Summary of supporting evidence

Physical Performance

Several studies have investigated the effects of TYR in relation to exercise performance in normal^9-11 and elevated/ high ambient temperatures.^12-15 All three of the studies conducted under temperate conditions failed to observe any beneficial effect of acute TYR ingestion on endurance performance^{9, 10} or strength and power performance.¹¹ These findings are not surprising due to the questionable amount of stress experienced during exercise in normal ambient temperatures and the relationship between stress and catecholamine turnover.

Recent studies have therefore focused on passive and active heat-stress based designs to examine the influence of TYR under extreme stress. Data has shown significant improvement in exercise capacity (15 ± 11%) after ingestion of 150 mg/ kg body mass of TYR, compared to placebo when cycling to exhaustion in a hot environment (30°C; 50% RH).¹² To date, this¹² is the first and only study to observe a positive effect of TYR on physical performance, despite the efforts of others¹³ who attempted to replicate this study. Indeed, others¹³ reported TYR did not influence exercise capacity or any aspects of cognitive function (reaction time, information processing or memory) in the heat, despite a significant increase in plasma TYR concentration. Indeed, others¹⁴ employed a pre-loaded time-trial design based on the theory that a benefit of TYR would be more apparent during self-paced exercise due to the greater influence of behavioural thermoregulation, motivation and arousal compared to constant load exercise.^{2, 14} However, this was not the case, as TYR ingestion (150 mg/ kg body mass) did not influence time-trial performance when performed in a hot environment (30°C; 50% RH).¹⁴ Others¹⁵ examined the TYR ingestion (300 mg/ kg body mass) during exposure to a 90 min soccer-simulation protocol [iSPT16] in a warm environment (25°C; 40% RH); TYR had a positive effect on cognitive function (vigilance) and readiness to invest mental effort, but did not influence physical performance.

Cognitive Performance (heat/cold/hypoxia):

The majority of literature assessing the effects of TYR is military based, with several investigations conducted by the US Army Research Institute^{8, 17-20} and other army institutes.^{4, 21} These have primarily focused on aspects of cognitive function (complex; working memory, vigilance, tracking and simple reaction time; etc.) and mood during exposure to acute stress, such as such as cold^{8, 17, 20} and hypoxia¹⁷, and paradigms involving both extended wakefulness²² and the physical/emotional stress nexus.²¹ Each of these aforementioned studies has demonstrated improvements in specific aspects of cognitive function after ingestion of TYR (100-300 mg/ kg body mass; N.B. when 300 mg/ kg body mass of TYR is administered, it is typically via two equal dosages 4 hours apart).

TYR supplementation has direct mechanistic evidence that it can offset heat-induced delays in reaction time during 90 min passive exposure to 45°C; 30% RH.⁵ This study also assessed higher levels of cognitive function using advanced brain imaging techniques (event related potentials; ERP), providing evidence that heat exposure causes an increase in P300 (reduced concentration) and M100 latency (reduced ability to react to a warning) and a decrease in M100 amplitude (linked with attention) which returned to near normal levels after ingestion of TYR. It was concluded that the higher DA and norepinephrine (NE) concentrations detected in the TYR trial might have maintained cognitive function by alleviating the decrements associated with heat-stress.⁵ This⁵ is the only TYRheat- stress based study to assess DA and NE concentrations in combination with cognitive testing and advanced brain imaging, currently the ‘best’ quality evidence regarding the efficacy of TYR during heat-stress to mediate undesirable heat-mediated cognitive function declines.