Mental disorders, particularly depression, account for a huge 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 negative 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 and its sequelae (eg. high blood pressure, blood glucose, body mass index) is now the leading risk factor for 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 herehere 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 other large cohort studies of maternal diet during pregnancy, finding that unhealthy diets were related to emotional/behavioural and cognitive outcomes. 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.

Importantly, in observational studies it is difficult to tease apart cause and effect. There remains 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. Most of the published prospective studies have investigated this and largely ruled it out; in fact, we published a study suggesting that a past experience of depression was associated with better diets over time. Meta-analyses have now suggested that a healthier diet is associated with an approximately 30% reduced risk for depression. However, randomised controlled trials are required to understand whether the relationship between diet and mental health is ‘causal’ rather than diet and mental health being simply associated with each other. Luckily, there have now been several trials that have examined that question.

Our randomised controlled trial, “SMILES”, was the first to attempt to answer this common question: “If I improve my diet will my depression improve?” The SMILES trial demonstrated the efficacy and cost-effectiveness of dietary improvement as a treatment for clinical depression, establishing – for the first time – that diet can be modified to successfully treat even severe clinical depression, in an approach that was also highly cost-effective. Other randomised controlled trials in depression have replicated this finding, and our meta-analysis, involving 16 randomised controlled trials and more than 45,000 participants, also confirms that dietary interventions improve depressive symptoms. We are now running large ‘effectiveness’ trials, that are examining the realworld impact of providing dietary and other lifestyle support to people with mental health problems.

This is a bit of the how and why:

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: unhealthy diet, lack of exercise, smoking, poor sleep, stress, and more. Many of these factors influence the gut microbiota, which in turn 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, brain function and health. Each of these factors is highly relevant to depression, reinforcing the idea of depression as a wholebody disorder.

There are two consequences of an unhealthy 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.

On the other hand, a diet high in saturated fats and refined sugars has a 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 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. 

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 ultra-processed, high fat-refined carbohydrate foods as well. On the other hand, some others will do the opposite: consume few ultra-processed foods, but not have sufficient nutrient-dense foods.

You can read more about the mechanisms that link diet to mental and brain health here.

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! 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.

Excitingly, this new knowledge is now reflected in more than 80 high-level policy documents around the globe. More importantly, it is now influencing clinical guidelines in psychiatry. The most recent clinical guidelines for the treatment of mood disorders by the Royal Australian New Zealand College of Psychiatrists places lifestyle modification (diet, physical activity, sleep, and smoking/substance cessation) as the ‘foundation’ of treatment, calling it essentially ‘non-negotiable’. Our Food & Mood Centre has led an international taskforce in developing Clinical Guidelines for Lifestyle-based Mental Health Care in Major Depressive Disorder, which you can access here.

For individuals the dietary recommendations are no different for any other aspect of health: the main constituents of diet should be plant foods (vegetables, fruits, legumes, wholegrains and nuts), healthy fats such as extra virgin olive oil, and moderate amounts of fish and lean red meat. At the same time, ultra-processed foods should be limited.

Given that mental disorders account for the largest burden of disability across the globe, it also throws into sharp relief the importance of addressing our food 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 ultra-processed food industry.