A Randomised Controlled Trial of Dietary Improvement for Adults With Major Depression (The ‘SMILES’ Trial)

ByCrossFitJune 2, 2020

Question: Can diet, independent of other changes in lifestyle, improve major depression?

Takeaway: Twelve weeks on a higher-quality diet led to significant improvements in depression severity. This small, controlled trial suggests further exploration of diet as a tool to improve mental health, alongside or independent of other tools to treat depression, is warranted.

Observational evidence has suggested a poor diet is associated with increased risk of depression, a high-quality diet is associated with reduced risk, and these effects are independent of reverse causality — e.g., the association between dietary quality and socioeconomic status (1). Data from animal trials has suggested improving dietary quality can ameliorate the symptoms of depression (2), and lifestyle interventions including dietary change have been found beneficial in humans (3). The SMILES trial was a small, randomized trial designed to test the specific impact of dietary change in the treatment of major depression.

Researchers randomized 67 New Zealand residents to one of two arms for the 12-week trial period. One group was prescribed the “ModiMedDiet,” based on Australian dietary guidelines. This diet focused on general improvements in dietary quality and encouraged a balanced diet high in fruits, vegetables, and other high-quality foods (4). Subjects in this group received four weekly dietary support sessions followed by three biweekly sessions. Controls received a “befriending” protocol, in which the same amount of time was spent talking, playing games, and completing similar activities with a member of the trial staff. This “befriending” protocol is commonly used as a control group in psychotherapy trials and is designed to control for differences in time allocated, client expectancy, and therapist interaction between the two groups. All subjects were 18 years or older, diagnosed with major depression, and scored low on a scaled evaluation of dietary quality prior to enrolling (5, 6). Subjects who were bipolar or had a similar condition were excluded. Subjects who were receiving treatment for depression continued treatment throughout the trial.

The primary outcome was improvement in the MADRS scale, a validated, interviewer-rated assessment of 10 items on a 6-point scale, with higher scores indicating more severe depression (7). The dietary intervention group showed an average of 7.1 points greater improvement in MADRS score. Participants also self-reported improvements in symptoms of depression and anxiety.

Figure 1: Mean MADRS scores before and after 12-week dietary and control (social support) interventions. The improvement in MADRS score was significantly greater in the dietary intervention group.

These results suggest a dietary therapy alone can lead to significant improvements in some measures of depression in as little as 12 weeks. These results are consistent with previous research suggesting diet can be an effective tool, both by itself and alongside other therapies, to ameliorate and/or reverse depressive states (8). Subjects following this diet did not lose significant amounts of weight, and no significant improvements were observed in any markers of cardiovascular risk or metabolic health. This suggests these improvements are independent of such factors. Instead, the link between diet and mood may be mediated by changes related to inflammation, oxidation, and the direct effects of diet on brain plasticity (9). Notably, the researchers found the “ModiMedDiet” was cheaper than the average of the diets participants had been following prior to enrollment, which suggests this intervention may be broadly applicable (10).

This trial was small and therefore limited in its ability to detect whether diet has a significant impact on measures of depression severity. Future trials can test the generalizability, robustness, and longevity of these effects. More importantly, given the established link between metabolic disease and depression, as previously reviewed on CrossFit.com, it is plausible that diets leading to greater metabolic improvements may simultaneously lead to greater improvements in depression.


  1. A systematic review and meta-analysis of dietary patterns and depression in community-dwelling adults; Relationship between diet and mental health in children and adolescents: a systematic review; Moving towards a population health approach to the primary prevention of common mental disorders; Nutritional medicine as mainstream in psychiatry; Association of Western and traditional diets with depression and anxiety in women; The association between habitual diet quality and the common mental disorders in community-dwelling adults: the Hordaland Health Study; Dietary pattern and depressive symptoms in middle age; Does reverse causality explain the relationship between diet and depression?; A prospective study of diet quality and mental health in adolescents; Association of the Mediterranean dietary pattern with the incidence of depression: The Seguimiento Universidad de Navarra/University of Navarra follow-up (SUN) cohort
  2. Nutrition, adult hippocampal neurogenesis and mental health
  3. The impact of whole-of-diet interventions on depression and anxiety: A systematic review of randomised controlled trials
  4. Specific dietary prescription: 5-8 servings per day of whole grains, 6 servings of vegetables per day, 3 servings of fruit per day, 3-4 servings of legumes per week, 2-3 servings of low-fat dairy per day, 1 serving of nuts per day, 2 servings of fish per week, 3-4 servings of lean red meat per week, 2-3 servings of chicken per week, up to 6 eggs per week, and 3 tablespoons of olive oil per day, with limits on consumption of all other foods, including snacks and sugary foods and drinks (less than 3 servings per week) and alcohol (less than 2 servings per day).
  5. Dietary screening tool identifies nutritional risk in older adults
  6. A low score on this scale indicates low protein, fiber, and vegetable intake and high intake of sweets, processed meats, and salty snacks.
  7. A new depression scale designed to be sensitive to change
  8. Dietary recommendations for the prevention of depression; Mediterranean diet, traditional risk factors, and the rate of cardiovascular complications after myocardial infarction: final report of the Lyon Diet Heart Study; Royal Australian and New Zealand College of Psychiatrists clinical practice guidelines for mood disorders
  9. Western diet is associated with a smaller hippocampus: A longitudinal investigation; The gut microbiome and diet in psychiatry: Focus on depression; So depression is an inflammatory disease, but where does the inflammation come from?; Oxidative & nitrosative stress in depression: Why so much stress?
  10. Assessing healthy diet affordability in a cohort with major depressive disorder

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