Depression may be a mental health condition, but it can also have wide-reaching physical implications across the body – especially within the gut. It’s something of a ‘chicken or egg’ situation in the sense that researchers aren’t sure whether changes in the gut trigger depression or if depression changes the gut, but it seems certain the two share an inextricable link.

That’s important, because understanding how this relationship works could not only give us more clarity over what causes depression – something that has long been a mystery – but also open new research avenues for potential treatments and interventions. So, what does the latest research say about the link between depression and the gut? Let’s take a closer look. 

One of the primary ways in which depression impacts the gut is through dysbiosis – a disruption to the gut microbiome that leads to an imbalance of ‘good’ and ‘bad’ bacteria.

In studies of people with depression, researchers have observed lower levels of good bacteria such as Lactobacillus – which can break down dietary fibre, produce anti-inflammatory short-chain fatty acids and help protect your body from chronic diseases – and increased levels of harmful inflammatory bacteria like Enterobacteriaceae. 

This bacterial imbalance can have a knock-on effect on overall health, but experts also believe it could feed into depressive symptoms, too. A 2021 study which saw faecal bacteria from humans with depression transplanted into rodents found that those rodents also developed depressive symptoms, demonstrating the bi-directional relationship between the gut and the brain. 

It’s for this reason that studies have investigated the potential role of diet – which is one of the key ways to improve the health of your gut microbiome – in the treatment of depression. One study, known as the SMILES trial, compared the impact of personalised nutrition to counselling on the impact of adults with depression. 

At the end of the study, 32% of those in the nutrition group were considered in remission from their depression, compared to 8% in the counselling group. 

The gut microbiome is responsible for more than just breaking down the food we eat – it plays a role in regulating everything from our immune response to our hormones.

In particular, the gut is responsible for the production of serotonin, also known as the happy hormone. While the serotonin theory of depression (which proposes that low levels of serotonin could drive depressive symptoms) has recently been called into question, some experts believe the impact of the gut on serotonin production could also play a role in depression.

“Changes in the gut microbiome can affect the production of serotonin, 95% of which is made in the gut, as well as our circadian rhythm, which can contribute to poor sleep,” Dr Romayne Gadelrab, a consultant psychiatrist in the South London and Maudsley NHS Foundation Trust and the private mental health centre The Soke, previously told Strong Women. “And that can have a knock-on effect on everything from hormone secretion to appetite regulation.” 

The gut microbiome isn’t the only part of the gut impacted during a depressive episode – the way the gut moves can be impacted too, and this can affect the way we process food and absorb nutrients from food, Dr Gadelrab explains. Typically, gut motility is slowed, which is why so many people with depression also suffer from constipation.

Studies have suggested this slowing has to do with a shortage of serotonin in the gut – which, as we’ve already covered, is the result of disruptions in the gut microbiome), as this neurotransmitter is responsible for the regulation of the enteric nervous system.

This nervous system controls all parts of the digestive process, including how fast or slow the gut moves, so when there is an absence of serotonin, this process becomes inhibited. 

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