The SMILES trial: what it tells us about diet and depression

Can what we eat affect how we feel? The SMILES trial, published in 2017, was the first randomised controlled trial to test whether improving diet could help support people with major depression.¹ It has helped shape a growing area of research known as nutritional psychiatry. Here is what the study found, and what it helps us understand to date.

The SMILES trial suggests that improving diet may be a helpful addition to depression care. It is not a replacement for medical or mental health treatment.

What was the SMILES trial?

The SMILES trial was run in Australia with 67 adults living with moderate to severe depression, and 56 completed the study. Most were already receiving treatment, such as medication, therapy or both, and continued with it during the trial. All participants had a lower quality diet at the start of the study.¹

Over 12 weeks, participants were placed into one of two groups:

• The diet support group had seven individual sessions with a dietitian, who supported them to follow a modified Mediterranean style diet.
• The social support group had the same number of sessions, but focused on friendly conversation and support instead of diet.

Including a social support group was important, as it helped show whether any benefits came from the diet itself rather than from the extra time and attention.

What did it find?

After 12 weeks, the diet support group had a greater improvement in depression symptoms than the social support group.¹

Of those who completed the study, about 32% in the diet support group reached remission, meaning their symptoms had reduced to a minimal level. This compared with 8% in the social support group.¹

Within the diet support group, people who made greater changes to their diet tended to see greater improvements in their symptoms.¹

A follow up analysis also found that the diet support was likely to be cost effective, with lower overall health care costs than the social support.²

What did the diet look like?

The diet was based on a Mediterranean style of eating, adapted to Australian dietary guidelines.¹ It focused on including more:

• Vegetables and fruit
• Whole grains
• Legumes, such as lentils, chickpeas and beans
• Nuts
• Fish, along with some lean meat, chicken and eggs
• Dairy foods, such as milk and yoghurt
• Olive oil

Participants were also encouraged to have "extras", such as sweets, fried foods, processed meats and sugary drinks, less often. The diet included recommended serves for each food group, and the dietitian supported each person to work towards these in a way that suited them.

What are the limitations?

The SMILES trial is an important study, but it has some limitations to keep in mind:¹

• It was small. With 67 participants, larger studies are needed to confirm the results.
• Participants knew what type of support they were receiving. This means expectations may have played a role in the results.
• More people dropped out of the social support group. This may have affected the comparison between groups.
• Everyone started with a lower quality diet. People who already eat well may not see the same benefit.
• It was short. The study ran for 12 weeks, so we do not know about longer term effects.
• Most participants were also receiving other treatment. The diet was tested as an addition to usual care, not on its own.

What has research found since?

Since SMILES, other studies have looked at similar questions. A 2019 review of 16 trials found that improving diet modestly reduced symptoms of depression, although the studies varied widely in design and the people included.³ A 2022 trial in young men aged 18 to 25 with depression also found that a Mediterranean style diet improved symptoms compared with social support.⁴

Not all research has found a benefit. A large European trial found that a food focused support program did not prevent depression in adults with mild symptoms.⁵ This suggests the effects of diet may depend on who is studied and how the support is delivered.

These findings are encouraging, but this is still a developing area of research. More large, longer term studies are needed.

Why might food affect mood?

Researchers are still exploring how diet may influence mental health. Some possible pathways include:⁶

• The gut brain connection. Fibre rich plant foods feed gut bacteria, which produce compounds that may influence the brain and mood.
• Inflammation. Ongoing, low grade inflammation has been linked with depression in some people. Eating patterns rich in plant foods, fish and olive oil are associated with lower levels of inflammation.
• Nutrients for the brain. The brain relies on a wide range of nutrients to function well.

Much of this research comes from animal and observational studies, so these pathways are promising but not yet fully understood. Diet is also likely to work alongside many other factors, such as sleep, movement, stress and social connection.

What this means in practice

Nutrition is not a cure for depression or a replacement for medical care, but research suggests it may be a helpful tool alongside other support. In the SMILES trial, diet support was added to existing care rather than used in place of it.

Depression can make planning, shopping and cooking feel much harder, so small, realistic changes are often the most sustainable place to start. In the SMILES trial, support was provided by a dietitian, which allowed changes to be tailored to each person.

Anyone struggling with their mental health is encouraged to speak with their GP. In Canada, support is available at any time by calling or texting 988, the Suicide Crisis Helpline.

Want support with nutrition and mood?

Nula Nutrition can support your nutrition alongside your doctor and wider healthcare team. The approach is always evidence based and tailored to you. Registered dietitian consultations are available for women's health in Calgary and online across Alberta, including Canmore, Banff, Edmonton, and Cochrane.

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Please note: This post is intended for general educational purposes only and does not constitute personalised medical or dietetic advice. Please speak with your GP or mental health professional about diagnosis and treatment of depression.

Written by Madison Hows RD APD

References

1. Jacka FN, O'Neil A, Opie R, et al. A randomised controlled trial of dietary improvement for adults with major depression (the 'SMILES' trial). BMC Med. 2017;15(1):23.

2. Chatterton ML, Mihalopoulos C, O'Neil A, et al. Economic evaluation of a dietary intervention for adults with major depression (the "SMILES" trial). BMC Public Health. 2018;18(1):599.

3. Firth J, Marx W, Dash S, et al. The effects of dietary improvement on symptoms of depression and anxiety: a meta-analysis of randomized controlled trials. Psychosom Med. 2019;81(3):265-280.

4. Bayes J, Schloss J, Sibbritt D. The effect of a Mediterranean diet on the symptoms of depression in young males (the "AMMEND: A Mediterranean Diet in MEN with Depression" study): a randomized controlled trial. Am J Clin Nutr. 2022;116(2):572-580.

5. Bot M, Brouwer IA, Roca M, et al. Effect of multinutrient supplementation and food-related behavioral activation therapy on prevention of major depressive disorder among overweight or obese adults with subsyndromal depressive symptoms: the MooDFOOD randomized clinical trial. JAMA. 2019;321(9):858-868.

6. Marx W, Lane M, Hockey M, et al. Diet and depression: exploring the biological mechanisms of action. Mol Psychiatry. 2021;26(1):134-150.