L-tryptophan is an essential amino acid. This means that the body cannot synthesis the amino acid, and so a dietary source must be provided for life to proceed. L-tryptophan has a number of interesting functions, one of which is its conversion to serotonin in the brain. Serotonin is an important neurotransmitter that may play a significant role in mood. One theory suggest that low levels of serotonin in certain parts of the brain may cause depressive symptoms. In order for L-tryptophan to be able to be synthesised into serotonin, it must first be absorbed and pass into the blood. From here it must then cross the blood brain barrier and enter the brain. Within neurons, the L-tryptophan is then converted to serotonin. The enzyme responsible for the conversion of L-tryptophan to serotonin is tryptophan hydroxylase, and this enzyme is dependent on the presence of iron. An iron deficiency can therefore inhibit the production of serotonin and this may explain the mood altering effects of iron deficient anaemia.
The L-tryptophan content of neurones is usually low and not be high enough to saturate the tryptophan hydroxylase enzyme. Therefore increasing neuronal levels of L-tryptophan has the potential to increase brain production of serotonin by providing more substrate to the converting enzyme. Animal experiment have demonstrated that administration of L-tryptophan to the blood of animals through either injections or oral routes does increase brain levels of L-tryptophan and this can then raise brain levels of serotonin. Restriction of L-tryptophan in the diets of animals has also been shown to lower blood levels of L-tryptophan, and this in turn can lower brain levels of L-tryptophan and of serotonin. Therefore the brain levels of L-tryptophan and serotonin are dependent on the blood levels of L-tryptophan, something that is also evidenced in humans. Very small increases in L-tryptophan, lower than can be seen through natural diurnal variation, can cause significant increases in brain levels of serotonin.
Because small fluctuations in blood levels of L-tryptophan can cause changes in brain levels of L-tryptophan, it may be of interest to consider the factors that naturally alter plasma blood levels of L-tryptophan. Blood levels of L-tryptophan are maintained by balancing the addition of L-tryptophan from the diet and the pools in tissues, and the removal of L-tryptophan by uptake to the tissues and from catabolism in the liver. The ingestion of dietary protein leads to the absorption of L-tryptophan and some of the L-tryptophan enters the blood. The size of this L-tryptophan load can depend on the type and amount of protein consumed, as some protein sources contain significantly more protein that others. The L-tryptophan degrading enzyme in the liver, tryptophan pyrrolase shows diurnal variation, and this may affect blood levels of L-tryptophan, with levels peaking in the mid afternoon. Insulin release causes a significant increase in plasma L-tryptophan levels and this can increase serotonin production.
Eat Well, Stay Healthy, Protect Yourself