Dietary tyrosine benefits cognitive and psychomotor performance during body cooling

Abstract

Supplemental tyrosine is effective at limiting cold-induced decreases in working memory, presumably by augmenting brain catecholamine levels, since tyrosine is a precursor for catecholamine synthesis. The effectiveness of tyrosine for preventing cold-induced decreases in physical performance has not been examined. This study evaluated the effect of tyrosine supplementation on cognitive, psychomotor, and physical performance following a cold water immersion protocol that lowered body core temperature. Fifteen subjects completed a control trial (CON) in warm (35 °C) water and two cold water trials, each spaced a week apart. Subjects ingested an energy bar during each trial; on one cold trial (TYR) the bar contained tyrosine (300 mg/kg body weight), and on the other cold trial (PLB) and on CON the bar contained no tyrosine. Following each water immersion, subjects completed a battery of performance tasks in a cold air (10 °C) chamber. Core temperature was lower (p = 0.0001) on PLB and TYR (both 35.5 ± 0.6 °C) than CON (37.1 ± 0.3 °C). On PLB, performance on a Match-to-Sample task decreased 18% (p = 0.02) and marksmanship performance decreased 14% (p = 0.002), compared to CON, but there was no difference between TYR and CON. Step test performance decreased by 11% (p = 0.0001) on both cold trials, compared to CON. These data support previous findings that dietary tyrosine supplementation is effective for mitigating cold-induced cognitive performance such as working memory, even with reduced core temperature, and extends those findings to include the psychomotor task of marksmanship.

Introduction

Stressful conditions that elevate brain catecholamine activity, such as cold or heat stress, altitude exposure, and tail shock in animals, are often associated with decreased cognitive performance [1], [2], [3], [4]. One reason may be depletion of central catecholamines, since both norepinephrine [5] and dopamine [6], [7] are important for acquiring and performing cognitive and motor skills. Catecholamines serve as neurotransmitters, so augmenting the availability of the amino acid tyrosine, the precursor for catecholamine synthesis, through dietary supplementation could help maintain brain function by sustaining brain neurotransmitter levels [8]. Tyrosine and other neutral amino acids are competitive binders to the transport receptor for crossing the blood brain barrier, therefore, when there is a higher ratio of tyrosine relative to total neutral amino acids the rate of tyrosine transport will increase [9]. Under stressful conditions that activate tyrosine hydroxylase, the rate-limiting enzyme that catalyzes the conversion of tyrosine to l-dopa, the enzyme becomes more responsive to increased intraneural tyrosine, whether through ingestion or injection of tyrosine [9], [10]. Animal studies have demonstrated that supplemental tyrosine increases brain levels of norepinephrine [2], [4], [11] and dopamine [10].

Soldiers are required to perform missions even at environmental extremes; therefore, identification of effective countermeasures to mitigate performance degradation due to environmental stress is important. One example is the 1995 hypothermia deaths of four Ranger students, where poor decision-making may have contributed to prolonged cold water immersion [12]. Both animal and human studies provide evidence that supplemental tyrosine is effective at limiting cold-induced decreases in cognitive performance [5], [11], [13], [14]. In humans, Banderet and Lieberman [13] found supplemental tyrosine was effective at decreasing symptoms such as headaches, cold sensation, and fatigue during a combined stress of cold and hypoxia. Performance on cognitive tests, including addition, coding, map/compass, pattern recognition, and reaction time was also better with tyrosine, compared to placebo. Shurtleff and Thomas [14] found that tyrosine offset the 30% reduction in performance on a Match-to-Sample test that involves short-term working memory after 60 min cold (4 °C) air exposure. Tyrosine had no effect during a 22 °C control condition, suggesting that tyrosine availability only became important during the cold condition when there was increased firing of catecholaminergic neurons. While these studies provide evidence for the effectiveness of tyrosine supplementation in a cold environment, none of the human studies used a cold stress that was sufficient to reduce core temperature. Furthermore, no studies have evaluated the effect of tyrosine supplementation on physical performance during cold stress.

The purpose of the present study was to evaluate the effect of tyrosine supplementation on cognitive, psychomotor, and physical performance during cold air exposure following a stressful repeated cold water immersion protocol that induces mild hypothermia [15]. Based on previous studies, it was hypothesized that cold-induced cognitive performance decrements would be mitigated with tyrosine. Marksmanship performance has been previously demonstrated to be sensitive to environmental stressors [16], [17], therefore, it was hypothesized that decrements in performance on this psychomotor task would also be mitigated with tyrosine. A secondary aim of this study was to identify physical performance tasks that are sensitive to body core cooling, and whether tyrosine reverses any observed physical performance decrements.