Depression is a major and growing problem in America and around the world.

We covered some of the alarming statistics at the outset of our recent article, Evidence Review Tags Omega-3s as Top Mood Aid.

Fortunately, there’s growing evidence that healthy diets can reduce the risk for depression, and the severity of symptoms.

You'll find more about this topic in Top 5 Foods for Boosting Mood, Can Food Really Lift Your Mood?, and Women's Depression Tied to Junky Diets.

However, most of the evidence stems from epidemiological (population) studies, which rely on comparisons between the generally reliable results of psychological tests and people’s rather unreliable answers to diet questionnaires.

Growing clinical evidence that healthy diets fight depression
Because people can see and taste what they’re eating, it’s difficult to conduct a controlled clinical trial to reliably test the mood effects of different diets.

Nonetheless, an international team recently reviewed the evidence and found 16 clinical trials, covering 45,286 participants, that provided what may be the best available evidence on the effects of diet on depression.

And their review led them to a positive conclusion: “Dietary interventions hold promise as a novel intervention for reducing symptoms of depression across the population.” (Firth J et al. 2019)

They noted that healthy diets contain compounds that may reduce the risk for and severity of depression by reducing oxidative stress, inflammation, and mitochondrial dysfunction, and by changing a person’s gut microbiota: 

  • “For example, vegetables and fruits contain, in addition to beneficial vitamins, minerals and fiber, [and] a high concentration of various polyphenols [antioxidants] that seem to be associated with reduced rates of depression … ”.
  • “Furthermore, vitamins (e.g., B vitamins), fatty acids (e.g., omega 3 fatty acids), minerals (e.g., zinc, magnesium), and fiber (e.g., resistant starch) as well as other bioactive components (e.g., probiotics), which are typically abundant in healthy dietary patterns, may also be protective …”.

On the other hand, as they wrote, “… [healthy] dietary interventions may also [reduce] the consumption of unhealthy food associated with increased risk for depression, such as processed meats, refined carbohydrates, and other inflammatory foods.”

Two recent, diet-focused depression trials added encouraging evidence
Two years ago, Australian researchers published a clinical trial designed to directly test the effects of diet on depression in adults, and we reported the encouraging results in Healthy Diets May Help Deflect Anxiety & Depression.

That same year, New Zealand researchers reported positive mood-lifting results from a small 14-day trial in which they divided 171 college students into three groups (Conner TS et al. 2017):

  • Group 1 – Diet-as-usual (control group)
  • Group 2 – Received text-message reminders to eat more fruits and vegetables plus a voucher to purchase some.
  • Group 3 – Received two daily servings of fresh fruits and vegetables to consume on top of their normal diet (or in place of less-healthful foods).

As the authors wrote, “We conclude that providing young adults with high-quality fruits and vegetables [i.e., group 3], rather than reminding them to eat more fruits and vegetables (with a voucher to purchase them), resulted in significant short-term improvements to their psychological well-being.”

Now, another Australian team just reported the similarly positive results of another small clinical trial — one whose design provides more confidence in its results, as we’ll explain.

New clinical trial tested effects of healthy diets in young adults
The new clinical trial, conducted among young adults suffering from some degree of depression, comes from a team led by Heather Francis of Australia’s Macquarie University.

The number of adolescents diagnosed with depression has risen by about 30 percent over the past 10 years, so it's fitting that her team's trial and the New Zealand trial described above involved college students.

In brief, the results of Dr. Francis's trial showed that depressed young adults eating unhealthy diets enjoyed reduced symptoms of depression after eating a healthy diet for three weeks.

Dr. Francis and her colleagues recruited 76 university students (17-35 years old) who exhibited moderate-to-high levels of depression and who were eating poor diets — i.e., ones high in processed foods, sugars, and saturated fats.

(Diets high in saturated fat aren’t necessarily unhealthful, but in this case high intakes of saturated fats signaled high intakes of junky foods such as fast food hamburgers and pepperoni or sausage pizza.)

They randomly assigned the participants to one of two groups for the three-week trial:

  • Regular diet (control group) – No diet instructions or provided foods
  • Diet change (test group) – This group got a sample healthy meal plan and recipes, an FAQ sheet, a basket of healthy foods, a $60 reimbursement for healthy grocery purchases, and two brief phone calls at seven and 14 days (to encourage participants and help them overcome any difficulties following the diet).

The participants randomly assigned to the diet-change group were asked to follow these eating guidelines:

  • Fruits: 2–3 servings daily
  • Fish: 3 servings per week
  • Vegetables: 5 servings daily
  • Nuts and seeds: 3 tablespoons daily
  • Whole grain cereals: 3 servings daily
  • Extra virgin olive oil: 2 tablespoons daily
  • Unsweetened dairy foods: 3 servings daily
  • Spices (turmeric and cinnamon): 1 teaspoon most days
  • Non-seafood protein (lean meat, poultry, eggs, tofu, legumes): 3 servings daily

Conversely, they were also asked to sharply reduce intake of refined carbohydrates (e.g. white flour products), sugars, fatty or processed meats, and soft-drinks.

The food basket given to the diet change group contained these healthy items: extra-virgin (i.e., antioxidant-rich) olive oil, natural nut butter, nuts and seeds (walnuts, almonds, pepitas, sunflower seeds), and two antioxidant-rich spices (cinnamon and turmeric).

Before and after the three-week trial, both groups were given standard tests designed to measure depression, anxiety, and mood, as well as their learning and reasoning performance.

Trial results showed rapid mood benefits from a healthy diet
Importantly, the Australian researchers verified adherence to a healthy diet by scanning participants’ palms with a spectrophotometer.

This device measures the degree of yellowness in someone’s skin, which reflects their intake of the antioxidants (especially carotenoids) in fruits and vegetables.

Based on the results of those palm scans, members of the diet-change group largely maintained their prescribed healthy diet throughout the three-week trial.

Encouragingly, the psychological tests administered by the researchers showed that the members of the diet change group showed significant improvements in mood, and that their depression-test scores rose into the normal range.

The diet change group also enjoyed significantly lower scores on anxiety tests versus the regular diet group, although no significant differences were detected between the groups on other measures, such as learning and reasoning.

In contrast, depression scores did not improve among members of the regular diet (control) group, and they remained in the moderate-to-high range.

The authors were able to contact 33 members of the diet change group three months after the end of the trial, to retest their psychological status and adherence to a healthy diet:

  • 7 reported that they’d maintained a healthy diet.
  • 7 reported that they hadn’t maintained much of the healthy diet.
  • 19 reported that they’d maintained some aspects of the healthy diet.

Surprisingly, despite wide variances in reported adherence to the healthy diet among these 33 participants, there were no significant differences in their depression-test scores three months after the end of the trial — which suggests that following a healthy diet for three weeks produces lasting mood benefits.

And, as the authors of the study noted, “One of the most interesting findings is the fact that diet change was feasible in this population. We anticipated that the symptoms of depression, including low energy, reduced motivation and apathy, would present as barriers to eating well. Despite these factors, there was a significant increase in the recommended foods and decrease in processed foods for the diet-change group but not the habitual diet group.”

“Furthermore”, as they wrote, “within the diet-change group, increase in recommended foods was associated with spectrophotometer readings. This provides objective evidence to support the participants’ self-reported compliance with the diet.”

The authors penned this conclusion: “Modifying diet to reduce processed food intake and increase consumption of fruit, vegetables, fish, and olive oil improved depression symptoms in young adults. These findings add to a growing literature showing a modest change to diet is a useful adjunct therapy to reduce symptoms of depression.”

Junky teen diets may predict future depression
As we reported in Women's Depression Tied to Junky Diets, studies have linked diets high in processed foods, sugar, and omega-6 fats to depression in women and tied whole-food diets to a reduced risk.

Now, the results of an innovative new study from the University of Alabama (UA) reveal that an adolescent’s typical diet may predict their risk for future depression.

The innovative part of the study was that — rather than relying on answers to diet questionnaires — the UA researchers tested the participants' urinary sodium and potassium levels to estimate the relative healthfulness of their diets.

The UA researchers followed 84 middle school students — primarily from low-income families — for a year and a half. Most of the participants were urban African American youth, who, according to the researchers “are at a higher risk for both poor diet and depression.”

At the start and end of the 18-month study, the researchers collected urine samples from the participants, tested the samples for sodium (salt) and potassium levels, and administered a standard screening test for depression.

Sodium in the urine is considered a reliable marker of how much salt a person regularly eats and, as the researchers wrote, high levels of urinary sodium “may reflect the consumption of processed, unhealthy foods.”

Conversely, high urinary levels of potassium reflect higher intakes of vegetables, fruits, whole grains, fish, and poultry, while low potassium levels are linked to diets high in fast and processed foods.

And the study's results linked higher sodium levels and lower potassium levels — which indicate junky, unhealthful diets — to increased depression at the end of the 18-month study.

Because higher sodium levels and lower potassium levels were not linked to greater to symptoms of depression at the outset of the trial, the UA researchers speculated that the depressing effects of an unhealthy diet build up over time.

As they wrote, “These results suggest that reducing the consumption of sodium-rich [i.e., junky] foods and increasing the consumption of potassium-rich foods (e.g., fruits, vegetables, fish, and whole grains) may help reduce the prevalence of depression in adolescents.”


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