Can Eating a Healthy Diet Really Help Treat Depression?

Young adults with depression may help reduce their symptoms by eating a healthier diet high in fruits and vegetables, rather than one heavy in sugar-laden, processed foods, according to a preliminary study.

But not everyone is convinced that a healthy diet can act as a full-fledged depression treatment. 

Indeed, one expert told Live Science that, based on the new research, which was published today (Oct. 9) in the journal PLOS ONE, scientists still can't say whether eating fruits and vegetables helps improve depressive symptoms any more than a"dummy pill," or a placebo intended to do nothing at all.

The link between diet and depression

Studies have long linked healthy diets, particularly those rich in fruits, vegetables, fish and whole grains, with a reduced risk of depression, according to a 2013 review in The American Journal of Clinical Nutrition.

However, although existing data suggest that there's some connection between poor dietary habits and depression, it's unclear whether there is a "cause-effect" relationship between the two, said Ana Ojeda, a licensed clinical psychologist at Nicklaus Children’s Hospital in Miami, who was not involved in the new study. In other words, past research offers no evidence that healthy eating can reverse depressive symptoms. 

To supply this evidence, scientists need to test dietary habits as they would antidepressant medications — by conducting randomized controlled trials, in which each participant is randomly selected to either receive a treatment or not. The latter group acts as a point of comparison, or control, to see how the treatment group changes throughout the trial.

Related: 8 Tips for Parents of Teens with Depression

To date, only one randomized controlled trial has looked at whether patients diagnosed with depression can find symptom relief through healthy eating, according to a 2019 review in the journal Psychosomatic Medicine. The study, known as the SMILES trial, found that adults who followed a recommended Mediterranean diet for 12 weeks scored better on a depression rating scale than participants who received social support for the same time period.

On average, participants in the SMILES trial were about 40 years old. "What was of interest in our [new] study was whether the findings in older adults could also apply to young adults who were otherwise healthy and of normal body weight," Heather Francis, co-author of the new PLOS ONE study and a clinical neuropsychologist and nutritional neuroscience researcher at Macquarie University in Sydney, told Live Science in an email.

Healthy and happy

Francis and her colleagues recruited 76 adults ages 17 to 35 who all consumed diets high in processed foods, saturated fats and refined sugars. The participants, who also scored "moderate to high" on a scale of depression symptoms used by doctors, were randomly split into two groups. One group received pointers to help improve their dietary habits, a small hamper of pantry items and money for grocery shopping. This "diet group" received two calls from the researchers during the three-week study, to check on their progress.

The control group participants received no food, money or nutritional guidance, and were asked to come back only when the trial concluded.

Following the three-week intervention, the diet group's depression ratings fell within normal range and the participants demonstrated significant improvements in their moods. Scores of those in the control group remained stable. Three months after the study ended, the researchers followed up with 33 of the diet-change participants and found that their moods had remained elevated — at least among the seven of them that had maintained healthy eating habits.  

Related: 7 Ways Depression Differs in Men and Women

The results suggest that "adherence to healthier foods for a period of time has a direct and positive impact on depressive symptoms," Ojeda said.

"These findings add to a growing literature to suggest that healthy diet can be recommended as an effective therapy to improve depression symptoms, as an adjunct to pharmacological and psychological therapy," Francis said. The benefit may stem from a reduction in harmful inflammation, she added — an elevated immune response that can take hold of body tissues as a result of poor dietary habits and is associated with a higher risk of depressive symptoms.

Holes in the research 

Ojeda was impressed by the study's "optimistic results" but notes that the study may  represent only a select subset of patients with depression. 

"Does this diet intervention reduce depression, generally, or only in teens with easy temperaments that can adhere to the plan?" she said. "We may find that kids with [more complex cases]...will not receive the same effect by modifying diet."  

Marc Molendijk, a clinical neuropsychologist at Leiden University in the Netherlands who was not involved in the study, found additional flaws in the work. 

Molendijk noted that the study didn't have an "active control," or a control group that received a different but potentially effective intervention (such as increased social support.)  "They just have a control group with which they do nothing," Molendijk said. Beyond changing what they ate, the diet group received monetary compensation and special attention from the researchers, while the control got nothing, he explained. These extraneous factors may have skewed the final results; there's no way to isolate the true effect of the dietary changes. 

The authors acknowledged this flaw in their design, saying in the paper that "there are difficulties in determining an appropriate active control." However, they assert that the shifts in depression ratings still suggest that "it was the change in diet per se that resulted in improved depression."

But was the improvement that impressive? Molendijk doesn't think so.  

"The effect of a placebo pill [seen in previous studies] is larger than the effect of the diet that these authors report on," he said. 

Related: Mediterranean Diet: Foods, Benefits & Risks

In clinical trials for antidepressant medications, participants often show significant symptom reduction in response to an inert placebo pill, sometimes experiencing up to 30 to 40 % improvement, according to a 2018 review in the journal Frontiers in Psychiatry. In practical terms, that means 8 out of 9 patients may experience equal symptom reduction from either a dummy pill or a true antidepressant drug, according to the review. Molendijk argues that the modest effect of the diet intervention does not exceed what would be expected of a placebo in any given antidepressant trial.    

Previously, Molendijk and his colleagues offered similar critiques of the SMILES trial, pointing out that during the recruitment process participants appeared to be told of the aspirational goals of the study and likely biased the final results.

"I would like to really make explicit that it, of course, would be fantastic if you could cure an illness like depression with diet," Molendijk said. It cannot hurt to eat healthy, he said, but people with depression shouldn't expect to be cured by leafy greens and whole grains because the intervention is "not scientifically proven." What's more, people with depression may blame themselves for not maintaining a healthier diet to begin with and thus worsen their already compromised mental health, he added.

"So far...for me, there's no convincing evidence at all," he said.

• 7 Ways to Recognize Depression in 20-Somethings 
• 9 Celebrities Who Spoke Up about Depression 
• 8 Tips for Dealing with a Depressed Spouse 

Originally published on Live Science.