No Evidence for Link Between Depression and Diet

Summary: While numerous researchers have concluded that diet can help prevent or alleviate symptoms of depression, a new study reports the evidence to support such claims is weak.

Source: Leiden University

Evidence that the right diet can prevent or alleviate depression is weak. In literature reviews on the subject, the authors often reach strong conclusions, nonetheless. This is what Florian Thomas-Odenthal, a master’s student in Psychology at Leiden University, discovered in his thesis research. The study was published in PLOS ONE.

“Healthy diet helps relieve depression” and “Mediterranean diet reduces risk of depression”: the link between diet and depression has become a popular news story in recent years. But the field is still young, and many of the academic articles that conclude there is a link between diet and depression are reviews: papers in which the author provides an overview of the current knowledge.

Psychology student Florian Thomas-Odenthal analyzed these reviews together with his supervisor Marc Molendijk from the Leiden Institute of Psychology and co-authors Willem van der Does and Patricio Molero. He found that the authors often draw much stronger conclusions than the underlying studies would justify.

Type of review

Thomas-Odenthal studied 50 review articles, distinguishing between narrative reviews (also known as literature reviews), systematic reviews and meta-analyzes. The difference between the three lies in the approach, he explains. “With a literature review, the author himself selects which studies and articles to include in the review. With a systematic review there is a fixed protocol for source selection, analysis and conclusion. And with a meta-analysis the protocol is even stricter, and it is a statistical summary of all the existing research.”

Conclusions too strong

He discovered that a third of the literature reviews reached a strong conclusion about the link between diet and depression, whereas none of the meta-analyzes found such a strong link.

Thomas-Odenthal: “We also did our own meta-analysis: if you take all the experimental evidence together, no strong link can be found between your diet and preventing depression, or for diet being able to help treat it.” The literature reviews are often too emphatic in their findings, therefore. “And those conclusions find their way to the public and practitioners.”

Fewer sources

There are a number of explanations for this discovery, says Thomas-Odenthal. “We saw that, on average, literature reviews contain 45% fewer studies as a source than meta-analyzes.” Furthermore, if the author selects the sources, it is easy for an imbalance can creep in, Thomas-Odenthal explains.

“An author unintentionally gives too much weight to his own research or research that supports his hypothesis. This is called confirmation bias and is probably an important reason why the conclusions in literature reviews are too strong.”

This shows plates of food
He found that the authors often draw much stronger conclusions than the underlying studies would justify. Image is in the public domain.

There is much less chance of this bias in systematic reviews and meta-analyzes—and therefore less chance of the conclusions being too strong. “We have now shown that for the link between diet and depression, but the effect of the review type on the strength of conclusions probably also applies to more topics.”

Be alert with reviews

The young psychologist thinks it is important that both researchers and journal editors become more aware of this. “With a systematic review or meta-analysis there is much less risk of the conclusion being too strong. Journal editors should ask themselves whether they still want to publish narrative reviews, particularly if there is not yet a meta-analysis on the topic.”

About this depression research article

Leiden University
Press Office – Leiden University
Image Source:
The image is in the public domain.

Original Research: Open access
“Impact of review method on the conclusions of clinical reviews: A systematic review on dietary interventions in depression as a case in point” by Florian Thomas-Odenthal et al. PLOS ONE.


Impact of review method on the conclusions of clinical reviews: A systematic review on dietary interventions in depression as a case in point

The recommendations of experts who write review articles are a critical determinant of the adaptation of new treatments by clinicians. Several types of reviews exist (narrative, systematic, meta-analytic), and some of these are more vulnerable to researcher bias than others. Recently, the interest in nutritional interventions in psychiatry has increased and many experts, who are often active researchers on this topic, have come to strong conclusions about the benefits of a healthy diet on depression. In a young and active field of study, we aimed to investigate whether the strength of an author’s conclusion is associated with the type of review article they wrote.

Systematic searches were performed in PubMed, Web of Science, Cochrane Database of Systematic Reviews, and Google Scholar for narrative reviews and systematic reviews with and without meta-analyses on the effects of diet on depression (final search date: May 30th, 2020). Conclusions were extracted from the abstract and discussion section and rated as strong, moderate, or weak by independent raters who were blind to study type. A benchmark on legitimate conclusion strength was based on a GRADE assessment of the highest level of evidence. This systematic review was registered with PROSPERO, number CRD42020141372.

24 narrative reviews, 12 systematic reviews, and 14 meta-analyses were included. In the abstract, 33% of narrative reviews and 8% of systematic reviews came to strong conclusions, whereas no meta-analysis did. Narrative reviews were 8.94 (95% CI: 2.17, 36.84) times more likely to report stronger conclusions in the abstract than systematic reviews with and without meta-analyses. These findings were similar for conclusions in the discussion section. Narrative reviews used 45.6% fewer input studies and were more likely to be written by authors with potential conflicts of interest. A study limitation is the subjective nature of the conclusion classification system despite high inter-rater agreements and its confirmation outside of the review team.

We have shown that narrative reviews come to stronger conclusions about the benefits of a healthy diet on depression despite inconclusive evidence. This finding empirically underscores the importance of a systematic method for summarizing the evidence of a field of study. Journal editors may want to reconsider publishing narrative reviews before meta-analytic reviews are available.

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