When exercising both fat and carbohydrates are used to provide the body with energy to make it through the workout. The intensity and duration of the workout determines which substrate is more heavily relied upon. Regardless of whether fat is the predominant energy source, carbohydrate availability is the limiting factor.
Where does my body get carbohydrates to use as energy?
There are 2 types of carbohydrate sources the body can rely on for energy during exercise. Endogenous carbohydrate stores, known as glycogen, are found in the muscle and liver. Exogenous carbohydrate, is sugar the body can use from the food that is eaten just before exercise. Sugar that is not used by the body immediately upon consumption may be stored as glycogen for future energy use.
Is glycogen a limiting factor during my long runs?
The American College of Sports Medicine says, “The depletion of muscle glycogen is associated with fatigue and a reduction in the intensity of sustained exercise, while inadequate carbohydrate for the central nervous system impairs performance-influencing factors such as pacing, perceptions of fatigue, motor skill, and concentration.” Yes, fat can supply hours of energy for the body, however carbohydrates are what support our brain and the ability for us to mentally continue. The “hitting the wall” phenomenon that many people describe during the middle of a long fun or workout is the bodies way of saying it is running low on sugar to support the brain.