• dietary patterns
• mental health
• nutritional treatment

It is well documented that the UK is in the midst of a mental health crisis that healthcare services are grappling with.1 This issue is not limited to the UK, and in 2018 it was estimated that more than one in six people in the European Union suffered from a persistent mental health issue.2 Mental health disorders include a wide range of conditions and severities, including but not limited to: anxiety disorders, depressive disorders, addiction disorders, bipolar disorders and schizoaffective disorders. Mental well-being was previously artificially dichotomised into either an absence or presence of mental illness, but there is now a greater appreciation for mental health and well-being existing on a continuum.

As mental health has become more widely understood and appreciated, many have looked beyond traditional interventions to more novel mechanisms. This is true of food, and nutrition. As such, the need for good-quality research and dedicated space in international journals has grown accordingly, making this special collection on Food, Mood and Mental Health, one of great interest but also one of great importance.

Mental ill health is the result of a complex, multifactorial aetiology, unique to each individual, but with a universally detrimental effect on society. It has been estimated that suffering from depression can shorten healthy life years, with an impact comparable to smoking.3 In 2009/2010, the estimated costs of mental health problems in England alone stood at over £105 billion, comprised of direct healthcare costs, loss of productivity and human suffering, an increase of 36% from a previous iteration of a study in 2002/2003.4 The COVID-19 pandemic has added further cause for concern with experts warning that up to 10 million people may require new or additional mental health support in the wake of the pandemic.5

Current main treatments for mental illnesses include medication and psychotherapy,6–8 provided in community, primary care, secondary care and in-patient settings. Like all drugs, those used to treat mental illnesses such as antidepressants, antipsychotics and antianxiety medications have unintended side effects, some of which notably reduce tolerance and adherence.9 10 As a result, many seek complementary approaches to modern medicine, including dietary interventions, to help maintain mental well-being and prevent and treat mental illness, while managing the unwanted side effects of medications.

Food is a central component of celebrations, culture and religious festivals globally with an ability to act as a gift, a donation in a time of need or offered as a sign of peace or solidarity. While culturally and behaviourally we may accept its importance for human mood and behaviour, hard evidence is less forthcoming. Accordingly, before diet and food-based interventions become a standard tool for those working in psychiatry, clinical psychology and within wider mental health services, further research is needed to elucidate the complex pathways involved. As with all emerging areas, there is a need to maintain the rigorous scientific standards. With BMJ NPH inviting further publications, there now exists a dedicated space for new evidence that will lead the way in the future. To date, the BMJ NPH 'Food, mood and mental health' special collection emerges insights from Lee et al 11 on potential effects of plant-based diets on depression, whilst Noonan et al 12 affirm that pre/probiotic therapy warrants further investigation in mental health, and Hayhoe et al 13 underpin the need for public health nutrition strategies to optimise mental well-being in children.

Evidence to date suggests diets of higher quality (such as adherence to Mediterranean eating patterns and diets lower in saturated fat, salt and sugar) tend to be associated with more favourable mental health outcomes throughout the life course.14–16 One prospective cohort study demonstrated an increased likelihood of depression among postmenopausal women, with progressively higher intakes of high glycaemic index foods.17

A subtle but important factor is the effect of nutritional status on mental health outcomes. After dietary consumption, how does one’s nutritional reserves impact their mental well-being? The issue that often arises with nutritional studies and mental health is the nature of the associations studied; it is difficult to know whether reverse causality or confounding is present, and we cannot conclude that an association rules out causation.

Low levels of vitamin C have been associated with lower mood and depression,18 described by self-assessment questionnaires. What remains unclear, is whether this is cause or effect. These studies suffer the same issues as many in nutrition; association does not imply causation and that, reverse causality (or confounding) might be at play. Yet it is also entirely plausible, that a higher quality diet may promote positive mental health outcomes and support individual well-being. In relation to vitamin C, however, there exists no convincing evidence to show benefit of supplementation in these groups.

This relationship between vitamin C and mood, showcases many of the challenges faced when studying the effect of food, or specific nutrients, on mental health outcomes. Many studies demonstrate correlations which cannot imply causation. Without placebo-controlled intervention trials demonstrating effect, nutrition is unlikely to make its way into a mental health clinician’s routine care. While there is an association between mental health and dietary patterns, efforts still need to be made to decipher whether it is the effects of mental illness leading to changes in dietary behaviours and subsequently markers of nutritional status, or whether nutritional deficiency and unfavourable dietary behaviours contribute to mental illness. In some cases, there can also be a circular relationship connecting the two possibilities. This collection seeks to begin to unravel some of these knots.

Despite the lack of robust data, there have been attempts to elicit the role of nutrition in mental illness. Patients with psychosis frequently have elevated homocysteine levels. One study19 theorised that reducing homocysteine would result in symptom improvement among patients with psychosis, through supplementation of vitamin B₆, B₁₂ and folic acid. Despite significantly reduced homocysteine levels among participants, it had no effect on symptoms of the condition.19 A meta-analysis on effects of dietary improvement on symptoms of depression and anxiety does hold promise for the potential role of dietary improvement to ameliorate symptoms of depression, but the majority of studies were not looking at participants with clinical levels of depression.20

Finally, consider the impact of food and diet on mental health in patients whose dietary intakes, and therefore, nutritional status, is affected by management of existing mental health conditions. Evidence has shown that patients who suffer from mental ill health are more likely to be nutrient deficient21 and suffer from obesity.22 The SHINE project23 highlighted the inequalities faced by patients with schizophrenia, whose lifespans are several decades shorter than the general population, mainly due to lifestyle and dietary factors.24

Patients on antipsychotic medications such as olanzapine and clozapine are at an increased risk of cardiovascular disease and adverse events.25 A contributory factor in this increased risk is the increase in appetite that results from the use of antipsychotic medications.26 These patients would benefit from nutritional intervention drawing from an evidence base with greater precision.

Individual adaptation of nutritional guidelines is important in all populations, and this may be truer of those suffering from mental illness. Introducing more readily accessible resources on adequate and beneficial nutrition for individuals with mental health conditions and their carers should be a priority in addressing mental health patients more holistically.

Recommendations to follow specific diets may not be achievable for all patients due to cost, accessibility or cooking literacy which needs to be considered in both research and clinical practice. One must also acknowledge the considerable genetic variabilities which exist. These may mean that even if two individuals were to experience the same mental illness, and follow the same dietary advice, they may still experience very different symptoms or severity of symptoms.

Implementing existing evidence on the benefits of a healthy dietary patterns for example, Mediterranean diet, alongside regular physical activity for those with mental health conditions may prove beneficial. Live More is one example of an initiative seeking to combine mental healthcare and physical activity by making exercise accessible to psychiatric inpatients, with positive outcomes in terms of recovery from illness and sustained lifestyle improvements.27 Accredited nutrition education for healthcare professionals, including those working in mental health services, should be made a priority across healthcare systems.

The therapeutic use of nutrition interventions in the management and improvement of mental health is an emerging area of practice with clear associations between diet and mental health. It should also be useful in educating and informing the next generation of clinicians, who unfortunately appear to have an existing deficiency of nutrition training within medical curricula.28

Rather than being two distinct concepts, mental and physical well-being should be interpreted as two sides of the same coin; one cannot exist without the other when considering the individual as a whole. What we eat plays a pivotal part in achieving overall health and within this domain ‘Food and Mood’ remains a promising in which we invite research, discussion and debate.