What Are the Causes of Low Serotonin Levels?

Serotonin is a neurotransmitter that has numerous physiological functions in the central nervous system. It is made from l-tryptophan, an essential amino acid found in many protein foods. Low serotonin levels in the brain occur from dietary deficiencies of certain nutrients, intake of a substance found in tea and psychological factors that occur in childhood. Serotonin deficiencies are associated with various medical conditions and psychiatric disorders such as obesity, anorexia, depression and insomnia.

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Low Dietary Levels of L-Tryptophan

Psychiatric research scientists at McGill University observed that low levels of l-tryptophan in the brain caused reduced synthesis of serotonin. Depleting l-tryptophan in the diet results in a transient decline in brain serotonin levels, whereas higher levels of dietary tryptophan can increase brain serotonin. L-tryptophan enters the brain through the blood brain barrier. The amount of L-tryptophan that crosses the blood brain barrier can be altered by the concentration or ratio of l-tryptophan to other amino acids in the plasma, particularly the branched chain amino acids such as leucine, isoleucine and valine. Consuming a meal high in protein and low in carbohydrates reduces the ratio of l-tryptophan to branched chain amino acids, and in turn, reduces the amount of l-tryptophan that enters the brain to synthesize serotonin.

Vitamin B6 Deficiency

Vitamin B6 is essential in the synthesis of serotonin from l-tryptophan 1. Levels of pyridoxal 5-phosphate, the active coenzyme of vitamin B6, regulate the amount of serotonin that can be synthesized in the brain. According to scientists at the University of Manitoba, low serotonin levels in various areas of the brain are associated with vitamin B6 deficiency. Vitamin B6 supplementation restores serotonin to normal levels throughout the brain.


Nutritional biochemists at the University of Shizuoka in Japan found that dietary intake of theanine reduces the synthesis of serotonin in the brain and increases degradation of serotonin. Theanine is an amino acid abundant in green tea leaves, and it can cross the blood brain barrier. Inside the brain cortex, theanine may inhibit nerve cells from releasing serotonin and causing anti-stress effects.

Childhood Abuse

Scientists at the New York State Psychiatric Institute discovered that physical or psychological adversity in childhood can lower serotonin function and levels in the brain. Adults with depression who reported childhood abuse had lower brain serotonin transporter-binding potential proportional to the number of available transporters compared with adults with depression who did not report childhood abuse 4. Serotonin abnormalities occurred across all regions of the brain. Childhood abuse predisposes a person to development of serotonin abnormalities and consequent psychiatric illness such as depression in adulthood.