L-tyrosine is an amino acid that is also a precursor in the biosynthesis of norepinephrine and dopamine. Supplementation of L-tyrosine increases norepinephrine levels, and counteracts some of the deficits induced by acute and chronic stress – such as short-term memory impairment, and increased blood pressure.
L-tyrosine’s effects are likely to be mediated by an increase in norepinephrine, a neurotransmitter that is associated with the fight or flight response, and is depleted by psychological and physical stress. L-tyrosine reaches peak blood concentration 1 hour after ingestion and falls back to baseline 4 hours later. In rats, tyrosine supplementation prevents norepinephrine loss and behavioral deficits associated with stress.
Multiple human studies have demonstrated the effects of L-tyrosine, in the context of stress-induced deficits in cognition and mood, and stress-induced blood pressure increases. In a study of young men exposed to 4.5 hours of cold and hypoxia (conditions found at high altitudes), administration of 100 mg/kg of L-tyrosine improved mood and cognitive performance deficits induced by the stress (see figure below). Follow-up studies of cold-induced deficits replicated this effect – that tyrosine supplementation can reverse cold-induced deficits in cognition.
L-tyrosine supplementation reverses deficits in symptoms of stress (A), mood deficits associated with stress (B), and cognitive deficits due to stress (C). In the figures, the first column is a temperature of 15 degrees Celsius, with an oxygen content similar to 4200 meters above sea level, with the second column being the same temperature, but with an oxygen content similar to 4700 meters. The striped bars represent the placebo measures, and the black bars represent the L-tyrosine supplemented measures.
In another study using acoustic stress (90 decibels of white noise), administration of 100 mg/kg of L-tyrosine improved performance on stress-sensitive cognitive tasks and reduced diastolic blood pressure. In a study of 21 military cadets undergoing training and combat simulation stress, the effect of L-tyrosine supplementation on mood, blood pressure, and norepinephrine metabolism were investigated. L-tyrosine supplementation led to improved performance on cognitive measures and a physical tracking task. It also reduced systolic blood pressure. However, this study was not rigorously designed, as there was not a within-subject comparison.