Is fatigue a brain-derived emotion and can we modulate it through nutrition?

Fatigue can be defined as an acute impairment of exercise performance which leads to an inability to produce maximalforce output possible due to metabolite accumulation or substrate depletion (St Clair Gibson et al., 2003). It includesboth an increase in the perceived effort necessary to exert a desired force or power output, and the eventual inability toproduce that force or power output (Davis & Bailey, 1997). Fatigue will not only occur at the peripheral level, since thereis ample evidence that also mechanisms at the central nervous system are implicated in the genesis of fatigue. Theseprocesses that lead to decrements in performance can occur at every level of the brain-muscle pathway and althoughliterature made a distinction between peripheral and central fatigue, one should be aware that both pathways are possiblyintegrated.A number of studies have attempted to postpone ‘central fatigue’ through nutritional interventions.Water intake and several amino acids, carbohydrate, caffeine, and other nutrients have been examined.It seems that not all studies have been conclusive, sometimes also because the study design was notwell developed. But it certainly indicates that there is a need for further research on this topic.There is evidence that dehydration results in a slight change in brain volume (Streitburger et al 2009),and dehydration and hyperthermia also appear to result in transient opening of the blood-brainbarrier, with possible influences on exercise performance (Meeusen 2014). It has also been shown thatstates of reduced water intake may adversely impact executive functions such as planning and visualspatialprocessing having possible implications for team sports.The Branched chain amino acids (BCAAs) leucine, isoleucine, and valine contribute directly and indirectly to a variety ofimportant biochemical functions in the brain, such as protein synthesis, energy production, and the synthesis of serotonin( 5-HT), dopamine, and noradrenaline, which are derived from the aromatic amino acids tryptophan, phenylalanine,and tyrosine. The ingestion of BCAAs causes a rapid elevation of their plasma concentrations and increases their uptakeinto the brain. It was hypothesized that by reducing the production of 5-HT in the brain, feelings of fatigue could be attenuatedand performance enhanced. However, there is limited or only circumstantial evidence to suggest that exerciseperformance can be altered by nutritional manipulation with BCAA supplements (Meeusen 2014).Tyrosine can be synthesized in the body from phenylalanine, and is found in many high protein foods such as soyproducts, chicken, turkey, fish, peanuts, almonds, avocados, milk, cheese, yogurt, and sesame seeds. A series of preclinicalanimal studies clearly indicate that tyrosine reduces many of the adverse effects of acute stress on cognitiveperformance in a wide variety of stressful environments. Although it has been difficult to demonstrate conclusively thattyrosine has beneficial effects in humans during exercise (Struder et al 1998, Chinevere 2002), partly due to ethical concerns,the majority of evidence suggests that tyrosine is useful as an acute treatment to prevent stress-related declinesin cognitive function (Meeusen 2014).The beneficial effect of carbohydrate supplementation during prolonged exercise could also relate to increased (ormaintained) substrate delivery for the brain, with a number of studies indicating that hypoglycemia affects brain function,and cognitive performance (Meeusen 2014). Glucose and lactate uptake by the brain increase out of proportion tooxygen when the brain is activated by exhaustive exercise (Dalsgaard et al 2002) influencing the ‘will’ to exercise. Brainglycogen could also play an important role during long-duration exercise. Matsui et al (2011) showed in a rodent studythat at the point of exhaustion brain glycogen was depleted, and that exercise training creates a supercompensation ofbrain glycogen (Matsui et al 2012). Those studies indicate that brain carbohydrate metabolism might also be an importantfactor influencing fatigue during endurance exercise. The role of carbohydrate and a possible direct link with thebrain was also shown by several mouth-rinse studies.Caffeine has long been recognized as an ergogenic aid. Current research supports a central nervous system effect mediatedby the antagonism of adenosine receptors as the most likely cause (Davis et al 2003). Adenosine inhibits the releaseof dopamine and, therefore, caffeine induces higher brain dopamine concentrations. Human studies using a variety ofexercise protocols have shown performance improvements after caffeine intake (Meeusen 2014).5 Is fatigue a brain-derived emotion and can we modulate it through nutrition?16Is fatigue a brain-derived emotion and can we modulate it through nutrition?We recently determined the effect of caffeine and glucose mouth rinse on cognitive performance and brain activity.Electroencephalography was applied throughout the experiment to record brain activity. Both the orbitofrontal and dorsolateralprefrontal cortex were activated during caffeine mouth rinse, potentially explaining the likely beneficial effecton reaction times. Glucose mouth rinse increased brain activity within the orbitofrontal cortex (De Pauw et al 2015).The dorsolateral prefrontal cortex is a brain area that has a large dopaminergic input explaining the ‘stimulating’ effectof caffeine, while the orbitofrontal cortex is involved in decision making. This brain region also contains the secondarytaste cortex in which the reward value of a taste is presented (Rolls 2004).Exercise and nutrition are both powerful means to influence the brain. The sports medicine profession are only at thestart of exploring and understanding what really happens in the brain during exercise, but it is clear that physical activityand nutrition have health-enhancing effects on the brain. In the near future, nutritional interventions will also focus onbrain activity during exercise (Meeusen 2014).References :1. Chinevere TD, Sawyer RD, Creer AR, et al. Effects of L-tyrosine and carbohydrate ingestion on endurance exercise performance. J Appl Physiol. 2002; 93(5): 1590–7.2. Dalsgaard MK, Ide K, Cai Y, et al. The intent to exercise influences the cerebral O2/carbohydrate uptake ratio in humans. J Physiol. 2002; 540(Pt 2): 681–9.3. Davis JM, Bailey SP. Possible mechanisms of central nervous system fatigue during exercise. Med Sci Sports Exerc. 1997; 29(1): 45–57.4. Davis JM, Zhao Z, Stock HS, et al. Central nervous system effects of caffeine and adenosine on fatigue. Am J Physiol. 2003; 284: R399–404.5. De Pauw K, Roelands B, Knaepen K, Polfliet M, Stiens J, Meeusen R. Effects of caffeine and maltodextrin mouth rinsing on P300, brain imaging, and cognitive performance. JAppl Physiol 118: 776–782, 2015.6. Matsui T, Ishikawa T, Ito H, et al. Brain glycogen supercompensation following exhaustive exercise. J Physiol. 2012; 590(Pt 3): 607–16.7. Matsui T, Soya S, Okamoto M, et al. Brain glycogen decreases during prolonged exercise. J Physiol. 2011; 589(Pt 13): 3383–93.8. Meeusen R. Exercise, Nutrition and the Brain. Sports Med 2014, 44 Suppl 1: S47-56.9. Rolls E. The functions of the orbitofrontal cortex. Brain and Cognition 2004; 55(1): 11–2910. St Clair-Gibson A, Baden DA, Lambert MI, et al. The conscious perception of the sensation of fatigue. Sports Med. 2003; 33(3): 167–76.11. 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