Tyrosine supplementation mitigates working memory decrements during cold exposure

Abstract

In rats, dietary supplementation with the amino acid tyrosine (TYR) prevents depletion of central catecholamines observed during acute environmental stress. Concomitant changes in the animals' behavioral responses to stress suggest that TYR might have similar effects on central catecholamines and cognition in humans exposed to environmental stress. This study aimed to determine if severe cold exposure impairs human cognition and if dietary supplementation with TYR would ameliorate such deficits. Volunteers (N = 19) completed three test sessions on different days (35 °C control/placebo, ∼ 10 °C/placebo, ∼ 10 °C/TYR) using a double-blind, within subjects design. During each session, volunteers completed two 90-minute water immersions and consumed a food bar (150 mg/kg TYR or placebo) before each immersion (total TYR 300 mg/kg). Cognitive performance, mood, and salivary cortisol were assessed. Cortisol was elevated in the cold (p < .01). Volunteers made fewer correct responses on a Match-to-Sample memory measure (p < .05) and reaction time (RT) and errors increased on a choice RT test (p < .01) in the cold. Self-reported tension (p < .01), depression (p < .05) and confusion (p < .01) also increased in the cold. When volunteers consumed TYR, correct responses increased on a Match-to-Sample memory measure (p < .05) and study time for the sample was shorter (p < .05), indicative of more rapid and accurate information processing. Finally, RT on the memory measure revealed a similar pattern across immersions for TYR and thermoneutral conditions, but not cold/placebo (p < .05). This study demonstrates cold exposure degrades cognitive performance and supplementation with TYR alleviates working memory decrements.

Introduction

Environmental stress can impair performance and produce adverse changes in behavior and mood. Animals exposed to high or prolonged levels of environmental stressors, such as extreme cold, show significant impairment on performance tasks [1]. In humans, significant decrements have been observed on several types of cognitive measures following cold stress, including vigilance, reaction time, reasoning skills, and short-term memory [2], [3], [4], [5], [6].

Reported changes in performance associated with environmental stress may result from the depletion of catecholamine neurotransmitters in the central nervous system following acute cold exposure. Exposure to cold increases brain catecholamine activity, e.g., norepinephrine, in animals [7], [8], [9], [10]. Since tyrosine is a substrate for catecholamine biosynthesis in the brain, under cold conditions, tyrosine availability might be a limiting factor for optimal central nervous system function. Consequently, dietary tyrosine supplementation might reduce some of the adverse effects of stress and central nervous system activation.

Tyrosine, a large amino acid found in substantial quantities in many animal and plant protein foods, is the metabolic precursor for synthesis of the catecholamine neurotransmitters, dopamine and norepinephrine, in the central and peripheral nervous systems [11], [12]. In rodents, administering tyrosine can increase catecholamine levels in the brain, under certain conditions. When catecholaminergic neurons are highly active, they release more neurotransmitter, causing tyrosine to be metabolized at an accelerated rate to sustain catecholamine synthesis. As the brain stores of tyrosine are depleted, the synthesis of catecholamines may become limited. Supplementing the diet with exogenous tyrosine may therefore be efficacious in restoring brain synthesis and release of catecholamines.

Supplementation with tyrosine has been shown, in animals, to limit or prevent the depletion of brain catecholamines that results from acute environmental stress [7], [9], [10], [13] and mitigate the associated behavioral deficits [13], [14], [15]. Studies with human volunteers have not consistently shown positive performance effects with tyrosine supplementation [12], [16]. The lack of conclusive evidence in human studies may be due to differences in the severity of stress (and consequently differences in brain catecholamine depletion) produced by the experimental paradigm or individual responses to acute stress. Previous work suggests that tyrosine reduces performance decrements in situations of high environmental stress [5] but has little effect under conditions of lower or moderate environmental stress [12].

The principle goal of this investigation was to evaluate the efficacy of the nutrient tyrosine as a countermeasure to cold-induced cognitive performance decrements. The study was designed using an environmental stressor previously shown to induce cognitive impairment in an effort to maximize the likelihood of stress-induced depletion of brain catecholamines and hence the potential for tyrosine administration to have significant beneficial impact. Participants were immersed to the chest in cold-water to decrease core body temperature, induce substantial cold stress and produce unambiguous performance decrements. Past work has demonstrated that cold-water immersion creates a stressful condition suitable for observing the effects of tyrosine [17]. The use of cold stress as an environmental manipulation is also of interest due to a lack of systematic information on the effects of body core cooling on cognitive performance in humans. We hypothesized that supplemental tyrosine could reduce the associated decrements in cognitive performance during cold stress.