I wrote a post a couple of days ago entitled "Twelve recent research studies on diet, psychological symptoms & wellbeing (1st post): overall dietary quality & depression".  I said that glancing back over recent research studies that I have noticed & downloaded to my personal database, I was struck by a whole series on the effects of diet on psychological state.  I've listed twelve that caught my eye in the last several months - the first post gave half a dozen on overall dietary quality & depression, whereas this post focuses more on specific dietary components.

Felice Jacka is a major figure in the recent surge of interest into the association between diet and psychological disorders.  There is more information about her work on her page at Deakin University, Melbourne and many of her research studies are accessible in full text at ResearchGate.  Examples include:

Jacka, F. N., M. Maes, et al. (2012). "Nutrient intakes and the common mental disorders in women."  J Affect Disord 141(1): 79-85. BACKGROUND: There is an increasing recognition of the role of nutrition in depression and anxiety.  Magnesium, folate and zinc have all been implicated in depressive illness, however there are few data on these nutrients in anxiety disorders and the data from population-studies are limited. AIMS: In a large, randomly-selected, population-based sample of women, this study aimed to examine the relationship between the dietary intakes of these three micronutrients and clinically determined depressive and anxiety disorders and symptoms. METHODS: Nutrient intakes were determined using a validated food frequency questionnaire. The General Health Questionnaire-12 measured psychological symptoms, and a clinical interview (Structured Clinical Interview for DSM-IV-TR, non-patient edition) assessed current depressive and anxiety disorders. RESULTS: After adjustments for energy intake, each standard deviation increase in the intake of zinc, magnesium and folate was associated with reduced odds ratio (OR) for major depression/dysthymia (zinc: OR=0.52, 95% confidence interval (CI) 0.31 to 0.88; magnesium: OR=0.60, 95% CI 0.37 to 0.96; folate: OR=0.66, 95% CI 0.45 to 0.97). There was also an inverse association between the intake of magnesium and zinc and GHQ-12 scores (zinc: zbeta=-0.16, 95% CI -0.29 to -0.04; magnesium: -0.14, 95% CI -0.26 to -0.03). These relationships were not confounded by age, socioeconomic status, education or other health behaviours. There was no relationship observed between any nutrient and anxiety disorders. CONCLUSION: These results demonstrate an association between the dietary intakes of magnesium, folate and zinc and depressive illnesses, although reverse causality and/or confounding cannot be ruled out as explanations.

Jacka, F. N., J. A. Pasco, et al. (2012). "Dietary intake of fish and PUFA, and clinical depressive and anxiety disorders in women."  Br J Nutr: 1-8.  Fish and PUFA consumption are thought to play a role in mental health; however, many studies do not take into account multiple sources of PUFA. The present study analysed data from a sample of 935 randomly selected, population-based women aged 20-93 years. A validated and comprehensive dietary questionnaire ascertained the consumption of n-3 and n-6 PUFA. Another assessed fish and energy intake and provided data for a dietary quality score. The General Health Questionnaire-12 (GHQ-12) measured psychological symptoms and a clinical interview (Structured Clinical Interview for DSM-IV-TR Research Version, Non-patient edition) assessed depressive and anxiety disorders. Median dietary intakes of long-chain n-3 fatty acids (310 mg/d) were below suggested dietary target levels. The only PUFA related to categorical depressive and anxiety disorders was DHA. There was a non-linear relationship between DHA intake and depression; those in the second tertile of DHA intake were nearly 70 % less likely to report a current depressive disorder compared to those in the first tertile. The relationship of DHA to anxiety disorders was linear; for those in the highest tertile of DHA intake, the odds for anxiety disorders were reduced by nearly 50 % after adjustments, including adjustment for diet quality scores, compared to the lowest tertile. Those who ate fish less than once per week had higher GHQ-12 scores, and this relationship was particularly obvious in smokers. These are the first observational data to indicate a role for DHA in anxiety disorders, but suggest that the relationship between DHA and depressive disorders may be non-linear.

Pasco, J. A., F. N. Jacka, et al. (2012). "Dietary selenium and major depression: A nested case-control study." Complement Ther Med 20(3): 119-123.  OBJECTIVES AND METHODS: Alterations in redox biology are established in depression; however, there are no prospective epidemiological data on redox-active selenium in depression. We aimed to determine if low levels of dietary selenium are associated with an increased risk for de novo major depressive disorder (MDD). In this nested case-control study, women aged 20 years or more were identified from a randomly selected cohort being followed prospectively for the Geelong Osteoporosis Study. Cases were individuals with incident MDD, identified using the Structured Clinical Interview for DSM-IV-TR (SCID-I/NP); controls had no such history. Dietary selenium intake was measured using a food frequency questionnaire at baseline, together with anthropometric and lifestyle measures. RESULTS: Eighteen women who developed de novo MDD were classified as cases; there were 298 controls. Low dietary selenium intakes increased the likelihood of developing MDD; OR 2.74 (95%CI 0.95-7.89). After adjusting for age and SES, compared with a high selenium intake, a low intake (<8.9 mug/MJ/day) was associated with an approximate trebling of the likelihood for developing de novo MDD; OR 2.95 (95%CI 1.00-8.72). Smoking, alcohol consumption and physical activity did not confound the association. CONCLUSION: These data suggest that lower dietary selenium intakes are associated with an increased risk of subsequent de novo MDD. We propose that selenium's function as an antioxidant, and as a constituent of selenoproteins that are important in redox homeostasis, warrants further investigation as a risk factor for depression, and suggest a potentially novel modifiable factor in the primary prevention and management of depression.

The suggestion that zinc is of importance in depression is reinforced by the findings from Yary, T. and S. Aazami (2012). "Dietary intake of zinc was inversely associated with depression."  Biol Trace Elem Res 145(3): 286-290. Depression is an important cause of morbidity, and World Health Organization has predicted that it will be the second leading contributor to the global burden of disease by 2020. Postgraduate students are at high risk for depression caused by the stress of examinations, the academic environment, and relationship problems with peers, lecturers, and family members. Physical inactivity, advancing age, unmarried status, and many other factors contribute to the development of depression in humans. Associations between symptoms of depression and the intake of nutrients such as magnesium have been investigated; however, the relationship between zinc intake and depression has not received as much attention. The purpose of the present study was to examine the relationship between dietary intake of zinc and depression in postgraduate students. This study was conducted on 402 participants with a mean age of 32.54 +/- 6.22 years, including 173 (43%) women and 229 (57%) men. In this study, we found an inverse relationship between dietary intake of zinc and depression. The results persisted even after we controlled for several potential confounding variables related to depression symptoms, including age, sex, years of education, smoking status (current and past), and physical activity. The results of this study show that long-term intake of zinc may modulate symptoms of depression.

While the benefits of fish oils are further supported, in relation to bipolar depression too, by recent research from Sarris, J., D. Mischoulon, et al. (2012). "Omega-3 for bipolar disorder: Meta-analyses of use in mania and bipolar depression." J Clin Psychiatry 73(1): 81-86. OBJECTIVE: Studies using augmentation of pharmacotherapies with omega-3 in bipolar disorder have been conducted; however, to date a specific meta-analysis in this area has not been published. Thus, we present the significant findings from meta-analyses of omega-3 in the treatment of bipolar depression and bipolar mania. DATA SOURCES: PubMed, CINAHL, Web of Science, and Cochrane Library databases were searched for clinical trials up to September 1, 2010, using the search terms bipolar disorder OR bipolar depression OR bipolar mania OR mania OR hypomania OR cyclothymia with the search terms omega 3 OR essential fatty acids OR polyunsaturated fatty acids OR DHA OR EPA OR fish oil OR flax oil. Clinical trial registries and gray literature (published or unpublished data not readily accessible via main databases) were also searched. DATA SELECTION: The analysis included randomized controlled studies 4 weeks or longer, with a sample size >10, written in English, using omega-3 for diagnosed bipolar depression or mania. No criteria were set for age, gender, or ethnicity. DATA EXTRACTION: A random-effects model was used. The model analyzed the standard mean difference between treatment and placebo between baseline and endpoint, combining the effect size (Hedges g) data. Funnel plot and heterogeneity analyses (I(2)) were also performed. DATA SYNTHESIS: The findings of 5 pooled datasets (n = 291) on the outcome of bipolar depression revealed a significant effect in favor of omega-3 (P = .029), with a moderate effect size of 0.34. On the outcome of mania, 5 pooled datasets (n = 291) revealed a nonsignificant effect in favor of omega-3 (P = .099), with an effect size of 0.20. Minor heterogeneity between studies on the outcome of bipolar depression was found (I(2) = 30%; P = .213), which was not present on the outcome of bipolar mania (I(2) = 0%; P = .98).  Funnel plot symmetry suggested no significant likelihood of publication bias.  Meta-regression analysis between sample size and effect size, however, revealed that studies with smaller sample sizes had larger effect sizes (P = .05).  CONCLUSIONS: The meta-analytic findings provide strong evidence that bipolar depressive symptoms may be improved by adjunctive use of omega-3. The evidence, however, does not support its adjunctive use in attenuating mania.

And despite the recent findings that multivitamin-multimineral use probably has neither beneficial nor detrimental effects on overall mortality (see Macpherson et al. "Multivitamin-multimineral supplementation and mortality: A meta-analysis of randomized controlled trials."), it is interesting and not that surprising, taking into account the research studies mentioned above, that a recent meta-analysis suggested that dietary supplements can improve psychological state - see Long, S.-J. and D. Benton (2013). "Effects of vitamin and mineral supplementation on stress, mild psychiatric symptoms, and mood in nonclinical samples: A meta-analysis."  Psychosomatic Medicine 75(2): 144-153. Objective Biochemical processes in the brain affect mood. Minor dietary inadequacies, which are responsible for a small decline in an enzyme's efficiency, could cumulatively influence mood states. When diet does not provide an optimal intake of micronutrients, supplementation is expected to benefit mood. This meta-analysis evaluated the influence of diet supplementation on mood in nonclinical samples.  Methods Databases were evaluated and studies were included if they considered aspects of stress, mild psychiatric symptoms, or mood in the general population; were randomized and placebo-controlled; evaluated the influence of multivitamin/mineral supplements for at least 28 days. Eight studies that met the inclusion criteria were integrated using meta-analysis.  Results Supplementation reduced the levels of perceived stress (standard mean difference [SMD] = 0.35; 95% confidence interval [CI] = 0.47-0.22; p = .001), mild psychiatric symptoms (SMD = 0.30; 95% CI = 0.43-0.18; p = .001), and anxiety (SMD = 0.32; 95% CI = 0.48-0.16; p < .001), but not depression (SMD = 0.20; 95% CI = 0.42-0.030; p < .089). Fatigue (SMD = 0.27; 95% CI = 0.40-0.146; p < .001) and confusion (SMD = 0.225; 95% CI = 0.38-0.07; p <.003) were also reduced.  Conclusions Micronutrient supplementation has a beneficial effect on perceived stress, mild psychiatric symptoms, and aspects of everyday mood in apparently healthy individuals. Supplements containing high doses of B vitamins may be more effective in improving mood states. Questions about optimal levels of micronutrient intake, optimal doses, and active ingredients arise.

To learn more about the various dietary ingredients mentioned in this post, a good place to start is the US National Institutes of Health's Office of Dietary Supplements - see, for example, their fact sheets on fish oils, folate, magnesium, selenium and zinc.