This is an excerpt from Plant-Based Sports Nutrition by D. Enette Larson-Meyer & Matt Ruscigno.
Although carbohydrate, fat, and, to a lesser extent, protein are used to fuel physical effort, carbohydrate is the only fuel that can sustain the moderate- to high-level effort that is required in most sports and athletic endeavors. Carbohydrate is also the preferred fuel for the brain and central nervous system and the only fuel these systems can use without weeks of adaptation that allows the brain to use products of fat metabolism, called ketones or ketone bodies.
As most athletes know, carbohydrate can be stored in skeletal muscle and liver in a starchlike form called glycogen. The body's glycogen stores, however, are limited. Glycogen can become depleted during continuous steady-state exercise lasting at least 60 minutes and during intense intermittent activities that include stop-and-go running, intense court play, and brisk hiking on difficult terrain. In fact, glycogen levels are likely depleted at the end of an intense soccer, basketball, or hockey game in team members who play the majority of the game.
Research has shown time and time again that muscle and whole-body fatigue develop at about the same time that glycogen stores become low. The reasons are relatively simple. First, active muscles that have been exhausted of their carbohydrate stores are forced to rely primarily on fat for fuel. Fat cannot be “burned” as rapidly or efficiently as carbohydrate, so you are forced to slow your pace and eventually stop exercising. What this means for you is that you produce less adenosine triphosphate (ATP) energy for a given amount of oxygen consumed when fat instead of carbohydrate is used as fuel. Second, the liver—exhausted of its carbohydrate stores—is now unable to serve as a storage reservoir for blood sugar and must struggle to maintain blood sugar level by converting protein (amino acid) sources to blood sugar. This process, termed gluconeogenesis, which means new-sugar formation, is slow and typically cannot keep pace with the rate at which the exercising muscle takes up sugar. The result is often low blood sugar, which is characteristically accompanied by lightheadedness, lethargy, and overall fatigue.
Although the body's enzymes—machinery for making blood sugar from amino acids—are typically regulated through training, athletes at any level can experience low blood sugar. Most likely you have experienced this feeling yourself—at least once—which in some athletic circles is called bonking or hitting the wall.