Neurotransmitters are signal messengers used in the central nervous system. The central nervous system including the brain uses electrical signals to allow communication between neurones. However, gaps (synapses) between the neurones prevent the electrical signals travelling directly from one neurone to the next. To overcome this problem the electrical signal must be converted to a chemical signal in the form of a neurotransmitter. The neurotransmitter then crosses the synaptic gap by diffusion before activating receptors on the other side of the synapse. This then activates the next neurone to allow propagation of the electrical impulse. Many neurotransmitters are synthesised from dietary components and so the quantity of their precursors in food can have an influence of central nervous system function. For example, the neurotransmitters dopamine, adrenaline and noradrenaline can be synthesised from the amino acid tyrosine and the neurotransmitter serotonin can be synthesised from the amino acid tryptophan.
Dopamine, adrenaline and noradrenaline are excitatory neurotransmitters. Increasing the amount of tyrosine in the diet may therefore increase the amount of dopamine, adrenaline and noradrenaline, which would in turn increase arousal and motivation. In contrast, serotonin is an inhibitory neurotransmitters. Increasing the tryptophan may therefore be expected to decrease arousal and motivation and increase restfulness. Because tyrosine and tryptophan are used to synthesise neurotransmitters studies have investigated how supplementation with these amino acids can affect behaviour. In one study 20 men were supplemented with a single dose of tyrosine (50 mg / kg) or tryptophan (100 mg / kg) in a double-blind placebo controlled study1. The results showed that supplementation of tryptophan significantly decreased alertness compared to the placebo or tyrosine supplementation. In contrast, supplementation of tyrosine significantly increased reaction times when compared to a placebo or tryptophan supplementation.
The increase in drowsiness associated with taking supplemental tryptophan is consistent with other studies that have shown similar findings. Tryptophan is a good aid to sleep because it increases brain concentrations of serotonin, which in turn are converted to melatonin, the hormone that initiates sleep. Likewise, the improved reaction times seen with tyrosine supplementation supports other studies that have shown that tyrosine can increase brain levels of the excitatory neurotransmitters dopamine, adrenaline and noradrenaline and that this in turn has beneficial effects on physical performance including reaction times. Unfortunately, tryptophan was removed from the market following alleged detrimental effects. However, the incident was later determined to be due to contamination of the product in the factory with other substances. Fortunately, another serotonin precursor 5-hydroxytryptophan is available in herbal form from the Griffonia simplicifolia plant. Tyrosine is still thankfully available commercially.
RdB
