reach five to 10 percent during the final minutes of work.