Central Fatigue Theories

The conversion rate of adenosine diphosphate (ADP) to ATP is a critical step in the supply of energy to working muscles. Inadequate carbohydrate availability lowers the rate of ADP to ATP conversion, making it impossible for muscles to continue exercise at a high-intensity level. In addition, the failure to convert ADP to ATP causes a buildup of ADP that also contributes to muscle fatigue.

A number of other factors involving the central nervous system are also involved in muscle fatigue. As a group, these factors are referred to as Central Fatigue Theory. All of these theories involve mechanisms that cause more than the usual amount of the amino acid tryptophan to pass the blood-brain barrier, which stimulates an increase in the amount of serotonin (5-HT) that is produced. 5-HT is a neurotransmitter that causes people to feel relaxed and, if enough is produced, to feel sleepy. For the athlete, this could translate into muscular fatigue. Put simply, mental fatigue leads to muscle fatigue.

Theory 1 Low blood sugar and low muscle glycogen stores stimulate the muscle break down for gluconeogenesis. This results in an increased catabolism of branched-chain amino acids (BCAAs), which causes a reduction in circulating blood BCAAs. BCAAs and tryptophan compete for the same receptor carriers that enable their passage through the blood-brain barrier. When BCAAs are high, the tryptophan passage to the brain is controlled. However, when the BCAA blood level is low (as happens when they are catabolized for energy), tryptophan can sequester more of the receptor carrier, and more tryptophan enters the brain. Tryptophan stimulates the formation of 5-HT. To prevent this from happening, blood and muscle glucose levels must be maintained to avoid gluconeogenesis.

Theory 2 Consumption of foods high in tryptophan (such as turkey) can increase the volume of tryptophan passing the blood-brain barrier, causing an increase in 5HT production. The increase in 5-HT leads to premature fatigue.

Theory 3 Fats compete for the same protein carrier in the blood as tryptophan. High fat intakes preferentially compete for this protein carrier, leaving a higher proportion of free tryptophan that can cross the blood-brain barrier. This causes an increase in 5-HT production, which may lead to premature fatigue.

While it may be logical that the intake of both BCAAs and carbohydrate reduces 5-HT and, therefore, inhibits both mental and physical fatigue, studies have not been conclusive because of the difficulty in distinguishing between brain and muscle effects. In addition, there may be interference from compounds such as caffeine, the ingestion of which has been shown to delay fatigue by temporarily stimulating the central nervous system.