Serotonin is a hormone produced by the body and a natural neurotransmitter that provides mood stabilisation and helps the brain feel a sense of happiness and contentment. Serotonin works in different parts of the body. As a neurotransmitter, it enables neurons to communicate with each other as well as with other cells in the body. It is also important for digestion, eating, sleep, an intrinsic sense of reward, and memory and learning. The normal range of serotonin in human blood is 101-283 nanograms per millilitre.
Serotonin is mainly made from one of the most important amino acids in the body known as tryptophan. It is an amino compound and a 5-hydroxy or 5-OH derivative of this amino acid.
Although serotonin is known for its cognitive and mood-stabilising functions in the brain, it is mainly produced in the gastrointestinal tract. It is estimated that about 90% of the body’s serotonin comes from production in the digestive tract. The cells that produce this serotonin in the intestine are known as enterochromaffin cells or EC cells for short.
Research has shown that EC cells produce serotonin with the help of gut bacteria. Therefore, significant imbalances of serotonin levels in the body are also observed in individuals with disorders of the gut microbiome. In addition, too low a serotonin level is observed in connection with low gastrointestinal motility and lower activation of blood platelets for the necessary blood clotting.
In addition to its regulatory function for a variety of functions in the digestive tract, serotonin plays another key role as both a neurotransmitter and a hormone in the nervous system. The neurons in the raphe nuclei are the main source of serotonin in the brain. From here, serotonin affects many areas of the brain, including sensorimotor functions in the cortical, subcortical and spinal areas that play a role in motor activity.
The brain’s main serotonergic system originates in the raphe nucleus, which is located in the brainstem, or hindbrain. These neurons project to different parts of the brain to carry out their functions. Most of the activity observed related to serotonin in the brain is inhibitory and antagonises the effects of other neurotransmitters such as choline, norepinephrine and dopamine. Serotonin circuits and receptors are located in the limbic system and influence sensations such as stress, impulsivity, aggression and anxiety. Other receptors are located in the cortex and influence rigid thinking, optimism and adaptability.
Sunlight, exercise, a healthy diet and certain foods such as eggs, cheese, turkey and nuts, meditation and a healthy gut microbiome can increase serotonin levels in the blood.
Unhealthy diets with low levels of the amino acid tryptophan, as well as increased consumption of caffeine and sugar, lead to lower blood serotonin levels.