Serotonin
A neurotransmitter involved in regulating mood, sleep, appetite and digestion, nicknamed the "feel-good hormone", mostly produced in the gut.
Serotonin regulates mood (it brings calm and well-being), sleep (it is the precursor of melatonin), appetite, digestion and many other functions. A deficit is associated with depression, anxiety and sleep problems.
The original angle (the gut, the real serotonin factory): contrary to the “brain molecule” image, ~90-95% of the body’s serotonin is produced in the gut by enterochromaffin cells, under the influence of the microbiome! Only a small fraction is in the brain.
This gut serotonin regulates digestive motility (transit) and communicates with the brain via the gut-brain axis. This explains the deep link between gut health and mood: a disrupted microbiome can affect serotonin production and therefore your spirits.
An important caveat: gut serotonin does NOT cross the blood-brain barrier directly to reach the brain. The gut-mood link goes through indirect routes (vagus nerve, precursors, inflammation, bacterial metabolites). It is subtler than a simple “transfer” of serotonin.
Modulation through diet:
- Tryptophan (the precursor): turkey, eggs, cheese, tofu, pumpkin seeds, banana
- Carbohydrates help tryptophan enter the brain (hence the comforting effect of sweet foods… but a trap if excessive)
- A healthy microbiome (fibre, fermented foods) supports gut production
- Light and exercise increase brain serotonin
On the theory of depression: the “depression = lack of serotonin” model is now being questioned (Moncrieff review, 2022). The reality is more complex (neuroplasticity, inflammation, context). Diet and lifestyle play a role, but depression is not a simple deficit to be “topped up”.