Mental disorders, particularly depression, account for the highest burden of global disability. Half of mental illnesses first manifest prior to 14 years of age and childhood disorders are linked to a range of long-term deleterious social, criminal and economic outcomes in adulthood. Despite an increase in the recognition and treatment of depression and anxiety, new data from around the globe suggest that rates may be increasing, rather than decreasing, particularly in young people. The 20th century has seen major shifts in dietary intakes globally, with a marked increase in the consumption of sugars, snack foods, take-away foods and high-energy foods. At the same time, the consumption of nutrient and fibre-dense foods is diminishing. These changes are particularly obvious in younger cohorts. Indeed, the latest data from the Global Burden of Disease Study tells us that unhealthy diet is now the leading cause of early death. Since 2009, our team, and others, have led many observational studies, across countries, cultures and age groups, showing that diet matters to depression. Better quality diets are consistently associated with reduced depression risk, while unhealthy dietary patterns – higher in processed foods – are associated with increased depression and often anxiety. You can read more about some of these studies here, here and here. This even seems to be the case right at the start of life. We led a very large study of more than 20,000 mothers and their children that showed that the children of mothers who ate an unhealthier diet during pregnancy had higher level of behaviours that are linked to mental disorders. This finding has since been replicated in two other large cohort studies. We also saw that the children’s diets during the first years of life were associated with behavioural markers of mental health, suggesting that maternal diet during pregnancy and early life diet are both important in modulating the risk for mental health problems in children as they grow. This is consistent with what we see in animal experiments where unhealthy diets fed to pregnant animals results in many changes to the brain and behaviour in offspring. This is very important to understand if we want to think about preventing mental disorders in the first place. You can read more about this in our article on Diet in Pregnancy. It is important to note that, at this stage, most of the existing data come from observational studies, where it is difficult to tease apart cause and effect. Of course, the possibility that the associations are explained by reverse causality – in other words, mental ill health promoting a change in diet rather than the other way around – is an important one to consider. Most of the published prospective studies have investigated this and largely ruled it out and, in fact, we published a study last year suggesting that a past experience of depression was associated with better diets over time. However, Nutritional Psychiatry is still lacking data from intervention studies due to the recency of the field; these sorts of studies are important in determining causality. There have been two very intriguing intervention studies in the last twelve months suggesting that dietary modification may prevent depression. In the first study, data from the very large PREDIMED intervention were investigated; this study was designed to examine whether adopting a Mediterranean-style diet, which we know is a very healthy way of eating, would reduce the risk of cardiovascular events in older adults with heart disease risk factors such as obesity and diabetes. This study showed that, indeed, a Mediterranean diet did protect against cardiac events and this important finding was published in the NEJM. Colleagues from the International Society for Nutritional Psychiatry Research (ISNPR) then looked at the depression data, which had also been collected as part of the trial. They realised that the sample size (around 4000 people) was too small to really be able to investigate the hypothesis properly – given the relatively early age of onset for depression, you need samples sizes in the tens of thousands to truly look at prevention of depression. However, even with their small sample size they were able to show a strong trend for the prevention of new cases of depression for those adopting a Mediterranean diet with nuts and this prevention effect was clearly significant for those with type 2 diabetes. In a large observational study from the US we have also shown that the potential protective effect of healthy diet on depression seems to be particularly pronounced in those with diabetes. Similarly, in a recent prevention study in the US, the investigators wanted to examine the potential of a form of psychotherapy to reduce the risk of clinical depression in older adults who were already suffering from some depressive symptoms. We know that, on average, people with elevated depressive symptoms are approximately four and a half times more likely to go onto develop full-blown clinical depression over the course of one year compared to people without such symptoms. The researchers weren’t aware of the link between diet and mental health, as it is a relatively new field. So they chose dietary counselling as the comparison to the psychotherapy, believing that dietary counselling would be psychologically ‘inert’ but would be a fair comparison. They were somewhat surprised to find that dietary counselling was just as effective as psychotherapy for reducing transition rates, which were between 8 and 9% (compared to the expected 20-25%) in both groups. What we urgently now need are more intervention studies testing the efficacy of dietary improvement as a treatment for depression. However, such trials, of course, have many difficulties associated with them. Dietary adherence is just one issue – it is not always easy for people to change their diet and they may not always accurately report what they have eaten. This is just one example of the myriad challenges associated with such a trial. Our current randomised controlled trial is the first to attempt to answer this common question: “If I improve my diet will my depression improve?” We have been running this trial since 2012 and will soon the results. If successful, dietary improvement may prove to be an effective and cost effective way to treat depression in some people. It is important to understand that researchers now believe that depression, in particular, is not just a brain disorder, but rather a whole-body disorder, with dysfunction of the immune system (chronic, low-grade systemic inflammation) as a very important risk factor. This ‘systemic inflammation’ arises as a result of many of the environmental stressors that are common in our lives: poor diet, lack of exercise, smoking, overweight and obesity, lack of sleep; lack of vitamin D etc. as well as stress. Many of these factors influence gut microbiota, which in turn profoundly influence the immune system. In fact, gut microbiota affect more than the immune system – they seem to be critical to almost every aspect of health including our metabolism and body weight and brain function and health. Each of these factors is highly relevant to depression, reinforcing the idea of depression as a whole body disorder. This is a bit of the how and why: There are two consequences of a poor diet that interact with the immune system and gut microbiota, as well as important aspects of brain function. These two consequences are related, but not necessarily the same thing. If we do not consume enough nutrient-dense foods such as fruits, vegetables, wholegrains, fish etc., this can lead to insufficiencies in nutrients, antioxidants and fibre, and this has a detrimental impact on our immune system as well as affecting gene expression, gut microbiota and other aspects of physical and mental health. Importantly, our gut microbiota are particularly reliant on an adequate intake of dietary fibre. On the other hand, a diet high in saturated fats and refined sugars has a very potent negative impact on brain proteins that we know are important in depression: proteins called neurotrophins, which protect the brain against oxidative stress and promote the growth of new brain cells. In fact, we published an important study last year that showed a clear relationship between the quality of older adult’s diets and the size of their hippocampus – a section of the brain that is central to learning, memory and mental health and that relies on neurotrophins to grow new cells. There also seems to be an impact of saturated fat on the stress response system, which is also important in both depression and anxiety. Processed foods and high fat diets are also particularly noxious for the gut. The reason we say that these factors are related, but not the same thing, is that you can have one without the other. For example, some people will consume an adequate amount of fruits and vegetables, but consume a lot of processed, high fat-refined carbohydrate foods as well. On the other hand, some others will do the opposite: consume few obvious ‘junk’ foods, but not have sufficient nutrient-dense foods (existing on a very limited diet of white bread and sausages, for example). The other aspect of the relationship between diet and mental health is the impact of poor mental health on dietary behaviours. There is no doubt that stress and uncomfortable emotions prompt us to reach for the biscuit tin and, in fact, experiments in animals tell us that consuming sweet and fatty foods can actually reduce the stress response. They don’t call them ‘comfort foods’ for nothing! However, a bit like smoking cigarettes or drinking too much, the short-term benefit is offset by the long-term damage done by these foods. Animal experiments also suggest that foods high in saturated fat and refined sugar are addictive, interacting with the dopamine system in the way that other addictive products do. Importantly, however, this tendency to crave unhealthy foods when we’re feeling down does not fully explain the links we repeatedly see between diet and mental health. The very large body of evidence that now exists suggests that diet is important to mental health in the same way as it is to physical health. The WHO has long said that ‘there is no health without mental health’. We now believe that the opposite is also true and that physical and mental health should be considered two sides of the same coin. In this sense, the same dietary and physical activity recommendations that are made to prevent and treat common physical diseases are also relevant for mental disorders. Thus, there is no longer a justification for not addressing the whole person when treating mental disorders. It may well be that a dietitian will soon become part of every multidisciplinary psychiatric team and that, in the future, referrals to dietitians will be common for people with mental disorders. For individuals the recommendations are no different for any other aspect of health: the main constituents of diet should be plant foods such as vegetables, salads, fruits, legumes (eg. chickpeas, lentils, tofu), wholegrains and raw nuts; fish and lean red meats; and healthy fats such as olive oil. At the same time, processed foods should be assiduously avoided. They contain high levels of unhealthy fats, sugars and refined carbohydrates, as well as other components that are increasingly being shown to have a detrimental impact on gut microbiota, such as artificial sweeteners and emulsifiers. We would also add here that fibre is key – gut microbiota act to influence health by fermenting fibre and we now believe that our microbiota may be the key to health. Plant foods have high levels of fibre and we should be aiming for 50 grams per day. Vinegars such as balsamic and apple cider also appear to be very beneficial to the gut, as are the fermented foods such as sauerkraut, kimchi, kombucha, tempeh and other such traditional foods. Interestingly, many alternative health practitioners have been advocating for these foods since the 1970s, but now the science is starting to catch up with the recommendations! There also seems to be a role for nutritional supplementation in some people under certain circumstances. For example, omega 3 fatty acids, found in fish, appear to be helpful for people suffering from quite serious depression. Similarly, there are studies starting to emerge to suggest that zinc or vitamin B supplementation may be helpful for some. There is also an amino acid called N-Acetyl Cysteine (NAC) that has been shown to be particularly helpful for people with depression, schizophrenia and bipolar disorders. There is also a lot of animal research that points to zinc as an important nutrient in mental health. Zinc supplementation appears to be helpful for depression in conjunction with other treatments, while dietary zinc intake is also protective for depression in the population. Zinc is a powerful antioxidant and also seems to exert beneficial effects on the gut. So what we need to do now is understand what supplements are useful for individuals under what circumstances. It may be, for example, that gut dysbiosis means that nutrients in the diet are not optimally absorbed or utilised, meaning that there is a need for supplementation even when dietary intake is adequate. These are the sorts of studies we need to now undertake. We need to recognise that addressing diet quality will have benefits for the many physical disorders that commonly accompany depression, such as heart disease, diabetes and obesity, and invoke the precautionary principle. When we have many lines of circumstantial evidence, from population studies and animal experiments, that repeatedly tell us that diet influences both the risk for depression and its underlying pathophysiology, it seems prudent to address unhealthy diet in those with depression – even without the evidence from randomised controlled trials. The critical new understanding that diet is of relevance to mental as well as physical health now gives us the opportunity to think about public health, prevention and treatment strategies that focus on dietary improvement. Interventions focused on improving diet and exercise, designed to prevent physical illnesses such as heart disease, diabetes and obesity, are also likely to help to prevent and treat depression and other mental disorders. This affords the possibility of taking public health approaches to the prevention of mental disorders in concert with those that exist for the common noncommunicable diseases. This is a very exciting possibility given the massive burden of illness imposed by mental disorders. Given that mental disorders account for the largest burden of disability across the globe, it also throws into sharp relief the importance of addressing obesogenic environments to make healthy eating the easiest, cheapest and most socially acceptable option for people, no matter where they live. This is the opposite of the situation most of the world now finds itself in. It reinforces the critical importance of policy changes to address health by addressing the activities of the global processed food industry.