“Let food be thy medicine, and medicine be thy food” – Hippocrates
The links between nutrition and physical health have long been known. For example, you are most likely aware that a healthy diet protects against non-communicable diseases such as diabetes, heart disease, stroke and cancer (World Health Organization, 2020). However, the association between nutrition and your mental health is less well known and understood. A new scientific field called nutritional psychiatry is beginning to show that “what we stick in our mouths matters to our mental health”, according to one of the field’s pioneers, Professor Felice Jacka (Fleming, 2019).
Jacka explains that “bloggers and self-styled dietary experts” have “brought nutrition research into disrepute” (Fleming, 2019). The aim of her research, and those of other colleagues in the field, is to apply scientific rigor to the many questions we all share regarding our diets, mental wellbeing and brain health. We will now take a look at a few of the key nutritional psychiatry studies.
Nutrition and mental health: Diet and depression meta-analysis
A meta-analysis pools results from multiple studies and is thus a powerful method of understanding the research in a particular field. A meta-analysis published in 2014 explored whether there was a correlation between dietary patterns and depression in adults (Lai et al., 2014). The authors identified 13 relevant studies involving more than 100,000 patients. When the data from all of this research was pooled, two distinct dietary patterns emerged – a ‘healthy diet’ and a ‘Western diet’. Using statistical processes, the authors concluded that people eating a healthy diet reduced their chance of experiencing depression by 16% (1 in 6).
This is quite remarkable given how serious depression is and that it can be difficult to treat. The mainstays of treatment are medication and psychotherapy (talk therapy). However, around 4 in 10 people don’t respond to antidepressants after a 12-week trial (Henssler et al., 2018) and more than half of patients receiving psychotherapy don’t experience a significant reduction in their symptoms (Cuijpers et al., 2021).
Nutrition and mental health: Cause or effect?
What the meta-analysis clearly established is that diet and mental health are correlated. However, the question it couldn’t answer is whether depression caused people to have a poor diet, or an unhealthy diet was an underlying cause of the depression. Scientists often say ‘correlation is not causation’ to make it clear that finding links (correlations) between phenomena is not the same as uncovering the cause. For example, if A and B are found to be linked, A could cause B, B could cause A, or some unknown factor (let’s call it C) could be the underlying cause of both A and B.
We know that mental and brain health are impacted by a complex mix of “biopsychosocial” factors. Biological factors relate to genes and medical conditions; psychological factors include personality traits and self-esteem; and social factors cover influences like education, employment, income, neighborhood and access to healthcare (Bruce, 2021; Food and Mood Centre & Deakin University, n.d.; Telloian & Grohol, 2021). Environmental factors including childhood abuse and significant life events also play a role.
The value of dietary research is that it relates to a modifiable lifestyle factor; we can’t change our genes, our personality, our past, and some aspects of our present circumstances, but we can certainly change our lifestyle behaviors such as the way we eat and our level of physical activity.
Professor Jacka became particularly interested in causation because if it turns out that poor nutrition is a cause of depression (rather than depression causing poor nutrition), then diet can be used to treat and prevent mental health conditions.
The SMILES trial: An overview
Jacka and colleagues ran the first randomized controlled trial (the gold standard of research designs) in the world investigating the question “If I improve my diet, will my mood improve?” The SMILES (Supporting the Modification of lifestyle In Lowered Emotional States) trial was published in 2017 (Jacka et al.).
SMILES was a 12-week study involving 67 people with moderate-to-severe, medically diagnosed depression. Participants were randomly divided into two groups; 33 received a diet intervention and 34 received social support (the control group). All patients continued with their usual treatment for depression (medication and/or psychotherapy) making the dietary changes or social support an adjunct (add-on) treatment. All participants completed a baseline assessment prior to starting the study that covered physical health (including a blood test), lifestyle, education, occupation and income. After 3 months, participants attended a follow up appointment where they completed the same measures as the baseline assessment.
Participants in the diet intervention received seven individual support sessions with a dietician. The goal of these sessions was to teach them about the recommended diet, a modified version of the Mediterranean diet, and to encourage adherence. Participants were provided with recipes, meal plans and a sample hamper containing the main components of the diet.
Compliance with the diet was calculated using a measure developed by the researchers called the Modi/MedDiet score, and depressive symptoms were measured using a standard psychiatric tool called the Montgomery-Asberg Depression Rating Scale (MADRS).
The SMILES trial: Interpreting results
So, what were the results? Participants in the dietary intervention group had a much greater reduction in their depressive symptoms over the 12-week study period compared with those in the social support group. This was calculated by comparing the follow-up and baseline MADRS scores. Importantly, these results were independent of changes in physical activity and were closely related to the extent of dietary change. In other words, those who improved their diet the most (as measured by the Modi/MedDiet score) experienced the greatest benefit in relation to their depression.
Even more impressive is the fact that almost a third (32%) of the participants in the diet group achieved remission of their depression compared with only 8% of those in the control group. Remission of depression – by changing what they put in their mouth!
The SMILES trial is a landmark study because it demonstrated the direction of causality. High quality nutrition caused an improvement in mental health. Here is how the authors summarized the implications of this finding:
“The results of this trial suggest that improving one’s diet according to current recommendations targeting depression…may be a useful and accessible strategy for addressing depression in both the general population and in clinical settings” (Jacka et al., 2017).
Or to return to the original question, “If I improve my diet, will my mood improve?”, the definitive answer from the SMILES trial is “Yes.”
Interested in mental health? Learn more about why having mental health literacy matters.
- Bruce, D. F. (2021). Causes of Depression. WebMD. Retrieved 7 October 2022 from https://www.webmd.com/depression/guide/causes-depression
- Cuijpers, P., Karyotaki, E., Ciharova, M., Miguel, C., Noma, H., & Furukawa, T. A. (2021). The effects of psychotherapies for depression on response, remission, reliable change, and deterioration: A meta-analysis. Acta Psychiatrica Scandinavica, 144(3), 288-299. https://onlinelibrary.wiley.com/doi/10.1111/acps.13335
- Fleming, A. (2019). Nutritional psychiatry: can you eat yourself happier? The Guardian. Retrieved 7 October 2022 from https://www.theguardian.com/food/2019/mar/18/can-you-eat-yourself-happier-nutritional-psychiatry-mental-health
- Food and Mood Centre, & Deakin University. (n.d.). Food and Mood: Improving Mental Health Through Diet and Nutrition. FutureLearn. Retrieved 7 October 2022 from https://www.futurelearn.com/courses/food-and-mood
- Henssler, J., Kurschus, M., Franklin, J., Bschor, T., & Baethge, C. (2018). Trajectories of Acute Antidepressant Efficacy: How Long to Wait for Response? A Systematic Review and Meta-Analysis of Long-Term, Placebo-Controlled Acute Treatment Trials. J Clin Psychiatry, 79(3). https://doi.org/10.4088/JCP.17r11470
- Jacka, F. N., O’Neil, A., Opie, R., Itsiopoulos, C., Cotton, S., Mohebbi, M., Castle, D., Dash, S., Mihalopoulos, C., Chatterton, M. L., Brazionis, L., Dean, O. M., Hodge, A. M., & Berk, M. (2017). A randomised controlled trial of dietary improvement for adults with major depression (the ‘SMILES’ trial). BMC Medicine, 15(1), 23. https://doi.org/10.1186/s12916-017-0791-y
- Lai, J. S., Hiles, S., Bisquera, A., Hure, A. J., McEvoy, M., & Attia, J. (2014). A systematic review and meta-analysis of dietary patterns and depression in community-dwelling adults. The American Journal of Clinical Nutrition, 99(1), 181-197. https://doi.org/10.3945/ajcn.113.069880
- Telloian, C., & Grohol, J. M. (2021). Depression Causes and Risk Factors. PsychCentral. Retrieved 7 October 2022 from https://psychcentral.com/depression/depression-causes#causes
- World Health Organization. (2020). Healthy diet. Retrieved 7 October 2022 from https://www.who.int/news-room/fact-sheets/detail/healthy-diet