The TANDIM trial published in the Alimentary Pharmacology & Therapeutics journal delivered some exciting findings relating to the Mediterranean Diet in people who have irritable bowel syndrome as well as symptoms of common mental disorders such as anxiety and/or depression.

Irritable bowel syndrome (IBS) is a common and debilitating condition characterised by abdominal pain and abnormal bowel habits such as altered stool frequency and/or consistency (e.g. diarrhoea and/or constipation). IBS and mental health symptoms are commonly found together, and this is not surprising given that the bidirectional gut-brain connection is now well established. In fact, almost 40% of people with IBS also have anxiety, and almost 20% with IBS have depression.

IBS symptoms can precede or come after the development of mental health symptoms. For example, people experiencing IBS may feel anxious about the unpredictability or severity of their bowel symptoms, which can lead to alterations in mood. On the other hand, in a large proportion of people, anxiety or depression symptoms are present and gut symptoms develop over time.

What’s causing the link between IBS and anxiety/depression symptoms?

The “gut-brain axis” describes the bi-directional communication network between our gut and brain. Abnormalities in this communication system explains the link between gut and mood symptoms. It explains why psychological therapies (such as anti-depressants and mindfulness) are proven ways to reduce IBS symptoms.

Dietary changes are also a key way that people with IBS can manage their symptoms. In particular, a low FODMAP diet, a diet that restricts specific types of carbohydrates from the diet, is recommended and used widely across the world.

The low FODMAP diet is very effective for reducing IBS symptoms for many people, however, there may be a number of reasons why it may not be effective for everyone, including those with anxiety or depression.

The low FODMAP diet involves restriction of prebiotic carbohydrates such as fructans and galacto-oligosaccharides in the initial stage of the diet. This has been shown to reduce beneficial Bifidobacterium in the gut. Interestingly, lower abundance of these bacteria have also been implicated in mental health disorders. The restrictive phase of the low FODMAP diet may also lead to poorer diet quality, which may be a drawback for those with co-occurring anxiety or depression given the evidence that a high quality diet may protect our mental health.

The Mediterranean diet

On the other hand, a Mediterranean diet has broad benefits for physical and mental health, does not focus on restriction, and may have beneficial effects on the gut microbiome. In many ways, the Mediterranean diet is similar to the Australian Dietary Guidelines, however the Mediterranean diet is higher in fruits, vegetables, wholegrains and healthy fats from nuts, seeds and olive oil. Protein sources predominately come from legumes, fish and white meat, whereas Australian Guidelines do not recommend eating specific types of protein.

Interestingly, a Mediterranean Diet is rich in FODMAP-containing foods such as onion, garlic and legumes. So, if someone was to switch to a strict Mediterranean Diet rapidly, this could exacerbate symptoms. Therefore, the important primary aim of this study was to assess if people with IBS and anxiety and/or depression could actually follow a Mediterranean diet.

The study 

In this study, adults with diagnosed IBS who had mild or moderate anxiety and/or depressive symptoms were recruited to a 6 week study. Participants were randomly allocated to receive Mediterranean diet counselling from a specialist dietitian or to continue their usual diet. Those in the Mediterranean diet group were provided individualised advice weekly for the duration of the trial.

Participants were encouraged to slowly adopt the Mediterranean diet through self-selected dietary goals. Some participants entered the trial already following a low FODMAP diet, and so some goals were related to reintroducing high FODMAP Mediterranean foods into their diet.

At 6 weeks, the Mediterranean Diet Adherence Screener score was higher in the Mediterranean diet group than controls, and there was a greater score increase than controls, demonstrating Mediterranean diet feasibility. Surprisingly, 83% of those in the Mediterranean diet group had a clinically significant improvement in their IBS symptoms, compared to only 37% in the control group. Additionally, those in the diet group had lower anxiety and depressive symptoms compared with the control group, and the degree of Mediterranean diet score improvement was associated with mental health symptom improvement. These findings are in line with our previous research in people with a clinical diagnosis of depression showing Mediterranean diet intervention can improve symptoms of depression.

There are many ways in which the Mediterranean diet could influence gut and mental health symptoms. For example, components of the diet such as the type/amount of fibre, beneficial plant chemicals and fatty acids, may beneficially affect the gut microbiome and the metabolites it produces, both comprising key communication routes between the gut and the brain via the gut-brain axis. The diet may also have anti-inflammatory effects, which could be important in IBS, anxiety and depression, as there is evidence that a proportion of people with these conditions have background low grade inflammation.

It’s important to note that this is a relatively small study, and that the study aim was to investigate whether people with IBS and anxiety/depression can follow a Mediterranean Diet. Therefore, we still need further larger studies to conclusively show the Mediterranean diet can improve gut and mental health symptoms in this population. However, it suggests the Mediterranean diet has promise as an alternative for people with IBS.

Importantly, just as for any other major dietary change, people should ideally be supported by a dietitian who can step you through with personalised advice.

If you think you have IBS, speak to your General Practitioner (GP) to rule out other gastrointestinal conditions such as coeliac disease or inflammatory bowel disease.

Written by Sophie Mahoney and Dr Heidi Staudacher.

The TANDIM trial was supported by Dr Heidi Staudacher’s Alfred Deakin Postdoctoral Research Fellowship, as well as seed funding provided by IMPACT Institute, Deakin University.