The question of what role nutrition plays in mental health has been the sauce (ahem) of research interest in recent times. As covered previously by oursElves in relation to dementia, the majority of this research has looked at individual nutrients like Omega-3 fish oil and other minerals, with the broader area of diet receiving less focus.

With depression estimated to be the leading cause of disability worldwide, and certain lifestyle behaviours like smoking and physical inactivity being linked to its development, the question of the role of diet in depression seems worth asking.

There are quite a few unanswered questions in fact:

• Could a healthy diet help prevent the onset of depression?
• Are those olive oil adverts showing elderly Italians sprinting around, lifting skirts and playing football filled with the joy of life on to something? That is, are particular traditional regional diets better than others?
• Equally, do unhealthy diets contribute towards depression?
• And does feeling depressed itself affect the dietary choices people make?

Researchers from Australia published the first systematic review to explore these questions in detail this summer in the BioMed Central Psychiatry journal. Examining 25 studies of varying design and measurement type, the authors found largely conflicting evidence of the associations between the quality and patterns of diet and depression, although limited evidence was found for Mediterranean and Norwegian diets as protective factors from depression.

Methods

The authors carried out a comprehensive literature search, identifying 25 studies with a total of 53,770 participants as suitable for review. The studies were undertaken in USA (11 studies), Japan, (3), Spain (2), and countries from Europe, Asia and Australasia (one from each country).

The studies were chosen because they involved:

• Cohort (optimal design), case-control or cross-sectional study designs
• The investigation of associations between self-reported dietary habits (rather than individual food groups) and depression (either self-report or through assessment measures)
• Population-based sample groups rather than from acute settings like hospitals

Criteria for study quality were assessed, with all studies meeting at least half the criteria and 18 of the 25 meeting around 90%. The ‘best-evidence synthesis’ comprised of the latter studies only. There was high heterogeneity between these studies, particularly in relation to measures of diet, meaning data pooling was not possible.

Most studies factored out socio-economic factors like age, sex, education, income and physical activity levels, although this was not done comprehensively across all studies.

Results

Traditional regional diets:

• Mediterranean: Limited evidence
  • 3 studies reported an association between increased adherence to a Mediterranean diet and reduced likelihood of self-reported depression
  • 1 showed no association
• Japanese: Conflicting evidence
  • 1 study showed an association between increased adherence to a Japanese diet and reduced odds of depressive symptoms
  • 1 showed no association
• Norwegian: Limited evidence
  • One study found a positive association in line with above diets for men but not women

Healthy diets (involving low-calorie or well-balanced meals or whole food diets):

• Conflicting evidence
  • 6 studies revealed a healthy diet reduced the likelihood of depression
  • 6 showed no association

Western or less healthy diets:

• Conflicting evidence
  • 3 studies reported positive association between consumption of a Western diet (including processed and take-away foods, and foods high in sugar and/or fat content) and incidence of depression
  • 7 studies reported no association

Depression as a predictor of diet quality:

• Conflicting evidence
  • 2 studies found that depressive symptoms predicted the consumption of an unhealthy diet (although one found this for white males only)
  • 1 study reported no association

Conclusion

Owing to the conflicting results and the heterogeneity across the studies reviewed, including the measurement and definitions of dietary quality, depression assessment and study samples, the authors conclude:

To elucidate whether true causal associations exist between diet and depression, further research is urgently required… Longitudinal studies that explore the role of diet in the development of mental disorders across the lifespan are [also] required.

Limitations

• 

  Only 2 of the studies analysed used a diagnostic interview to measure depression, with the majority using either self-report or symptom measures

• Most of the studies were cross-sectional in design, meaning direction of causality could not be determined
• No studies examined a comparison of diets between countries, which may elucidate greater variation (although it may be difficult to control for numerous confounding variables)

It’s unclear whether the conflicting evidence is a result of the heterogeneity between studies, difference in statistical methods used, or the difficulty in controlling for all potential confounding socio-economic variables. It is also somewhat unclear, given the high number of studies showing no association (before even accounting for publication bias) whether further exploration in this area is necessarily as urgently needed as claimed by the authors. Nevertheless it is clear that in order to develop a greater understanding of the relationship between diet and depression, more high-quality research is required.

Links

Quirk SE, Williams LJ, O’Neil A, Pasco JA, Jacka FN, Housden S, Berk M, Brennan SL. The association between diet quality, dietary patterns and depression in adults: a systematic review (PDF). BMC Psychiatry 2013; 13: 175-187.