Tyrosine
Tyrosine is one of the 22 amino acids used by the body to construct proteins however its interest in cognitive enhancement comes from the fact that it is the precursor to the neurotransmitters dopamine and noradrenaline. Both of these neurotransmitters are involved in attentional and reward pathways in the brain making them potential targets for enhancing neural function. The research regarding the cognitive enhancing effects of tyrosine seem to show that it is only effective under stressful conditions. Several studies show that tyrosine administration is effective at reducing cognitive deficits caused by stressful or demanding situations but that neural enhancement during normal (not stressful) conditions did not occur. It should also be noted that very large doses are required for effective results.
Most beneficial for - only people in stressful situations
Effective dose - 100-300mg/kg
Length of action - Positive effects reported one hour and observed up to 5 hours after administration
Safety - Found naturally in large amounts in the diet so unlikely to be dangerous
Type cognition effected - working memory, memory and reaction time
Enhancers - not known
Supporting Human Studies
Tyrosine improves cognitive performance and reduces blood pressure in cadets after one week of a combat training course (Deijen et al, 1999)
Twenty one military cadets received 10g of tyrosine or a placebo daily for 6 days during a demanding training course. Cognitive, mood, blood pressure and norepinephrine tests were performed before the course and after day 6. The group receiving the tyrosine had improved results on both memory and tracking tasks. They also had lower systolic blood pressure compared to the placebo group. No effects on mood were observed.
Tyrosine Improves Working Memory in a Multitasking Environment (Thomas et al, 1999)
Ten men and 10 women underwent a complex simultaneous working memory, arithmetic skills, and visual and auditory monitoring task and a simple simultaneous working memory and visual monitoring only task. They received tyrosine (150mg/kg) or a placebo 1 hour before commencing the tasks. Tyrosine significantly improved working memory performance however had no effect on the other parameters measured during the complex task but no effect on either working memory or visual monitoring on the simple task. Cortisol was not affected during either task however both blood pressure and heart rate were increased in both.
Effects of Tyrosine, Phentermine, Caffeine d-amphetamine, and Placebo on Cognitive and Motor Performance Deficits During Sleep Deprivation (Magill et al, 2003)
Young healthy men received tyrosine (150mg/kg), one of a number of other drugs or a placebo. This was administered at 15:30 following a night of sleep deprivation. Tests of visual scanning, running memory, logical reasoning, mathematical processing, the Stroop task, four-choice serial reaction time, time wall take, pursuit tracking, visual vigilance, Trails (B) task and long-term memory were evaluated at standardized intervals before, during and after sleep deprivation and drugs. Improvements with medication following sleep deprivation were seen in running memory, logical reasoning, mathematical processing, tracking and visual vigilance. Although less effective than d-amphetamine, tyrosine improved performance on several tests.
The effects of tyrosine on cognitive performance during extended wakefulness (Neri et al, 1995)
Twenty male subjects underwent 9 iterations of cognitive test over a 13 hour period. They had been awake for 11 hours before the tests began and were not allowed to sleep for the duration of the experiment. Six hours after the experiment began half the subject received a tyrosine supplement while the other half a placebo. The group receiving the tyrosine demonstrated improved performance in psychomotor and attention tests compared to the placebo whilst under sleep deprived conditions.
Tyrosine reverses a cold-induced working memory deficit in humans (Shurtleff et al, 1993)
Eight male subjects received either tyrosine (150mg/kg) or a placebo 2hours before undergoing memory tests. Subject performed the tests either at ambient room temperature or 30 minutes after cold exposure at 4 degrees C. Cold exposure significantly reduced memory performance compared to room temperature. Tyrosine administration significantly attenuated the hindered performance caused by the cold exposure. Tyrosine had no effect on cognitive performance at ambient room temperatures.
Effect of tyrosine on cognitive function and blood pressure under stress (Deijen et al, 1994)
Fifteen healthy subjects were tested for cognitive function and blood pressure under stressful conditions over two days. On day one they received 100mg per kg of tyrosine and on day two a placebo. Compared to the placebo the tyrosine was found to improve cognitive performance under stress 1 hour after administration. It was also found to decrease diastolic blood pressure 15 minutes after ingestion but return to normal after 1 hour.
Tyrosine supplementation mitigates working memory decrements during cold exposure (Mahoney et al, 2007)
Nineteen volunteers received either a placebo or 300mg/kg of tyrosine both at normal temperature and at 10 degrees C. The cold temperature was designed to induce stress on the subject. After this there performed cognitive, mood and salivary cortisol tests. The cold was shown to increase cortisol levels indicating its effectiveness as a stress inducer. The cold increased the number of memory and reaction time errors compared to normal temperatures. Increases in negative mood were also reported with the cold exposure. Those receiving the tyrosine supplement showed more accurate and rapid cognitive performance compared to the placebo in the cold exposed tests.
Treatment with tyrosine, a neurotransmitter precursor, reduces environmental stress in humans (Banderet et al, 1989)
In this double blind, placebo controlled trial subjects received either tyrosine (100mg/kg) or a placebo before being exposed to cold and hypoxia for 4.5 hours. They then underwent tests for mood and cognitive performance. The tyrosine significantly decreased symptoms, adverse moods and performance impairments caused by the stress exposure.
Dietary tyrosine benefits cognitive and psychomotor performance during body cooling (O'Brien et al, 2007)
Fifteen subjects completed one control trial at 35 degrees C and two at 10 degrees C. Subjects took either tyrosine (300mg/kg) or a placebo before each before each trial and then completed cognitive and psycho-motor tasks. The cold exposure was shown to decrease cognitive and psycho-motor test performance between 14-18% compared to normal temperatures. The tyrosine prevented these cognitive and psycho-motor deficits completely.
Supporting Animal Studies
Diet restriction in mice causes a decrease in hippocampal choline uptake and muscarinic receptors that is restored by administration of tyrosine: interaction between cholinergic and adrenergic receptors influencing cognitive function (Avraham et al, 2001)
The cognitive effects of diet restriction was performed on rats. This was found to significantly impair cognitive functioning in water maze tests. Tyrosine administration was shown to reverse the cognitive deficits caused by the diet restriction. Tyrosine supplementation was also associated with increased choline uptake and density of m1 receptors (receptors thought to be involved in memory function).
Tyrosine administration prevents hypoxia-induced decrements in learning and memory (Shukitt-Hale et al, 1994)
In this experiment 27 rats performed water maze tests both under normal atmospheric pressure and at simulated 5950m above sea level. Some received tyrosine (400mg/kg) before the experiment while others a placebo. The hypoxic conditions created by the high altitude significantly reduced performance on the water maze compared to normal atmospheric conditions. The administration of tyrosine reversed the cognitive impairments caused by hypoxia.
Contradictory Studies
None
Combinational Studies
None
Safety Studies
None
Most beneficial for - only people in stressful situations
Effective dose - 100-300mg/kg
Length of action - Positive effects reported one hour and observed up to 5 hours after administration
Safety - Found naturally in large amounts in the diet so unlikely to be dangerous
Type cognition effected - working memory, memory and reaction time
Enhancers - not known
Supporting Human Studies
Tyrosine improves cognitive performance and reduces blood pressure in cadets after one week of a combat training course (Deijen et al, 1999)
Twenty one military cadets received 10g of tyrosine or a placebo daily for 6 days during a demanding training course. Cognitive, mood, blood pressure and norepinephrine tests were performed before the course and after day 6. The group receiving the tyrosine had improved results on both memory and tracking tasks. They also had lower systolic blood pressure compared to the placebo group. No effects on mood were observed.
Tyrosine Improves Working Memory in a Multitasking Environment (Thomas et al, 1999)
Ten men and 10 women underwent a complex simultaneous working memory, arithmetic skills, and visual and auditory monitoring task and a simple simultaneous working memory and visual monitoring only task. They received tyrosine (150mg/kg) or a placebo 1 hour before commencing the tasks. Tyrosine significantly improved working memory performance however had no effect on the other parameters measured during the complex task but no effect on either working memory or visual monitoring on the simple task. Cortisol was not affected during either task however both blood pressure and heart rate were increased in both.
Effects of Tyrosine, Phentermine, Caffeine d-amphetamine, and Placebo on Cognitive and Motor Performance Deficits During Sleep Deprivation (Magill et al, 2003)
Young healthy men received tyrosine (150mg/kg), one of a number of other drugs or a placebo. This was administered at 15:30 following a night of sleep deprivation. Tests of visual scanning, running memory, logical reasoning, mathematical processing, the Stroop task, four-choice serial reaction time, time wall take, pursuit tracking, visual vigilance, Trails (B) task and long-term memory were evaluated at standardized intervals before, during and after sleep deprivation and drugs. Improvements with medication following sleep deprivation were seen in running memory, logical reasoning, mathematical processing, tracking and visual vigilance. Although less effective than d-amphetamine, tyrosine improved performance on several tests.
The effects of tyrosine on cognitive performance during extended wakefulness (Neri et al, 1995)
Twenty male subjects underwent 9 iterations of cognitive test over a 13 hour period. They had been awake for 11 hours before the tests began and were not allowed to sleep for the duration of the experiment. Six hours after the experiment began half the subject received a tyrosine supplement while the other half a placebo. The group receiving the tyrosine demonstrated improved performance in psychomotor and attention tests compared to the placebo whilst under sleep deprived conditions.
Tyrosine reverses a cold-induced working memory deficit in humans (Shurtleff et al, 1993)
Eight male subjects received either tyrosine (150mg/kg) or a placebo 2hours before undergoing memory tests. Subject performed the tests either at ambient room temperature or 30 minutes after cold exposure at 4 degrees C. Cold exposure significantly reduced memory performance compared to room temperature. Tyrosine administration significantly attenuated the hindered performance caused by the cold exposure. Tyrosine had no effect on cognitive performance at ambient room temperatures.
Effect of tyrosine on cognitive function and blood pressure under stress (Deijen et al, 1994)
Fifteen healthy subjects were tested for cognitive function and blood pressure under stressful conditions over two days. On day one they received 100mg per kg of tyrosine and on day two a placebo. Compared to the placebo the tyrosine was found to improve cognitive performance under stress 1 hour after administration. It was also found to decrease diastolic blood pressure 15 minutes after ingestion but return to normal after 1 hour.
Tyrosine supplementation mitigates working memory decrements during cold exposure (Mahoney et al, 2007)
Nineteen volunteers received either a placebo or 300mg/kg of tyrosine both at normal temperature and at 10 degrees C. The cold temperature was designed to induce stress on the subject. After this there performed cognitive, mood and salivary cortisol tests. The cold was shown to increase cortisol levels indicating its effectiveness as a stress inducer. The cold increased the number of memory and reaction time errors compared to normal temperatures. Increases in negative mood were also reported with the cold exposure. Those receiving the tyrosine supplement showed more accurate and rapid cognitive performance compared to the placebo in the cold exposed tests.
Treatment with tyrosine, a neurotransmitter precursor, reduces environmental stress in humans (Banderet et al, 1989)
In this double blind, placebo controlled trial subjects received either tyrosine (100mg/kg) or a placebo before being exposed to cold and hypoxia for 4.5 hours. They then underwent tests for mood and cognitive performance. The tyrosine significantly decreased symptoms, adverse moods and performance impairments caused by the stress exposure.
Dietary tyrosine benefits cognitive and psychomotor performance during body cooling (O'Brien et al, 2007)
Fifteen subjects completed one control trial at 35 degrees C and two at 10 degrees C. Subjects took either tyrosine (300mg/kg) or a placebo before each before each trial and then completed cognitive and psycho-motor tasks. The cold exposure was shown to decrease cognitive and psycho-motor test performance between 14-18% compared to normal temperatures. The tyrosine prevented these cognitive and psycho-motor deficits completely.
Supporting Animal Studies
Diet restriction in mice causes a decrease in hippocampal choline uptake and muscarinic receptors that is restored by administration of tyrosine: interaction between cholinergic and adrenergic receptors influencing cognitive function (Avraham et al, 2001)
The cognitive effects of diet restriction was performed on rats. This was found to significantly impair cognitive functioning in water maze tests. Tyrosine administration was shown to reverse the cognitive deficits caused by the diet restriction. Tyrosine supplementation was also associated with increased choline uptake and density of m1 receptors (receptors thought to be involved in memory function).
Tyrosine administration prevents hypoxia-induced decrements in learning and memory (Shukitt-Hale et al, 1994)
In this experiment 27 rats performed water maze tests both under normal atmospheric pressure and at simulated 5950m above sea level. Some received tyrosine (400mg/kg) before the experiment while others a placebo. The hypoxic conditions created by the high altitude significantly reduced performance on the water maze compared to normal atmospheric conditions. The administration of tyrosine reversed the cognitive impairments caused by hypoxia.
Contradictory Studies
None
Combinational Studies
None
Safety Studies
None