During times of stress, brain levels of tryptophan increase suggesting that serotonin is somehow involved in the adaptive process to stabilise mood during times of perceived threat. Prolonged stress however, can lead to depression which is often characterised by reduced levels of serotonin. Serotonin therefore, might be an important component of the defence against stress. Drugs such as selective serotonin reuptake inhibitors (SSRI) artificially increase levels of serotonin in depressed individuals in order to stabilise mood, but they are associated with severe side effects and their benefits are controversial. A nutritional approach aims to first remove the stress and then increases precursors of serotonin to boost brain levels. In order for this to be effective it is necessary to have an understanding of the metabolic synthesis of serotonin and factors that affect it.
Serotonin is synthesised in neurones from the dietary amino acid tryptophan in a series of steps (figure 1). Ingested tryptophan is absorbed to the circulation and crosses the blood brain barrier, where it competes for absorption with other large neutral amino acids (LNAA). Carbohydrate rich-protein poor diets tend to increase the tryptophan in the brain and increase production of serotonin. This is because carbohydrate induced insulin release facilitates the uptake of all LNAA into skeletal muscle, excluding trytptophan. The resulting high tryptophan to LNAA ratio in the blood increases brain concentrations of tryptophan and serotonin. In contrast, a protein rich diet results in a low trytptophan to LNAA ratio in the blood because protein foods are poor sources of tryptophan (1-2 %) but are rich in the LNAA (leucine, valine, isoleucine, phenylaanine and tyrosine).
Figure 1. The synthesis of serotonin from L-tryptophan.
Research published in the American Journal of Nutrition in 20001, fed highly stress venerable individuals a whey protein diet with a high tryptophan content (α‑lactalbumin) or a casein protein diet to determine the change in tryptophan to LNAA ratio in response to acute stress. The stress involved performing a complex arithmetic test. The researchers measured the cortisol concentrations of the subject to test the reaction to stress and took profile of their mood state. The results showed that the tryptophan to LNAA ratio was 48 % higher after ingestion of the α-lactalbumin whey protein diet compared to the casein diet. Following the test, those consuming the high tryptophan diets had a decrease in cortisol and an improvement in mood. This research suggests that the tryptophan content of the diet can have a direct impact on mood and the response to stress.
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