Diet and mental health
Professor Felice Jacka is Professor of Nutritional and Epidemiological Psychiatry in the School of Medicine at Deakin University and Director of the Food & Mood Centre. The Centre’s focus is on the development of new, nutrition focused preventive strategies and treatments for mental and brain disorders. Professor Jacka has pioneered a highly innovative program of research that examines how individuals’ diets and other lifestyle behaviours affect the risk of mental health problems.
1. You are at the forefront of a new field of nutritional psychiatry – where are we now and what do you see in the future of this new field?
We and now others have led many observational studies, across countries, cultures and age-groups, showing that an unhealthy diet is associated with an increased risk for depressive disorders, while healthy diets are protective. These relationships are not explained by income, education, body weight, exercise or other factors. We are now at the stage of testing out interventions and achieving positive results, which is very exciting.
The Global Burden of Disease Study (GBOD) tells us that an unhealthy diet is now the leading risk factor for early death across middle and high-income countries, while at the same time mental disorders – particularly depression – account for the leading burden of illness and disability globally. We are now showing that good nutrition may help to both prevent and treat depressive illness, which has large implications for public health and clinical practice; indeed, it is changing the way we understand and conceptualise depression. In addition to conducting more trials, we are now conducting extensive research on the mechanisms involved, with a particular focus on the gut microbiome. We know there are many ways gut bacteria can influence brain health, including neuroinflammation, and dietary change can influence composition and functioning of the gut microbiota within hours. Diet also plays an important role in promoting the health of the gut lining and preventing ‘leaky gut’.
I founded the International Society for Nutritional Psychiatry Research (ISNPR) in 2013 and convened its first major international conference in Washington last year. We are at the forefront of a global movement that recognises physical and mental health as one. It’s an exciting time to be at the forefront of a field with so much potential to improve public health.
2. The Mediterranean diet for depression has been grabbing international media attention - what did you find?
Two landmark studies were published by Australian researchers last year (2017). Our SMILES randomised trial showed a third of intervention participants with clinical depression experienced full remission of their depression compared to 8% of those in the social support control group. The intervention involved seven sessions with a dietitian to implement a Modified Mediterranean Diet (‘ModiMedDiet’). The more people improved their diets, the more their depression improved, independent of body weight.
Interstate colleagues also published results from HELFIMED RCT showing a Mediterranean style dietary intervention supplemented with fish oil achieved a greater reduction in depression and improved mental Quality of Life (QoL) scores at 3 months, and these scores were sustained to 6 months. The Mediterranean Diet group consumed more vegetables (and more diversity of vegetables), fruit, nuts, legumes and wholegrains and less unhealthy snacks and red meat/chicken. Increased Mediterranean Diet Score correlated with reduced depression.
3. What do we know about how diet can protect against depression?
People who eat a diet high in wholefoods such as fruits, vegetables, nuts, wholegrains, fish and unsaturated fats are up to 35% less likely to develop depression than people who eat less of these. A high intake of highly processed foods fatty and sugary foods with little nutritional value has been associated with a 60% increase in the risk of developing depression. And now we have intervention trial data to show following a healthy diet is an effective treatment.
Importantly, this style of eating need not cost more. A detailed cost analysis of the SMILES trial showed the participants’ intervention diet actually cost less than their usual diet - they saved an average of $28 per week.
4. In your view, do you think unhealthy foods are having a negative effect because of their ingredients, or because they are replacing healthy foods in the diet?
It may be a bit of both, however the biggest effect we have shown so far is the beneficial effects of diet quality. A healthy diet rich in plant foods such as vegetables, fruits, legumes, nuts and wholegrains, and healthy fats such as olive oil, avocado, seeds and oily fish are helpful, and we need to eat more of these. There is also support for the idea that too many highly processed foods are not good for physical or mental health, although the mechanisms are not entirely clear. One possible mechanism is emerging research in animals that has identified added emulsifiers in processed food affecting the gut lining making it ‘leaky’. This leads to a chronic activation of the immune system (inflammation) that predisposes to depression, as well as a host of other chronic diseases. High blood glucose (sugar), which is very common nowadays, also promotes leaky gut. Finally, while fats from sources such as olive oil, nuts, seeds, fish and avocados are beneficial to gut health, saturated fats promote the growth of less healthy bacteria and leaky gut.
5. Is there evidence to suggest an adverse effect of dietary added sugar in depressive disorders?
Not sugar per se, but too many highly processed foods appear to be detrimental, including foods with high levels of added salt, fats and sugars. And when it comes to added sugar, the food context is important. For example, we know dark chocolate, along with fruits, vegetables, spices, tea, coffee and red wine, contains valuable polyphenols. Like prebiotics, polyphenols travel to the large bowel and benefit the gut microbiota and may play a part in influencing mood. However, more research is needed on polyphenol efficacy, safety and dosage; we’re not yet at the stage of prescribing chocolate for depression (sorry!). But we do see that high blood sugar is a problem for gut health, particularly in relation to the promotion of leaky gut.
When reducing added sugars in the diet, care is also needed in choosing better alternatives. New evidence suggests artificial sweeteners adversely affect the gut microbiome and may promote weight gain by altering glucose regulation. Choosing less highly processed foods of all kinds, including those labelled ‘no added sugar’ is prudent.