L-tryptophan is an essential amino acid required for normal human metabolic regulation. There is current interest in L-tryptophan because it has been shown that low levels of plasma tryptophan may be involved in the development of depression. This relates to the ability of tryptophan to cross the blood brain barrier, where it is converted to serotonin, a neurotransmitter than may be involved in the regulation of mood. The relationship between serotonin and mood is complex and not fully understood, but some studies suggest that low tryptophan levels may be a cause of aggression in humans, whilst high levels may have a calming effect. This relationship between tryptophan and aggression also likely relates to the subsequent conversion of tryptophan to serotonin. Some evidence suggests that low levels of serotonin are involved in aggression towards self, which may imply that self harmers have low serotonin levels. Additionally, impulsive aggression is also related to low serotonin levels.
A number of studies have investigated the effects of tryptophan depletion on aggression. For example one study1 used healthy male volunteers and manipulated the tryptophan content of their diet in order to alter plasma levels of tryptophan. Tryptophan loading was achieved by consuming a drink containing high amounts of tryptophan, whilst tryptophan depletion was achieved by consuming a drink devoid of tryptophan. The 10 grams of tryptophan in the loading drink significantly increased the plasma levels of tryptophan in the subjects above the fasted and tryptophan depletion states. The researchers then provoked the subjects by using a computer game against an imaginary adversary, that the subject could not see. This resulted in monetary loss from a sum attributed to the subject, and the retaliatory behaviour in the game was taken as a measure of aggression. The results showed that following ingestion of the tryptophan depleted beverage, aggressive behaviour was significantly increased compared to the no food control.
However, there was no significant difference between the tryptophan loading condition and the no food control. This therefore suggests that lowering plasma levels of tryptophan, through limiting intake of tryptophan, may result in an increase in aggressive behaviour, but that consuming additional tryptophan may not have an additional calming effects when compared to the fasting condition. This suggest that under normal fasting conditions, the plasma levels of tryptophan may be optimal and that addition of further L-tryptophan to the diet is not able to further increase the calming effect of amino acid. Tryptophan flux into the brain across the blood brain barrier is facilitated by the release of insulin and the researchers did sweeten the drink suggesting that insulin release may have occurred. However, it could be that consumption of carbohydrates with this drink may have facilitated a greater uptake of tryptophan to the brain, and improved the calming effect of tryptophan.
RdB
