1. What is Serotonin?

Serotonin is a neurotransmitter. It is commonly known as what is targeted by some antidepressants and is associated with depression and mood. A neurotransmitter is a chemical which carries messages between nerve cells, for example, in the brain and other parts of the body. Neurotransmitters regulate bodily systems by sensing our external environment and our internal state and communicating that to relevant bodily systems. Serotonin is also called 5-hydroxytryptamine (5-HT) [1].

2. What does Serotonin do?

Serotonin does so much more than regulate mood. In addition to this serotonin regulates memory, appetite, addiction, pleasure, sleep, pain, blood clotting, bone health, pupil dilation, immune function, heart rate, blood pressure, breathing, digestion, gastric emptying (pooping), metabolism, insulin secretion, glucose (sugar) uptake, fat breakdown/ build-up in the liver, peeing, ovary development, uterine blood flow and muscle tone and penis erectile function [1].

3. Where does Serotonin come from?

95% of serotonin is produced in our gut by the gut microbiome [2]. As serotonin is primarily produced in the gut, it sends signals to the brain via the GBA (Gut Brain Axis) [3]. Serotonin synthesis requires the amino acid tryptophan as a starting ingredient and the rate-limiting enzyme tryptophan hydroxylase for the chemical conversion, which takes place. Tryptophan regulates communication across the GBA, in both directions, with the help of B vitamins ( BH4, B6 and B9 (folic acid)). Tryptophan has a neuroprotective activity. [1].

4. Disease associated with poor serotonin?

Serotonin is commonly associated with depression. However, the mechanism through which it works is not very straightforward. One research review showed no convincing evidence that lower serotonin levels caused depression. This was true for studies which compared serotonin activity between people with/ without depression, as well as studies which looked at low levels of tryptophan, which is required to produce serotonin [3]. Research has highlighted that Genetics impacts susceptibility to depression and suicide, and there is a genetic element to serotonin transporters in the body [4].

Another research review found an association between high serotonin and good mood, and low serotonin being associated with poor mood. Mood level, then, can be related to depression. There is a circular system when it comes to depression, which can make falling into a depression over time happen before you know it, if you are not paying attention to your mood, well-being and practising self-awareness. Being in a general state of well-being and happiness offers some protection against mental disorders and related physical disorders. For example, negative mood and hostility could increase a person’s level of social isolation, and social isolation could further increase negative mood. Mood is linked to mortality rates for physical diseases, such as heart disease, through levels of agreeableness and social support, which together play a role in outcomes [4].

Other conditions associated with serotonin include:

Anxiety, Depression, Migraine, Psychosis, Schizophrenia, Hypertension, Heart failure, Pulmonary hypertension, Obesity, Cognitive disorders, Sleep disorders, GI motility disorders, Nausea and Vomiting, Obsessive Compulsive Disorder, Post-Traumatic Stress Disorder, Neurodegenerative disorders such as Parkinson’s and Alzheimer’s, Multiple Sclerosis, Autism Spectrum Disorders, Irritable Bowel Syndrome, Inflammatory Bowel Disease, age related gastro intestinal dysfunction [5].

5. How to Regulate Your Serotonin?

In terms of natural ways to control serotonin in the brain, some tools include: changing our thoughts, light therapy, exercise, diet, and spending time outdoors [4].

Changing our thoughts:

For example, through meditation, visualisation, and psychotherapy. There is a long-standing theory that thought control can change brain metabolism. Some studies, for example, have demonstrated changes in blood flow, dopamine release and serotonin synthesis. Hence, this further supports the circular nature of the relationship between mood and serotonin. Serotonin influences mood, and mood influences serotonin [4].

Bright Light:

Generally accepted as a treatment for SAD (Seasonal Affective Disorder), which is a depression which is related to winter, where natural light declines. There is evidence that light therapy could be an effective treatment for non-seasonal depression, poor mood related to menstruation and pregnancy.

There is a relationship between serotonin production and the hours of sunlight on a day. This was shown to be due to light in particular and not related to cycles, for example, the circadian rhythm, which you might have heard before, which relates to sleep. In humans, bright light (3000 lux) treatment in healthy women completely counteracted low mood, which had been caused by acute tryptophan depletion. We as humans would have, as recently as a few generations ago, spent most of the day outdoors.

On a cloudy day, outside light can be 1000 lux plus, a level not generally seen indoors. A study carried out at latitude 45 degrees North, which is about halfway between the equator and the north pole, i.e. around southern France, Northern Italy, Northern States of North America, looked at levels of light exposure in people working at least 30 hours weekly. This study found only about 30 minutes of exposure to 1000 lux plus light in winter, and for summer, this was still only at 90 minutes. This suggests we are living in a light-deprived society [4].

Exercise:

Exercise is a recommended treatment for mild depression by the National Institute for Health and Clinical Excellence in the UK. Regular and consistent aerobic exercise is the most effective. Exercise increases tryptophan and serotonin brain levels, neuronal activity and synthesis. [4].

Diet:

Tryptophan is the amino acid which is a building block for serotonin; it is classified as a drug in Canada and some European countries. Purified tryptophan increases brain serotonin. Tryptophan is the least abundant amino acid in protein.

When we eat a meal, there is competition between amino acids in terms of which can be digested, and due to the low level of tryptophan typically found in a proteinaceous food source like turkey in relation to other amino acids, tryptophan uptake is reduced. Tryptophan levels in food are impacted by how processed that food is; wild chickpeas are richer in tryptophan than cultivated chickpeas. [4].

Other foodstuffs associated with the serotonin pathway:

REFERENCES:

[1] Bamalan OA, Moore MJ, Al Khalili Y. Physiology, Serotonin. [Updated 2023 Jul 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK545168/
[2] https://integrishealth.org/resources/on-your-health/2021/january/facts-about-the-human-body-you-never-knew (accessed 15th Feb 2025)
[3] J. Moncrieff, R.E. Cooper, T. Stockmann, et al.,’The serotonin theory of depression: a systematic umbrella review of the evidence’ Molecular Psychiatry, Vol.28, 2023, p.3243–3256
[4] Young SN. “How to increase serotonin in the human brain without drugs”, Journal of Psychiatry Neuroscience, vol. 32(6), 2007, p.394-399.
[5] Roth W, Zadeh K, Vekariya R, Ge Y, Mohamadzadeh M.,”Tryptophan Metabolism and Gut-Brain Homeostasis”, International Journal of Molecular Sciences, vol. 22(6), 2021.
[6] Liaqat H, Parveen A, Kim SY., “Neuroprotective Natural Products' Regulatory Effects on Depression via Gut-Brain Axis Targeting Tryptophan”, Nutrients, vol14 (16) 2022.