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Why the SMILES Trial is a Landmark for Mental Health Nutrition
The idea that food can affect mood isn’t new, but the SMILES trial gave us the first step to a solid answer to the question: Can diet help depression?
Published in 2017, this was the first randomized controlled trial (RCT) to investigate whether a structured dietary intervention could reduce symptoms of clinical depression.
While the Mediterranean diet is often promoted for heart health, the SMILES trial brought attention to its potential role in managing depression.
And while it wasn’t a perfect study (no RCT is), its findings were both hopeful and hard to ignore.
As healthcare professionals increasingly look for collaborative, whole-person approaches, this trial made a compelling case for including dietitians on the mental healthcare team.
It also pushed nutritional psychiatry into a new era: one grounded in rigorous, intervention-based research.
To unpack its significance, I’m using the CRAA(M)P framework, a practical tool for evaluating research with a critical eye. It’s a way to look beyond the headlines and assess how this study holds up: its strengths, limitations, and implications for those of us working in mental health care.
TL;DR: What the SMILES Trial Found
- The SMILES trial was the first RCT to test dietary changes as treatment (not prevention) for clinical depression.
- A modified Mediterranean diet significantly reduced depression scores vs. a control group.
- Nearly 1 in 3 people in the diet group achieved remission, compared to just 8% in the control group.
- The intervention included dietitian-led sessions, motivational interviewing, and food access support.
- Results suggest nutrition is a valid, evidence-based part of depression care, especially when led by trained dietitians.
Evaluating the SMILES Trial Through the CRAA(M)P Test
Currency
The SMILES trial was published in 2017, which may feel dated at first glance. But its findings continue to be supported by emerging research, keeping it highly relevant in conversations around diet and depression.
Relevance
This study doesn’t dance around the question; it tackles it head-on. Can dietary change improve symptoms of depression? For those of us working at the intersection of food and mental health, it’s exactly the kind of research we were waiting for.
Authority
The lead author, Professor Felice Jacka, is a central figure in the field of nutritional psychiatry. She’s a Distinguished Professor, president of the International Society for Nutritional Psychiatry Research, and was awarded the Medal of the Order of Australia for her contributions.
Her work is widely cited, and the rest of the authorship team is equally well-published.
Accuracy
The trial references 48 sources and includes a balanced literature review that acknowledges both supporting and conflicting evidence. It avoids cherry-picking, which strengthens its credibility.
Methodology
This was a 12-week, single-blind, parallel-group RCT comparing a modified Mediterranean diet to a social support control (TAU – Treatment As Usual). Both groups met with clinicians at the same frequency, helping to isolate the dietary intervention as the variable of interest.
The study does suffer from a small sample size.
Purpose
There’s no commercial agenda here. Funding came from independent sources and donations, with no product placement or industry involvement.
Study Design and Methods in Detail
Participants
The trial included adults diagnosed with current moderate to severe major depressive episodes that have failed at least one treatment attempt. Participants were carefully screened to reduce confounding diagnoses: those with bipolar disorder, severe physical illnesses, or other major psychiatric comorbidities were excluded.
Intervention
The dietary group received seven individualized sessions with a dietitian over 12 weeks. Sessions incorporated motivational interviewing and mindful eating strategies—an important detail that reflects how behavior change support was baked into the intervention. Participants also received food hampers to help reduce financial and logistical barriers to adherence.
Diet
The prescribed eating pattern was a modified Mediterranean diet based on Australian and Greek dietary guidelines. It emphasized whole grains, vegetables, fruits, legumes, nuts, olive oil, fish, and lean meats, while limiting refined foods, sweets, and alcohol.
- Daily Goals:
- 5-8 servings of whole grains
- 6 servings of vegetables per day
- 3 servings of fruit per day
- 2-3 servings of low-fat, unsweetened dairy
- 1 serving of nuts
- 3 Tbsp olive oil
- Weekly Goals:
- 3-4 servings of legumes
- 2 servings of fish
- 3-4 servings of lean red meat
- 2-3 servings of chicken
- Up to 6 eggs.
- No more than 3 servings of “extras” (sweets, refined cereal, fried food, fast food, processed meats, sugary drinks, and EtOH [beyond 1-2 servings of wine per day])
- Average macronutrient distribution: 18% protein, 40% fat, 37% carb (remaining from EtOH and fiber)
Control Group
The control group received social support sessions with the same visit frequency as nutrition intervention sessions. These sessions were intentionally non-directive and avoided both psychotherapy techniques and nutrition advice. This design helped ensure that the main difference between groups was the dietary component.
Outcome Measures
The primary outcome was change in depression severity, assessed using the Montgomery–Åsberg Depression Rating Scale (MADRS), a validated clinician-rated tool. The study also measured dietary adherence, quality of life (well-being, self-efficacy, and physical activity), physical markers (like weight and blood pressure), and biochemical data (e.g., inflammatory markers, lipids).
Blinding
While participants couldn’t be fully blinded (common in behavioral trials), they weren’t explicitly told the study’s hypothesis, which helped reduce expectation bias. Only the clinicians knew the participants’ assigned group; researchers and statisticians analyzing the data were blinded to group allocation.
SMILES Trial Results: What Did They Find?
The SMILES trial showed a clear benefit of dietary intervention for people with moderate to severe depression. On average, the diet group saw a 7.1-point reduction on the MADRS, compared to 3.9 points in the control group. That 3.2-point difference may sound modest, but it exceeds the commonly accepted threshold for clinical relevance.
In fact, a ≥6-point reduction on the MADRS is often considered a meaningful improvement at the individual level, and nearly a third of participants in the diet group (32%) achieved full remission (defined as a MADRS score below 10).
By comparison, only 8% of the control group reached remission, reinforcing that the dietary changes had more than just a placebo effect.
Interestingly, while diet quality improved significantly only in the intervention group, there were no major shifts in wellbeing or self-efficacy scores, suggesting that the improvements in depression weren’t just due to participants “feeling good” about taking action or receiving support.
Another important finding: food costs actually went down for the intervention group. That challenges the assumption that healthier eating is always more expensive, a key concern for clients and practitioners alike.
Finally, the study found a dose-response relationship: for every 10% increase in dietary adherence, there was a 2.2-point improvement in MADRS scores. That strengthens the case that food wasn’t just part of the overall picture; it was the driver of the results.
Strengths and Limitations
As the first randomized controlled trial to test a dietary intervention for clinical depression, the SMILES trial marked a major step forward for mental health nutrition.
Its rigorous methodology, including a well-matched control group, blinding of researchers and statisticians, and detailed, individualized dietary counseling, gives the results weight. The study also placed its findings in a balanced context, acknowledging limitations in the existing literature rather than overclaiming impact.
Most importantly, SMILES demonstrates how dietitians can play a direct, therapeutic role in mental health care, not just as nutrition educators, but as key contributors to recovery.
That said, no study is perfect, and this one has its caveats.
With only 67 participants and a 12-week timeframe, it’s hard to know how well the findings hold up over time or in more diverse populations. Like many nutrition studies, it relied on self-reported food intake, which can introduce bias and error.
And despite efforts to mask the true hypothesis, expectation effects—especially in a dietary intervention—can’t be fully ruled out.
Some researchers have also raised concerns about recruitment transparency and the use of subjective outcome measures. These critiques are valid but have largely been addressed through author responses and, importantly, replication in later trials, which have shown similar outcomes.
Implications for Clinical Practice
The SMILES trial offers more than just encouraging numbers.
It provides concrete evidence that nutrition can be a powerful tool in the treatment of depression, not just prevention. That makes food not just relevant, but clinically significant in mental health care.
This study reinforces what many of us in the field already see in practice: when individuals improve the quality of their diet, depressive symptoms can improve too, sometimes substantially. And that change doesn’t require perfection, restriction, or expensive superfoods.
It comes from accessible, health-promoting dietary patterns and genuine, supportive counseling.
The trial’s use of motivational interviewing and mindful eating techniques reflects what works in real life.
Changing how someone eats, especially during a depressive episode, requires more than handing out a list of foods to eat or avoid. It takes empathetic, patient-centered care, and that’s where dietitians are uniquely equipped to lead.
Of course, practical challenges remain. Food access, affordability, time, and executive functioning can all be barriers, especially for people living with mental illness. But this study shows those barriers aren’t insurmountable.
With flexible strategies and realistic goals, we can help patients make meaningful progress. And we can do it without adding to their overwhelm.
For therapists, primary care providers, and other members of the care team, SMILES underscores the value of including dietitians as part of a multidisciplinary mental health approach. Nutrition support doesn’t replace psychotherapy or medication, but it can enhance both.
Referring patients to a trained dietitian isn’t just a “nice to have.” It’s evidence-based care.
The Bigger Picture: Biological Mechanisms and Future Directions
While SMILES focused on clinical outcomes, it opens the door to understanding why dietary changes may help improve mood. Several plausible mechanisms are already under investigation, including reduced inflammation, improved antioxidant status, enhanced neuroplasticity, and modulation of the gut–brain axis.
Each of these pathways points to the deep biological connections between what we eat and how we feel. And while we don’t yet know exactly which mechanisms are most influential, the convergence of evidence is growing.
And it’s compelling.
Still, we need more research. Larger trials with more diverse participants, longer follow-up periods, and varied dietary approaches will help clarify how best to use nutrition in treatment.
SMILES was a starting point, not the final word
As the field of nutritional psychiatry continues to grow, so does the need for dietitians who are confident and competent in mental health care. That means not just knowing the science, but being able to navigate complex emotions, co-occurring conditions, and the realities of access and stigma.
This is where our profession has an opportunity, and a responsibility, to lead.
Final Thoughts
The SMILES trial marked a turning point in nutritional psychiatry. It was the first to show that targeted dietary support can improve symptoms of major depression, not just in theory, but in clinical practice.
For dietitians and therapists alike, it’s a call to action. We don’t need to wait for perfect evidence to start integrating nutrition into mental health care; we already have a solid foundation. What we need now is collaboration, continued learning, and a willingness to adapt evidence to real-world contexts.
Nutritional psychiatry is still evolving, but SMILES gave it a running start. It’s up to us to keep that momentum going.
References and Further Reading
- Jacka FN et al. (2017). A randomised controlled trial of dietary improvement for adults with major depression (the SMILES trial). BMC Medicine. Full text available here
- Critiques and responses
- Molendijk M. et al. (2018). The SMILES trial: do undisclosed recruitment practices explain the remarkably large effect?
- Jacka F. et al. (2018). The SMILES trial: an important first step
- Related studies
- HELFIMED trial: Parletta N. et al. (2019). Mediterranean-style dietary intervention supplemented with fish oil improves mental health in people with depression. Nutritional Neuroscience.
- AMMEND trial: Bayes J. et al. (2022). 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
- Analysis of PREDIDEP: Cabrera-Suáreza B. et al. (2023). Mediterranean diet-based intervention to improve depressive symptoms: analysis of the PREDIDEP randomized trial
- A Meta-Analysis!: Firth J. et al. (2019). The Effects of Dietary Improvement on Symptoms of Depression and Anxiety: A Meta-Analysis of Randomized Controlled Trials
- Check out my Resource Library
Want to explore more studies like SMILES?
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Frequently Asked Questions about the SMILES trial
The SMILES trial was the first randomized controlled trial (RCT) to investigate whether dietary changes could improve symptoms of major depressive disorder.
Conducted in Australia, the study showed that participants who received nutritional support and followed a modified Mediterranean-style diet experienced significantly greater reductions in depression symptoms compared to those receiving social support alone.
The trial was led by Professor Felice Jacka and her team at Deakin University’s Food & Mood Centre. It was published in 2017 in BMC Medicine and has become a cornerstone in the field of nutritional psychiatry.
Participants in the intervention group were supported in adopting a whole-foods dietary pattern similar to the traditional Mediterranean diet. This included plenty of vegetables, fruits, whole grains, legumes, nuts, olive oil, and moderate amounts of fish and lean meats, while reducing ultra-processed and sugary foods.
The trial found that dietary changes led to a clinically significant reduction in depression symptoms for many participants. However, it’s important to view nutrition as one part of a multidisciplinary treatment approach, not a standalone “cure” for depression.
The study used the Montgomery–Åsberg Depression Rating Scale (MADRS) to measure changes. The diet group saw significantly greater improvements compared to a social support control group.
Yes. Despite being published in 2017, the SMILES trial remains highly influential. It helped spark a wave of new studies on diet and mental health. Ongoing research is now exploring how dietary patterns may impact depression, which populations benefit most, and how to best integrate nutrition into mental health care.
Source: MoodMattersDietetics.com
