Mediterranean diet

A poor-quality diet with a high intake of processed and ultra-processed foods reduces psychological resilience and is a risk factor for stress and stress-related disorders. The Mediterranean diet, consisting of fruit, vegetables, and fish, is widely acknowledged as one of the best nutritional approaches for improving health and longevity (Godos et al. Reference Godos, Grosso, Ferri, Caraci, Lanza, Al-Qahtani, Caruso and Castellano2023). In recent years, evidence for such beneficial effects has extended beyond metabolic and cardiovascular health to include the brain and mental health. At a population level, adherence to a Mediterranean diet is associated with a lower risk of depression and cognitive disorders (Butler and Morkl Reference Butler and Morkl2023). In people suffering from depression, switching to a Mediterranean-style diet is an effective treatment strategy. The Mediterranean diet is a safe and sustainable dietary pattern which should form part of the toolkit of any mental health professional wishing to incorporate nutritional approaches.

Overall, patients with depression have a poor quality diet, which may play a role in the pathophysiology. There is a demonstrated a significant correlation between poor diet and severity of depressive symptoms (Loughman et al. Reference Loughman, Staudacher, Rocks, Ruusunen, Marx, Apos, Neil and Jacka2021). A recent study from Australia used a randomised controlled trial design to investigate the efficacy of a dietary programme for the treatment of major depression (Jacka et al. Reference Jacka, ONeil, Opie, Itsiopoulos, Cotton and Mohebbi2017). Structured dietary support, focusing on improving diet quality using a modified Mediterranean diet, was compared to a social support control condition. Sixty-seven patients were recruited, fulfilling the criteria for major depression and scoring 75 or less out of a possible score of 104 on a Dietary Screening Tool. This score indicates a poor baseline diet. If patients were on antidepressant medication or undergoing psychotherapy, they were required to be on the same treatment for at least 2 weeks prior to study entry. The dietary intervention group showed a significantly greater improvement in depression scores between baseline and 12 weeks than the social support control group. Overall, the results of this trial suggest that improving diet may be a useful strategy for treating depression or at least as an adjunctive to conventional therapies. The large effect size may partially be due to expectation bias. However, meta-analyses of randomised controlled trials of dietary interventions for depression and anxiety conclude that such interventions have a significant positive effect.

Dietary components

From a dietary viewpoint, the predominantly plant-based nature of the Mediterranean diet makes it abundant in antioxidants, polyphenols, and fibre, and it is low in saturated fats and animal proteins (Meiners et al. Reference Meiners, Ortega-Matienzo, Fuellen and Barrantes2025). Polyphenols are a large family of naturally occurring organic compounds abundant in plants, giving fruit and vegetables their colour. They are present in all the main foods of the Mediterranean diet: olive oil, nuts, red wine, legumes, vegetables, fruits, and whole-grain cereals. Polyphenols have antioxidant and anti-inflammatory effects, and it is thought that long-term consumption may protect against a wide variety of chronic diseases. While saturated and trans-fats are considered unhealthy fats, unsaturated fats (monounsaturated or polyunsaturated) are recognised as having multiple health benefits. Olive oil is a rich source of monounsaturated fatty acids, while nuts and fish provide a plentiful supply of polyunsaturated fatty acids (PUFAs), especially omega-3 and omega-6. An essential feature of the Mediterranean diet regarding fat intake is the relative proportion of omega-3 to omega-6 PUFAs. The Western diet typically has a high omega-6-to-omega-3 ratio. The average Western diet has an omega-3: omega-6 ratio of up to 1:20, while the recommended ratio is much lower, 1:4 or less. Increasing omega-3 PUFAs is regarded as essential for maintaining stress resilience. We have little or no capacity to produce PUFAs. We are entirely dependent on a dietary supply. The brain is a fatty organ, and omega-3 plays an important structural role. It is also known to increase gut microbial diversity, which has positive health benefits. Fish is the primary source of omega-3 and is an essential dietary component for anyone wishing to maintain good mental health.

Diet has a direct impact on the brain and indirectly affects brain function through gut microbes, an axis known as the brain-gut-microbiota. Until recently, gut microbes were considered commensal. We fed them, but they had little impact on our physiology. We now know this is not the case. Gut microbes are essential in regulating brain function, and the most important determinant of gut microbes is diet. Microbes communicate with the brain through multiple parallel channels, which include the vagus nerve. Interestingly, many psychotropic drugs negatively impact the gut microbiota, most notably atypical antipsychotics. Depressed individuals not only show a decrease in microbial diversity but also often activation of the hypothalamic-pituitary-adrenal axis. We showed that patients attending a Cork clinic with a current episode of depression had decreased gut microbial diversity, and when such a microbiota was transplanted into rodents, they exhibited depression-type behaviours and an alteration in tryptophan metabolism (Kelly et al. Reference Kelly, Borre, O.’ Brien, Patterson, El Aidy, Deane, Cryan and Dinan2016). The microbiota is involved in controlling inflammatory responses. Certain bacteria can reduce pro-inflammatory cytokines in the bloodstream (Zamora-Pineda et al. Reference Zamora-Pineda, Kalinina, Osborne and Knight2022), which may benefit cases of depression where these cytokines are elevated. Previously, tryptophan, the building block for serotonin, was thought to come entirely from the diet. However, it has now been established that gut microbes, such as bifidobacteria, can produce tryptophan and thus play a role in regulating serotonin. The gut microbiota produces hundreds of molecules which the brain and other organs require. We cannot metabolise fibre, but microbes can, producing short-chain fatty acids such as butyrate, propionate, and acetate (Tapping Reference Tapping1996). Butyrate is a potent histone deacetylase inhibitor (HDAC) and can thus influence gene activity in the brain. A low-fibre diet will be associated with reduced short-chain fatty acid production, a situation associated with multiple health risks, including depression.

An EU-funded consensus report focused on the optimal diet for individuals with a history of depression or those who are currently depressed (Dinan et al. Reference Dinan, Stanton, Long-Smith, Kennedy, Cryan, Cowan, Cenit, van der Kamp and Sanz2019). The statement stressed the importance of fresh fruit and vegetables, fermented foods, nuts, and fish. The recommendations are worthy of consideration.