The role of nutrition in mental health

In one of the first studies to ignite interest in the possibility of a link between diet and mental health, Joseph Hibbeln¹ demonstrated a strong, inverse correlation between national levels of fish consumption and national prevalence rates of major depression, the highest prevalence psychiatric disorder, across nine countries. In the ensuing years, a significant body of evidence was generated, suggesting a role for long-chain omega-3 polyunsaturated fatty acids. This relationship has significant biological plausibility, as these dietary-derived lipids are the predominant structural fatty acid in the grey matter of the brain². Following on from this interest, researchers in the field of psychiatry began to generate evidence regarding a role for the Bgroup vitamins in depression³ as these vitamins are also essential for numerous processes in the brain. More recent studies have examined nutrients that are particularly salient to inflammatory and oxidative processes, such as dietary selenium⁴ and magnesium⁵. While psychological stress is known to increase the production of pro-inflammatory cytokines, the Relationship appears to be bi-directional, with inflammation, a marker of immune activation, now suggested as a direct contributor to the pathophysiology of depressive illness⁶. Inflammation is accompanied by an accumulation of highly reactive oxygen species, and increased oxidative stress is also implicated as a factor in depressive illnesses⁷. Diet and nutrition are potent modulators of inflammation and oxidative processes.

However, there are clear limits to studying individual nutrients or food components in relation to disease, given the importance of taking into account the complex combinations and synergistic interactions among nutrients, polyphenols, phytochemicals and fibre in our daily diets. Overall dietary quality, assessed using composite measures of dietary intake, is clearly associated with prevalence of, or risk factors for, many disease outcomes, such as cardiovascular disease, cancer and diabetes. However, it is only in the last 12 months that several studies have been published, in high impact scientific journals, describing relationships between overall dietary quality and the high prevalence mental disorders, depression and anxiety.

In the first of these studies, Sanchez-Villagas et al.⁸ identified that low adherence to a Mediterranean diet, long recognised as a healthful pattern of eating, increased the risk for depression over time in middle-aged Spanish adults. The second study in middleaged public servants identified that those with higher scores on a healthful dietary pattern (vegetables, fruits and fish) were less likely, and those scoring higher on a ‘western’ (unhealthy foods) dietary pattern more likely, to experience depression over the follow-up period9. In each of these studies, the relationships between diet and depression were not explained by socioeconomic factors or other health behaviours, nor by ‘reverse causality’ (dietary changes as a result of depressive symptoms). Similarly, our group10 has reported that women who scored higher on a dietary pattern comprising vegetables, fruits, lean red meats (predominantly grass fed in Australia), wholegrains and fish were less likely to have clinically significant depressive and/or anxiety disorders, while those scoring higher on a western dietary pattern were more likely to have a depressive disorder and higher levels of psychiatric symptoms. Results of each of these studies point to a critical role for nutrition in the mental health of the population.

Mental illness has major social, psychological, and biological consequences. Psychiatry has not investigated lifestyle modification as a preventative strategy for psychiatric symptoms and disorders, and hence lacks a coherent evidence-based public health message based on reversible lifestyle factors. Research funding also remains heavily focused on treatment rather than prevention. These recent scientific discoveries open the door to the possibility of public health messages and strategies that focus on the primary prevention of the common mental health problems, depression and anxiety. The key challenge will be to formulate such messages effectively and create environments that support an increase in the consumption of nutrient-dense foods, such as fruit and vegetables, and a decrease in the consumption of high-energy, low nutrient-density foods, at an individual and population level.