4.6 Mental health

As well as having physical consequences to the health of the body, dysbiosis has been linked to a number of mental health conditions.

Recent studies suggest that the microbiota-gut-brain axis plays a key role in depression and anxiety (Zhu, Tu and Chen, 2022). You may recall that the gut microbiome communicates with the brain via a number of routes, but particularly via:

• Nerves – nerve endings in the gut wall are triggered by substances produced by the gut microbiome and signals are relayed to the brain via the vagus nerve.
• Hormones, neurotransmitters, and other molecules (such as serotonin, SCFAs, and other important neurotransmitters involved in signalling in the brain and nervous system) – which are made by the gut microbiome and enter the bloodstream to travel to the brain (and other organs).

An altered F/B ratio, with a reduction in Firmicutes and increase in Bacteroidetes, is frequently seen in individuals with anxiety, while the reverse is more common in individuals experiencing depression. Other mental health disorders such as bipolar disorder, schizophrenia and post-traumatic stress disorder are associated in changes in the amount of other bacterial species (Xiong et al., 2023).

Some studies have suggested that dietary modifications are particularly beneficial in improving the symptoms of anxiety, even more so than pharmaceutical-grade probiotics (Yang et al., 2019). Prebiotics, dietary fibre and particularly probiotics (which you will learn about in the next section) are considered a potential therapeutic strategy for reducing anxiety, depression or improving dysbiosis (Bibbò et al., 2022; Cooke, Catchlove and Tooley, 2022).