We now know that depression is associated with a chronic, low-grade inflammatory response and activation of cell-mediated immunity, as well as activation of the compensatory anti-inflammatory reflex system. It is similarly accompanied by increased oxidative and nitrosative stress (O&NS), which contribute to neuroprogression in the disorder. The obvious question this poses is ‘what is the source of this chronic low-grade inflammation?’ This review explores the role of inflammation and oxidative and nitrosative stress as possible mediators of known environmental risk factors in depression, and discusses potential implications of these findings. A range of factors appear to increase the risk for the development of depression, and seem to be associated with systemic inflammation; these include psychosocial stressors, poor diet, physical inactivity, obesity, smoking, altered gut permeability, atopy, dental cares, sleep and vitamin D deficiency. The identification of known sources of inflammation provides support for inflammation as a mediating pathway to both risk and neuroprogression in depression. Critically, most of these factors are plastic, and potentially amenable to therapeutic and preventative interventions. Most, but not all, of the above mentioned sources of inflammation may play a role in other psychiatric disorders, such as bipolar disorder, schizophrenia, autism and post-traumatic stress disorder.

A number of cross-sectional and prospective studies have now been published demonstrating inverse relationships between diet quality and the common mental disorders in adults. However, there are no existing prospective studies of this association in adolescents, the onset period of most disorders, limiting inferences regarding possible causal relationships. In this study, 3040 Australian adolescents, aged 11–18 years at baseline, were measured in 2005–6 and 2007–8. Information on diet and mental health was collected by self-report and anthropometric data by trained researchers. There were cross-sectional, dose response relationships identified between measures of both healthy (positive) and unhealthy (inverse) diets and scores on the emotional subscale of the Pediatric Quality of Life Inventory (PedsQL), where higher scores mean better mental health, before and after adjustments for age, gender, socio-economic status, dieting behaviours, body mass index and physical activity. Higher healthy diet scores at baseline also predicted higher PedsQL scores at follow-up, while higher unhealthy diet scores at baseline predicted lower PedsQL scores at follow-up. Improvements in diet quality were mirrored by improvements in mental health over the follow-up period, while deteriorating diet quality was associated with poorer psychological functioning. Finally, results did not support the reverse causality hypothesis. This study highlights the importance of diet in adolescence and its potential role in modifying mental health over the life course. Given that the majority of common mental health problems first manifest in adolescence, intervention studies are now required to test the effectiveness of preventing the common mental disorders through dietary modification.

Background: Recent evidence suggests that diet modifies key biological factors associated with the development of depression; however, associations between diet quality and depression are not fully understood. We performed a systematic review to evaluate existing evidence regarding the association between diet quality and depression. Method: A computer-aided literature search was conducted using Medline, CINAHL, and PsycINFO, January 1965 to October 2011, and a best-evidence analysis performed. Results: Twenty-five studies from nine countries met eligibility criteria. Our best-evidence analyses found limited evidence to support an association between traditional diets (Mediterranean or Norwegian diets) and depression. We also observed a conflicting level of evidence for associations between (i) a traditional Japanese diet and depression, (ii) a “healthy” diet and depression, (iii) a Western diet and depression, and (iv) individuals with depression and the likelihood of eating a less healthy diet. Conclusion: To our knowledge, this is the first review to synthesize and critically analyze evidence regarding diet quality, dietary patterns and depression. Further studies are urgently required to elucidate whether a true causal association exists.

Objective: Recent evidence suggests a role for diet quality in the common mental disorders depression and anxiety. We aimed to investigate the association between diet quality, dietary patterns, and the common mental disorders in Norwegian adults. Methods: This cross-sectional study included 5731 population-based men and women aged 46 to 49 and 70 to 74 years. Habitual diet was assessed using a validated food frequency questionnaire, and mental health was measured using the Hospital Anxiety and Depression Scale. Results: After adjustments for variables including age, education, income, physical activity, smoking, and alcohol consumption, an a priori healthy diet quality score was inversely related to depression (odds ratio [OR] = 0.71, 95% confidence interval [CI] = 0.59-0.84) and anxiety (OR = 0.77, 95% CI = 0.68-0.87) in women and to depression (OR = 0.83, 95% CI = 0.70-0.99) in men. Women scoring higher on a healthy dietary pattern were less likely to be depressed (OR = 0.68, 95% CI = 0.57-0.82) or anxious (OR = 0.87, 95% CI = 0.77-0.98), whereas men were more likely to be anxious (OR = 1.19, 95% CI = 1.03-1.38). A traditional Norwegian dietary pattern was also associated with reduced depression in women (OR = 0.77, 95% CI = 0.64-0.92) and anxiety in men (OR = 0.77, 95% CI = 0.61-0.96). A western-type diet was associated with increased anxiety in men (OR = 1.27, 95% CI = 1.14-1.42) and women (OR = 1.29, 95% CI = 1.17-1.43) before final adjustment for energy intake. Conclusions: In this study, those with better quality diets were less likely to be depressed, whereas a higher intake of processed and unhealthy foods was associated with increased anxiety. FFQ = food frequency questionnaire; HADS = Hospital Anxiety and Depression Scale; OR = odds ratio; CI = confidence interval; SD = standard deviation; Kj = kilojoule; PA = physical activity; SES = socioeconomic status; MI = myocardial infarction.