Central and peripheral factors in fatigue.

The causes of fatigue during muscular exercise include factors that reside in the brain (central mechanisms) as well as the muscles themselves (peripheral mechanisms). Central fatigue is largely unexplored, but there is increasing evidence that increased brain serotonin (5-HT) can lead to central (mental) fatigue, thereby causing a deterioration in sport and exercise performance. Although there are also strong theoretical grounds for a beneficial role of nutrition in delaying central fatigue, the data are much more tenuous. Dietary supplementation with branched-chain amino acids (BCAA) in low doses produces small and probably inconsequential effects on peripheral markers of brain 5-HT synthesis (plasma free tryptophan/BCAA), whereas larger doses are likely to be unpalatable, reduce the absorption of water in the gut, and may increase potentially toxic ammonia concentrations in the plasma. Alternatively, carbohydrate supplementation results in large reductions in plasma free tryptophan/BCAA and exercise time to fatigue is significantly longer, but it is difficult to distinguish between the effects of carbohydrate feedings on central fatigue mechanisms and the well-established beneficial effects of carbohydrate supplements on the contracting muscle. These data support the exciting possibility that relationships exist among nutrition, brain neurochemistry and sport performance. However, while the evidence is intriguing and makes good intuitive sense, our knowledge in this area is rudimentary at best.