Fuel Substrates

There are many different factors that influence fuel substrate selection during exercise. During exercise, fats and carbohydrates are the primary fuel with protein supplying a small amount of the total energy.Carbohydrates contribute four calories per gram. They provide a rapidly available form of energy. Carbohydrates form glycogen which is stored in the muscles and liver. Total glycogen stores are small and can be used in only a few hours as a result of prolonged exercise. During high intensity exercise carbohydrates are the main fuel source. Fats contribute nine calories per gram. Fats are classified in four general groups: fatty acids, triglycerides, phospholipids, and steriods. Fatty acids are the primary type of fat used by muscle cells for energy. Fats are the main source during low intensity exercise.Proteins use four calories per gram and are composed of many tiny subunits ca...