Can Diet Help Treat Depression? What the SMILES Trial Found

Originally published 2017-03-04.

Yes, there is good evidence that improving your diet can help with depression, at least as part of wider treatment. The best-known study is the SMILES trial, run by Deakin University in Australia and published in BMC Medicine in 2017. After 12 weeks of help from a dietitian, 32% of adults with major depression reached remission, compared with 8% of people who received social support instead. It was a small trial, and it does not show that food can replace therapy or medication, but it changed how researchers think about food and mood.

Below we explain what the trial did, what it found, what it cannot tell us, and what sensible, practical steps follow from it.

Why diet and depression are worth studying

Depression is one of the most common health problems in the world. The World Health Organization estimates that about 322 million people live with it, around 5% of adults, with women affected more often than men. It is a leading cause of disability, and many people do not respond fully to the first treatment they try.

Long before SMILES, large observational studies across many countries had noticed a pattern: people who ate more vegetables, fruit, whole grains, fish and olive oil tended to have lower rates of depression, while diets heavy in processed food, sugar and refined grains went with higher rates. The trouble with such studies is that they cannot show cause and effect. Feeling low can make people eat worse, rather than the other way round.

That is why the SMILES team set out to run a randomised controlled trial, the kind of experiment that can test whether changing diet actually changes symptoms. Professor Felice Jacka, who led the work as director of Deakin's Food and Mood Centre, described it as the first such trial to test whether improving diet quality could treat clinical depression.

How the SMILES trial worked

SMILES stands for "Supporting the Modification of lifestyle In Lowered Emotional States". The researchers recruited 67 adults with moderate to severe major depression who also had a fairly poor diet. Many were already being treated: participants could stay on antidepressants or continue psychotherapy, as long as their treatment had been stable beforehand.

The volunteers were split at random into two groups:

• Dietary support (33 people): seven one-hour sessions with a clinical dietitian over 12 weeks. They were coached towards a modified Mediterranean-style pattern the team called the ModiMedDiet. There was no calorie counting and no focus on weight loss.
• Social support, or "befriending" (34 people): seven one-hour sessions of the same length, spent chatting or playing games with a trained person. This controlled for the benefit of simply having regular, friendly attention.

Depression was measured with the Montgomery-Åsberg Depression Rating Scale (MADRS), a standard clinical tool, by assessors who did not know which group each person was in.

What people in the diet group ate

The ModiMedDiet was built from familiar, affordable foods rather than supplements or exotic "superfoods". Participants were encouraged to eat more of:

• vegetables and fruit, every day and in variety
• whole grains and legumes such as beans, lentils and chickpeas
• fish, plus modest amounts of lean red meat and chicken
• olive oil as the main fat
• raw, unsalted nuts
• eggs and some dairy

At the same time, they were asked to cut back on so-called extras: sweets, refined cereals, fried and fast food, processed meats and sugary drinks. By the end, the diet group was eating more from six of the eleven recommended food groups and less junk food than at the start.

What the results showed

After 12 weeks, depression scores fell considerably more in the dietary support group. The average difference between the groups on the MADRS was about 7 points, which is clinically meaningful. Remission, defined as a MADRS score below 10, was reached by 10 of 31 people (32.3%) in the diet group who were assessed, against 2 of 25 (8.0%) in the befriending group.

Put another way, roughly one extra person reached remission for every four who received dietary support. Other measures of anxiety, depression and overall illness severity also improved more in the diet group. Because the programme never asked anyone to diet for weight loss, the improvement cannot simply be put down to people slimming down and feeling better about their bodies.

The limits of a single small trial

SMILES was a carefully designed study, but it has real weaknesses, and the authors acknowledged them:

• Small numbers. With 67 people enrolled and only a dozen remissions in total, a few individuals could swing the result.
• No blinding for participants. People knew whether they were being coached on diet, so hope and expectation may have played a part.
• Selected volunteers. Everyone started with a poor diet, and the diet group's eating was worse at baseline. The benefit might be smaller for people who already eat well.
• Short follow-up. Twelve weeks says nothing about whether improvements last.

Later trials have produced broadly encouraging but mixed results, and reviews generally conclude that diet is a promising add-on rather than a stand-alone cure. That is a more modest message than the "proven treatment" headlines that circulated in 2017, but it is still good news, because diet is something people can change.

Where antidepressants and therapy fit in

Some coverage at the time framed the trial as evidence against medication. That is not what it showed: many participants were on antidepressants throughout. The WHO lists psychological therapies, such as cognitive behavioural therapy and behavioural activation, as first-line treatment, and recommends antidepressants alongside therapy for moderate and severe depression.

A few common claims deserve a calm correction. Antidepressants are not addictive in the way alcohol or opioids are, although stopping them suddenly can cause withdrawal-like discontinuation symptoms, which is why doses should be reduced gradually with medical advice. Regulators such as the US Food and Drug Administration do carry a warning about a possible rise in suicidal thoughts in young people starting these drugs, which is why close follow-up in the first weeks matters. Untreated depression is itself dangerous.

If you are struggling, or thinking of changing or stopping any treatment, talk to a doctor first.

Practical takeaways

Why might food affect mood at all? Researchers are investigating several routes, including inflammation, the gut microbiome and the nutrients the brain needs to build and maintain nerve cells. None is fully proven, but together they make the link biologically plausible. For everyday life, the lessons are simple:

• Build meals around vegetables, legumes, whole grains and olive oil, with fish a couple of times a week.
• Treat sweets, fast food and sugary drinks as occasional rather than daily.
• Keep regular eating and sleeping habits, cut down on alcohol and move a little every day; the WHO suggests even a short walk helps.
• Ask your doctor about a referral to a dietitian if changing how you eat feels overwhelming. In SMILES, the coaching was part of what worked.

Food will not fix everything, and nobody should feel blamed for being depressed. But for many people, a better plate is one more tool, alongside proper care, that can help them feel better.

Sources: The Mental Elf, 27 March 2017: Diet and depression, the SMILES trial; Jacka et al., BMC Medicine, 2017: A randomised controlled trial of dietary improvement for adults with major depression; World Health Organization: Depression fact sheet; The Mind Unleashed, which first shared this story.