Original ArticlesTyrosine improves cognitive performance and reduces blood pressure in cadets after one week of a combat training course
Introduction
Psychosocial and physical stress are known to increase the release of both peripheral and central (brain) norepinephrine (NE) 16, 29. Peripheral and central release of catecholamines are controlled by two separate systems, because peripherally released catecholamines cannot pass the blood—brain barrier. In the frontal cortex, the transmission of noradrenergic neurons is increased by stressful events 16, 21. Noradrenergic projections from the locus coeruleus (LC), which show an increased electrical activity during stress, provide a main innervation to the frontal cortex 1, 31. The activity of noradrenergic neurons in the LC plays an important role in attention processes, alertness, motor activity and the regulation of emotional processes [28]. In animal studies, stress-induced depletion of brain NE was followed by reduced explorative and motor behavior and by behavioral depression 22, 41. Also in humans central NE has been found to be important in maintaining attention. A clonidine-induced inhibition of NE release resulted in an increase in the number of lapses of attention in healthy males, which was reversed by the antagonist idazoxan [34].
With respect to dopamine (DA), a variety of tasks, including active avoidance, passive avoidance and the radial-arm maze, have been used in experimental animals to show the involvement of DA systems in learning and memory. Systemic DA receptor blockade impaired learning in different tasks suggesting the role of DA blockade in producing learning deficits 6, 7, 24. Neurotoxic depletion of catecholamines (CA) in the prefrontal cortex of young adult monkeys produced impairment of spatial memory that was reversed by treatment with the DA agonists L-dopa and apomorphine [12]. With respect to humans, some investigators found an increased incidence of dementia in patients with Parkinson’s (PD) disease, a syndrome characterized by atrophy and degeneration of DA neurons 18, 42. The existence of a deficit in visuospatial working memory in PD also indicates the involvement of DA in intellectual functioning [10]. However, in animal studies, where the concentration of NE and DA was measured after stress induction, no changes in the concentration DA was found [41]. Therefore, considerably more severe stress seems to be required to alter DA levels in the brain than NE levels. The release of DA may be more related to coping behaviors than to the uncontrollability of the stressor, which appears to be the crucial determinant of the NE response [22].
The large neutral amino acid L-tyrosine, which is the precursor of NE and DA, has been found to enhance NE synthesis [23] and may thus prevent stress-induced NE depletion in the animal brain. In mice, brain tyrosine was found to reach its maximum concentration 1 h after oral ingestion and returned to baseline level after 4 h [39]. In addition, in rats receiving a tyrosine-rich diet, neither NE depletion nor behavioral impairment was found after stress induction 11, 22. Similar results have been found in humans. Young men who were exposed to cold and hypoxia exhibited fewer stress symptoms, such as headache, tension and fatigue, and showed fewer psychomotor impairments after being supplemented with 100 mg/kg tyrosine [5]. In a more recent human study, Shurtleff, Thomas, Schrot, Kowalski, and Harford [33] found that the administration of 150 mg/kg tyrosine prevented the cold-induced (4°C) impairment of short-term memory that was observed in a placebo group. Positive effects of L-tyrosine on mental performance in human subjects were also found in a study from Deijen and Orlebeke [15]. The supplementation of 100 mg/kg L-tyrosine in healthy subjects improved the mental performance under stress (noise of 90 dB) as compared to a placebo condition. In addition, L-tyrosine decreased diastolic blood pressure (DBP) 15 min after ingestion. This was consistent with the blood-pressure-reducing effects found in animals 37, 43. Hence, laboratory studies strongly suggest that supplementation with tyrosine may serve to reduce the cognitive, behavioral and physiological effects of exposure to stress.
Also, protein-containing diets that provide tyrosine have been found to increase the plasma tyrosine/large neutral amino acids (LNAA) ratio and brain tyrosine levels. Plasma tyrosine concentration, expressed relative to the plasma concentrations of the same transport competitive amino acid (tyrosine/LNAA), reflects the amount of tyrosine available to the brain for catecholamine (CA) synthesis. The acute consumption of a high-protein meal was found to almost double the serum tyrosine level and tyrosine ratio in rats 19, 20. In humans, plasma tyrosine levels and the plasma tyrosine ratio were measured in subjects who consumed a protein-rich diet. Plasma tyrosine levels rose significantly during the day when the diet was consumed [26]. In another study oral protein (albumin) was found to increase the ratio in plasma of tyrosine to other LNAA in healthy fasting females by 20–60% [27]. Finally, a protein breakfast was found to cause a significant rise in cerebrospinal fluid tyrosine in patients who were suffering from normal pressure hydrocephalus [38].
The present study was designed to determine whether the administration of L-tyrosine would also be effective in reducing the effects of “real-life” stress. The study was carried out with a group of cadets of the military academy who had to complete a combat training course as part of their training program. As this course involved psychologically, as well as physically highly demanding conditions (including sustained operations and sleep loss), it was considered to be an appropriate environment to study the stress-reducing properties of L-tyrosine. The study focussed specifically on the effects of L-tyrosine on cognitive functioning after stress-induction. We hypothesized that supplementation with tyrosine would reduce the stress-induced cognitive, psychomotor and mood impairments and would lower blood pressure.
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Subjects
Thirty-two cadets of the Royal Military Academy (Koninklijke Militaire Academie; KMA) volunteered to participate in the study. Sixteen subjects were randomly assigned to the tyrosine group, and the remaining 16 subjects were assigned to the placebo group. In the course of the study, 11 subjects dropped out due to injuries, leaving 21 subjects for analysis. This left 10 subjects belonging to the tyrosine group and 11 subjects to the placebo group. The tyrosine group included one female and nine
Perceptual-motor tasks
The number of correct responses on the MCT was significantly higher at the posttest in the tyrosine group than in the placebo group (F(1,18) = 4.11, p < 0.05). In contrast, the number of incorrect responses observed in the tyrosine and placebo groups was not significantly different (see Fig. 1).
In addition, the tyrosine group had a better RMS tracking score on the TT at the posttest than the placebo group (F(1,18) = 6.14, p < 0.05) (see Fig. 2). The mean RMS values for the 14 separate
Discussion
The aim of the present study was to investigate whether supplementation with a drink containing tyrosine during a highly demanding combat training course would reduce the negative effects of stress and fatigue on cognitive performance and emotional well-being. In addition, the effects of tyrosine on blood pressure and on the concentration of the NA metabolite MHPG were assessed. During the posttest, which took place 6 days after the start of the combat course and 5 days after tyrosine
Acknowledgements
This study was performed by order of the Department of Behavioral Sciences of the Royal Netherlands Army.
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