SMILES Trial: A Depression Diet Blueprint

When we think about treating depression, diet isn't always the first thing that comes to mind. However, the SMILES trial, an Australian study published in 2017, changed the way we view the role of food in mental health. This groundbreaking study was one of the first randomized controlled trials to show that a healthier diet could significantly improve depression symptoms.

Using the results of this study, we can create a so called blueprint for a”depression diet," using food as a natural tool to improve mental health.

What Is the SMILES Trial?

The SMILES trial (Supporting the Modification of Lifestyle in Lowered Emotional States) aimed to see if a Mediterranean-style diet could help people with moderate to severe depression (1).

Researchers divided participants with depression into two groups:

1. Dietary Support Group: Participants in this group received nutritional counselling and guidance to follow a modified Mediterranean diet rich in whole foods like vegetables, fruits, whole grains, legumes, nuts, and fish.

2. Social Support Group: This group received social support sessions but no dietary guidance.

The trial lasted 12 weeks, and researchers assessed changes in depressive symptoms at the end.

The Results: Mood Follows Food

At the end of the trial, participants who followed the Mediterranean diet saw a significant improvement in their depression symptoms. In fact, about 32% of those in the dietary support group achieved remission vs. only 8% of participants in the social support group. What's more interesting, is that these improvements in depressive symptoms were independent of changes in weight, BMI or physical activity. This highlights the role of dietary improvement as a practical and powerful strategy for treating major depressive episodes.

What did participants eat for better mood? A depression Diet Blueprint

1. Vegetables (5-8 servings/day)

Vegetables are rich in vitamins, minerals, antioxidants, and fiber, all of which support mental well-being by protecting the brain from oxidative stress, reducing inflammation and improving gut health. Vegetables such as leafy greens, are also rich in B vitamins like folate, which are important players in the formation of mood-regulating brain chemicals called neurotransmitters (2).

One serving of vegetables is typically defined as:

• 1/2 cup of cooked vegetables (such as steamed broccoli or sautéed spinach)

• 1 cup of raw vegetables (such as leafy greens or chopped carrots)

2. Fruit (3 servings/day)

Fruits are rich in vitamins like vitamin C, antioxidants, and fiber, which help reduce oxidative stress and inflammation—key factors that contribute to depression. Certain fruits, like berries, are particularly known for their mood-enhancing properties due to their high levels of flavonoids, which have been shown to improve cognitive function and reduce symptoms of anxiety and depression.

One serving of fruit is typically defined as:

• 1 medium-sized piece of fruit (such as an apple, banana, orange, or pear)

• 1/2 cup of fresh, frozen, or canned fruit (without added sugar)

• 1/4 cup of dried fruit (such as raisins or apricots)

3. Whole Grains (5-8 servings)

The study encouraged high-fiber whole grains including foods such as oats, quinoa, brown rice, and whole wheat. Whole grains offer a slow release of blood sugar, which can help stabilize mood and energy levels. Check out a handy guide for navigating the glycemic index of foods here.

One serving of whole grains is typically defined as:

• 1/2 cup of cooked whole grains, such as brown rice, quinoa, barley, or oats

• 1 slice of whole grain bread

• 1/2 cup of whole grain pasta

• 1 small whole grain tortilla

• 1 ounce (about 1/4 cup) of whole grain cereal or granola

  
  

4. Legumes (3-4 servings/week)

Legumes, such as beans, lentils, and chickpeas, are high fermentable fibers. Fermentable fiber refers to a type of fiber that our gut bacteria can break down, or "ferment," to produce beneficial compounds called short-chain fatty acids (SCFAs). These SCFAs, particularly butyrate, support gut health by reducing inflammation and strengthening the gut lining. A healthy gut is linked to better mental health, as gut bacteria communicate with the brain through the "gut-brain axis." Bacteria in our gut also make neurotransmitters like GABA and serotonin, which play key roles in our mental and emotional health.

One serving of legumes is typically defined as:

• 1/2 cup of cooked beans, lentils, or peas (such as black beans, chickpeas, kidney beans, or lentils)

• 1/4 cup of dried legumes (before cooking)

5. Low-fat and unsweetened dairy foods (2-3 servings/day)

Dairy is a rich source of calcium, which is crucial for nerve function and the regulation of neurotransmitters that affect mood and stress levels. Dairy also contains vitamin D, deficiencies in which has been linked to depressive symptoms.

One serving of dairy typically looks like:

• 1 cup (8 ounces) of milk or unsweetened plant-based milk (such as almond, soy, or oat milk, preferably fortified with calcium and vitamin D)

• 1/2 cup of plain, unsweetened yogurt

• 1 ounce (about 1 slice or 1 small wedge) of cheese

6. Raw unsalted nuts (1 serving per day)

Nuts are rich in omega-3 fatty acids, antioxidants, B vitamins and magnesium. Omega-3s, especially in walnuts, support brain cell function and reduce inflammation, while antioxidants protect against oxidative stress linked to mood disorders. Nuts, particularly almonds, also provide B vitamins, like folate and B6, which are essential for neurotransmitter production, including serotonin and dopamine. Moreover,  nuts like Cashews, almonds, and peanuts are good sources of magnesium, a mineral essential for regulating the nervous system and supporting the body's stress response.

A typical serving size of nuts is about 1 ounce (28 grams), which roughly equals:

• 1/4 cup of whole nuts (like almonds, cashews, or walnuts)

• 1 small handful of nuts

7. Fish (2x/week)

Fish, particularly fatty fish like salmon, anchovies, herring, mackerel and sardines, play a key role in reducing depression and supporting mental health due to their high levels of omega-3 fatty acids (particularly EPA and DHA). These omega-3s support brain function, reduce inflammation and improve neurotransmitter function.

8. Lean red meat (3-4x/week) and chicken (2-3x/week)

As mentioned in my recent blog post on high protein diets for depression, protein sources like lean meat provide the building blocks for creating neurotransmitters. Therefore, a deficiency in protein leads to an imbalance in neurotransmitter levels and consequently, depression symptoms.

A typical serving of protein is generally considered to be around:

• 3-4 ounces (85-113 grams) for meat, poultry, or fish

9. Eggs (up to 6 per week)

Eggs are rich in choline. Choline is crucial for producing acetylcholine, a neurotransmitter that supports learning and memory. Due to its role in learning, inadequate choline intake can lead to symptoms like poor concentration, forgetfulness, and memory loss - symptoms often present with depression. Given that the body produces only small amounts of choline, dietary sources like eggs are necessary to meet daily needs for optimal mental health and brain function.

10. Olive oil (3 tbsp/day)

Olive oil is very high in monounsaturated fats. The monounsaturated fats, especially oleic acid, help reduce inflammation in the brain, which is often linked to depression.

Which foods did participants limit?

Processed foods and sugars including sweets, refined sugars, fried foods, fast food, processed meat and sugary drinks (max 3x/week)

Processed foods and sugar can worsen mental health by causing inflammation and disrupting blood sugar levels. Chronic inflammation releases chemicals called cytokines, which can cross the blood-brain barrier and can interfere with the production of neurotransmitters like serotonin and dopamine. As a result, excess intake of highly refined foods are associated with mental health disorders, such as depression. Additionally, sugar causes spikes and crashes in blood sugar, leading to energy fluctuations that can negatively affect mood and cognitive function. While The SMILES trial suggests limiting certain foods to help support mental health and manage depression, remember it’s not about total elimination—moderation is key. The focus is on mindful, moderate consumption rather than strict restriction

How to build a SMILES-inspired "happy plate"

step 1:

• Non-starchy vegetables: Fill half of your plate (approx 2 cups) with colorful, non-starchy vegetables, like leafy greens, bell peppers, and tomatoes.

step 2:

• Lean or plant-based protein: Reserve a quarter for a lean protein (approx the size of your palm), like chicken, salmon, or a serving of beans or lentils. As a guide, aim for 20-30g of protein at a meal and 10-15g of protein at a snack. Use this hand pocket guide to know how much protein common foods contain.

step 3:

• High fiber, unrefined carbs: Use a quarter of your plate (approx 1/2 cup) for whole grains, such as quinoa, brown rice, and barley OR a fruit like an apple or orange OR a starchy vegetable like potatoes, corn and cassava.

step 4:

• Healthy Fats: Add a drizzle (approx 1-2 tbsp) of olive oil, nuts or seeds, or an avocado for healthy fats.

Curious about how to apply this balanced meal approach to your lifestyle in a way that is easy and practical? Come work with me!

Final Thoughts

The SMILES trial highlights that a nutrient-dense diet can have a profound impact on mental health. These dietary changes, though seemingly simple, provide essential nutrients that support brain function, stabilize blood sugar, reduce inflammation, and promote gut health—all factors that contribute to a healthier mood. So, remember: small changes to what you eat each day can be powerful steps toward better mental health.

written by Dorothy Perelman, RD,