Stress has been shown to cause neurochemical and behavioural changes that are related to alterations in the neurotransmitter balance in the brain. During times of stress, catecholamine levels in some brain areas increase, and in other areas there is a decline, but overall absolute levels fall. This causes behavioural changes such as reduced alertness, poor attention, increased hostility and increased anxiety which ultimately impairs physical performance. The large neutral amino acid L-tyrosine, is the precursor to noradrenaline and dopamine, and supplementation with L-tyrosine has been shown to decrease stress levels in animals. The benefits of L-tyrosine has been hypothesised to be due to the increased substrate need of highly active catecholaminergic neurones. Supplementation with L-tyrosine increases plasma levels which accelerates flux across the blood brain barrier and increases substrate availability for catecholaminergic neurones.
The symptoms of nordrenaline deficiency are similar to those seen in both battle fatigue and in periods of heavy overtraining during prolonged physical activity. Researchers1 investigated the effects of L-tyrosine supplementation on a 4.5 hour exposure to cold and hypoxia in 23 male US army personnel. Subjects were exposed twice to 15°C and a simulated 4500 m altitude as well as a controlled condition of 20°C and a simulated 500 m altitude. Subjects were tested in each condition with either 100 mg per kg body weight of L-tyrosine or a placebo. The results showed that supplementation with L-tyrosine significantly decreased the symptoms (adverse mood and performance impairment) compared to the placebo. The most significant results were seen in those subjects who experienced the greatest stress response, which suggests that L-tyrosine supplementation is most effective when catecholamine neurone activity is highest.
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