Biochemistry of Exercise, Description

 

The human body responds rapidly to the increased energy demands of exercise, in particular, the utilization of carbohydrates (sugars) and lipids (fats) as substrates that are oxidized to provide energy to muscle cells. Energy turnover in muscle increases more than 100-fold during exercise.

Oxidation of glucose and fatty acids for muscle energy increases in proportion to the intensity of the exercise. The metabolic pathways that provide the energy for muscle contraction are controlled by complex interrelationships among hormones and other signaling molecules.

 During moderate-to-hard endurance exercise, carbohydrates supply about 50% of energy requirements, primarily from glycogen stored in muscle tissue. Glycogen is the storage form of glucose, the primary carbohydrate utilized by the human body. Glycogen is broken down to glucose. Glycolysis is the breakdown of glycogen and glucose to supply cellular energy. As muscle glycogen stores become depleted, the liver releases more glucose into the bloodstream to fuel glycolysis in muscle.

The remaining energy is derived from the oxidation of fatty acids, including those already in the bloodstream as well as those mobilized from fat stores. As the intensity of the exercise decreases, for example, to a pace where a conversation could be held, less carbohydrate is used, and fat becomes the principal energy source.

When carbohydrates are limited, certain amino acids from muscle protein can be converted to glucose to be consumed as fuel. Although their energy contribution during short-term intense exercise is negligible, during prolonged exercise, amino acids provide 3–6% of the body's energy requirements. When carbohydrate availability is limited, amino acids may provide as much as 10% of the body's fuel.