2.6.2 Gut microbiome and hormone signalling

In addition to the mucosal cells lining the inner surface of the gut wall, there are specialised hormone-producing cells interspersed between them. These are known as enteroendocrine cells (EECs) where entero = gut and endocrine = hormone.

EECs are scattered all along the length of the GI tract and produce a wide range of hormones. Some of the hormones produced by the EECs act locally to affect the functioning of the gut, such as modifying gut motility. Other hormones enter the bloodstream and travel to the brain. The metabolites made by the microbiome can stimulate the enteroendocrine cells (Figure 15), to trigger the same signalling pathways resulting in messages in the brain modifying satiety, hunger, and mood.

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Figure 15 The location of enteroendocrine cells in the gut wall and signalling to the brain

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A schematic illustration shows the communication between the gut microbiota and the brain. The illustration shows a layer of intestinal epithelial cells with microvilli surrounding the intestinal lumen. An EEC is embedded in the epithelium. The intestinal lumen contains numerous types of intestinal microbiota. The neurotransmitters, short-chain fatty acids, and immune cells also pass from the intestinal lumen through the epithelial cells into the blood circulation to travel to the brain. The immune cells and metabolites also relay signals to the brain via the vagus nerve. The vagus nerve creates a synaptic contact with the EEC and relays stimulating signals to the brain. The brain then regulates mood, cognition, and emotion.

Figure 15 The location of enteroendocrine cells in the gut wall and signalling to the brain

The EECs in the gut are also the largest producer of serotonin – a hormone which is important in the regulation of mood in the brain. However, some species of microbiota bacteria also directly produce and secrete serotonin and together they make approximately 95% of the body’s serotonin. Serotonin is associated with feelings of happiness, calmness, and focus, as well as the initiation of sleep. Reduced concentrations of serotonin have been linked to depression, anxiety, mood disorders and not being able to enjoy things. Recent evidence has indicated that changes in the gut microbiome can alter the amount of serotonin produced, which may provide a previously unknown mechanism for how diet and the gut microbiome can affect mood and mental health (Stasi, Sadalla and Milani, 2019).

As you can see, there are several mechanisms by which the gut microbiome could potentially help to regulate mood, cognition, emotion, and even pain.