Depression is a common mental disorder, which involves a depressed mood or loss of pleasure or interest in activities for long periods of time.[1] 40% of people with depression also have a long-term condition, such as cardiovascular disease, diabetes or musculoskeletal disorders [2], and, therefore, there is an established link between poor physical and mental health.

Treatment for depression will usually involve a combination of self-help, talking therapy and medication. [3] From a lifestyle perspective, there is good evidence that physical activity can help prevent and reduce symptoms of depression.[4,5] The research on dietary interventions for the treatment of depression is less clear.

There is an abundance of observational data that demonstrates diet quality is a possible risk or protective factor for depression. [6-9] The available evidence suggests that diets higher in fruits, vegetables, legumes, wholegrains and lean proteins are associated with a reduced risk for depression. In contrast, dietary patterns that include high intakes of nutrient-poor processed foods are associated with an increased risk of depression. [6-9] However, these findings do not allow us to determine cause and effect, and given the symptoms of depression, it is reasonable to suggest that depression may contribute towards unhealthy dietary behaviours. Therefore, it is important to look at intervention studies.

What is the evidence?

The most commonly cited randomised control trial investigating improving dietary quality and outcomes in patients with depression is the SMILES Trial. [10] One of the main strengths of this study is that all of the participants had been diagnosed with depression.

The participants were split into two groups: the dietary intervention group, who followed an inclusive, predominantly wholefoods Mediterranean-style diet for 12 weeks and the control group, who received social support. At the end of the study, there was a significant improvement in depression symptoms in the intervention group vs the control group, with a large effect size. These findings were supported by the HELFIMED trial [10], where the intervention group followed a Mediterranean-style diet + fish oil supplement and after six months, there was a significant improvement in depression symptoms in comparison with the control group.

Whilst these findings are interesting, we must be cautious in how we communicate this to the public. In both the SMILES and HELFIMED trials, the intervention groups received a lot of support, which introduces other factors that could influence the results. For example, in the SMILES trial, there was a therapeutic approach, and patients received seven one-hour sessions with a Registered Dietitian, where the professional would use behaviour change techniques, such as goal setting, motivational interviewing and mindful eating techniques.

In both studies, participants received food hampers, which doesn’t reflect the type of support available via healthcare services. In the HELFIMED trial, the intervention group attended cooking classes and had dietitian-led support throughout. Introducing these other factors into the intervention group makes it harder for us to determine whether it was diet alone that influenced the result. Being in a supportive environment, building rapport with a healthcare professional, or learning a new skill may also have influenced the results.

Additionally, there is a clear potential for expectation bias to influence the results, particularly in the SMILES trial. The recruitment advertisements used in this study suggested to participants that food improves mood, and so they entered the study with a preconceived idea that dietary intervention would improve symptoms. Interestingly, there was a much higher dropout in the control group (nine vs two) in comparison with the dietary group. This could potentially be explained by the expectation of participants.

More evidence required…

Findings reported in a meta-analyses of the available RCT evidence [11] demonstrate a small but significant effect of dietary intervention on depression symptoms. However, 15 of the 16 trials included in this study are in non-clinical populations. Therefore, more evidence is needed to determine whether diet quality as an isolated intervention can have a clinically significant improvement on symptoms in patients diagnosed with depression.

End note

In summary, a healthy diet is important for overall health, but it is not an appropriate stand-alone treatment for depression.

Given the relationship between poor physical health and mental health disorders, it is important that mental health practitioners encourage and support health-promoting behaviours in patients with depression.

Due to the limitations in the available evidence, there is a need for well-designed RCTs to fully understand the role of diet in the treatment of depression.