Research update - Diet and depression: exploring the biological mechanisms of action

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Dakota Rhys-Jones - Research Dietitian, 03 December 2020

A recent review paper published by colleagues at Deakin University’s Food and Mood Centre explored diet and depression, and the mechanisms of action behind this. Our recent blog series focuses on mental health, as we know that there is a higher prevalence of anxiety and depression in those with IBS. Here we have our summary of some of the key points.

Mechanisms of action

  • Inflammation and oxidative stress (1)
 Increased levels of inflammation and oxidative stress have been implicated in mental health (2). Oxidative stress describes the imbalance of antioxidants and free radicals in the body. Free radicals are unstable and reactive molecules in the body that can cause damage to our cells and DNA, and antioxidants act to protect the cell by ‘mopping up’ the free radicals.

Stress in all of its forms, from psychological stress to external stressors such as poor diet, smoking and physical inactivity are described to promote those pro-inflammatory states and encourage oxidative stress. Additionally, the review suggests that more than 60% of individuals with depression exhibit excessive cortisol (a stress hormone) production (1).

  •  Obesity (1)

The relationship between obesity and depression is complex – we know that those with depression are more likely to become obese, and those with obesity are more likely to become depressed. A typical ‘western diet’ which is energy dense, high in refined carbohydrates, trans fats, with limited consumption of wholegrains, fruits and vegetables in seen in those with obesity. This promotes pro-inflammatory pathways in the body, and reduced levels of the neurotransmitters serotonin and dopamine have been seen following this diet. These are chemicals that are involved in mood regulation and contribute to feelings of happiness, as well as a number of other biological functions.

  • Gut microbiome (1)

While research regarding the gut microbiome is extremely popular, it is a relatively new field of science with a lot to still uncover, particularly when it comes to depression. There is evidence from animal studies that suggest direct links between diet, the gut microbiota and depression. Markers of poor gut health (such as imbalances of bacterial species) are seen following high calorie or western style diets. Researchers have found that in animals following these diets can prompt symptoms such as anxiety, decreased memory, depression and poorer cognitive flexibility. Pre and probiotics, which are well known for their modulation of the gut microbiome, have been used to reduce these behaviours and improve gut health. While these results are promising, human data are limited, and more studies are needed to translate these findings.

Dietary therapies - the Mediterranean diet

A Mediterranean diet, comprised of plenty of wholegrains, fruit, vegetables, healthy fats such as nuts and olive oil, fish, moderate consumptions of dairy and red meat, and limited processed foods, is considered an ‘anti-inflammatory’ diet (3). In relation to mental health, an important study named the SMILEs trial, was the first randomised control trial to use the Mediterranean diet for those with depression (4). Compared to controls, researchers found those on a Mediterranean diet with depression significantly improved their depressive symptoms (4).

A number of components of the diet may be contributing to the anti-inflammatory effects of this diet. Firstly, a Mediterranean diet is high in antioxidants, in the form of both nutrients and polyphenols. Vitamin C (found in fruits and vegetables) and Vitamin E and polyphenols (olive oil, nuts) are abundant in the Mediterranean diet and behave as potent antioxidants that protect cells from damage (5, 6). Secondly, omega-3 polyunsaturated fatty acids EPA and DHA are well known for their anti-inflammatory properties, which are predominantly found in fish and marine foods. Lastly, the diet is rich in plant-based foods, and is high in fibre, which we know is important to maintain a healthy gut environment.

Relevance for individuals with IBS

Where does all of this information leave someone with IBS who may be following a low FODMAP diet? We know that the Mediterranean diet is abundant in prebiotics, which is essentially synonymous with the term FODMAP and may not be practical for someone with IBS. However, there are plenty of higher FODMAP foods that play a role in the Mediterranean diet that can be eaten safely as part of a low FODMAP diet (i.e. canned legumes or nuts). Check out our app for safe serving sizes, and work with your dietitian if you wish to plan some more Mediterranean style meals, without causing the unwanted symptoms. 


1.            Marx W, Lane M, Hockey M, Aslam H, Berk M, Walder K, et al. Diet and depression: exploring the biological mechanisms of action. Molecular Psychiatry. 2020.

2.            Miller AH. Beyond depression: the expanding role of inflammation in psychiatric disorders. World Psychiatry. 2020;19(1):108-9.

3.            Willett WC, Sacks F, Trichopoulou A, Drescher G, Ferro-Luzzi A, Helsing E, et al. Mediterranean diet pyramid: a cultural model for healthy eating. Am J Clin Nutr. 1995;61(6 Suppl):1402s-6s.

4.            Jacka FN, O’Neil A, Opie R, Itsiopoulos C, Cotton S, Mohebbi M, et al. A randomised controlled trial of dietary improvement for adults with major depression (the ‘SMILES’ trial). BMC Medicine. 2017;15(1):23.

5.            Tapsell LC. Foods and food components in the Mediterranean diet: supporting overall effects. BMC Medicine. 2014;12(1):100.

6.            Traber MG, Stevens JF. Vitamins C and E: beneficial effects from a mechanistic perspective. Free Radic Biol Med. 2011;51(5):1000-13.

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