Tyrosine and its effect on cognitive function and load-carriage performance in the heat

A thesis submitted to the University of Bedfordshire in partial fulfilment of the requirements for the degree of Masters of Science by ResearchProlonged exercise-heat-stress impairs both exercise performance and cognitive function. Military based operations are often performed in hot environmental conditions and thus performance and safety may be compromised which could be potentially life threatening. Acute ingestion of tyrosine (TYR), a catecholamine precursor, has been shown to improve aspects of cognitive function and mood during exposure to stressful environments, in military and sport specific settings. Currently, there is limited research exploring the optimal dose of TYR relative to blood values, to prescribe pre-exercise or before exposure to a stressor. Therefore, the purpose of experimental chapter 1 was to investigate the effects of acute TYR ingestion strategies (0, 150 and 300 mg.kg body mass-1 TYR administered in 2 equal doses, 4 h apart) on serum TYR concentrations at rest. Twenty-one healthy males were randomly allocated to one of three groups (n = 7 per group); HIGH (300 mg.kg body mass-1 TYR in total), LOW (150 mg.kg body mass-1 TYR in total) and CON (sugar free squash placebo). Ingestion of TYR was double blinded and was administered in a drink form (dissolved in 250 mL sugar free squash) in two separate doses at both 0900 and 1300. Participants consumed a standardized breakfast (0800) and lunch (1200) prior to consumption of TYR and remained in the laboratory from 0900-1700 having blood drawn every hour from a cannula. Measures of gastric discomfort were also recorded. Significant differences in serum TYR concentrations were observed between groups (p 0.05). No significant differences in gastric discomfort were observed between groups (p > 0.05). This study demonstrates that ingestion of a single dose of 150 mg.kg body mass-1 TYR may be sufficient to elevate serum TYR concentrations and that the peak in TYR concentration typically occurs 2 h post ingestion (without the need for a second identical dose 4 h later). Experimental chapter 2 was designed to investigate the effects of TYR ingestion on steady state exercise, cognitive function and time-trial performance in the heat, utilising the identified dose from the findings of experimental chapter 1. Eight recreationally active, healthy males visited the laboratory on four occasions (two familiarisation and two experimental conditions). In a double-blind, counter-balanced, crossover design participants ingested a placebo [PLA (250 mL sugar free squash)] or TYR (same as PLA plus 150 mg.kg body mass-1 TYR powder) 1 h pre-exercise. Participants completed a 60 min walk at 6.5 km.h, followed by a 2.4 km time-trial carrying a 25 kg backpack in 40°C; 30% RH. Cognitive function (vigilance, dual-task and simple reaction time) was assessed at 5 time-points; pre-ingestion, pre-exercise, 30 min into exercise, post 60 min exercise and post time-trial. Traditional physiological (HR), perceptual (RPE, TSS) and temperature (rectal and skin temperature) measures were recorded throughout exercise. A significant increase from pre-post exercise (p 0.05). Furthermore, no significant difference was observed in time-trial completion time (F1,14 = 547.9 , p = 0.74) between TYR (19.78 ± 3.44 min) and PLA (20.29 ± 3.55 min). No significant differences were observed in any of the physiological, perceptual or temperature measures between conditions (p > 0.05). The main findings presented within this thesis indicate that although it was identified that a single dose of 150 mg.kg body mass-1 was sufficient to significantly elevate serum TYR concentrations, ingestion of this dose did not influence cognitive function or time-trial performance in the heat. This is surprising since ingestion of similar doses have significantly improved aspects of cognitive function previously during exercise-heat-stress. In conclusion it appears that under the conditions of the present study, TYR is not a useful ergogenic aid. Future research should aim to elucidate the central effects of TYR to enable a better understanding of its mechanistic properties. Key words: Heat-stress; central fatigue; tyrosine; cognitive functio