You are what you eat, but when it comes to your mental health, what you eat can have a profound impact on your mood and how you feel. Welcome to the rapidly moving world of nutritional psychiatry which is uncovering the key links between diet and mental health. And it could just be that it is our gut microbes, through acting as psychobiotics, that are the stars of the show here, so long as they are kept well fed. 

A nutritious diet isn't just good for your body. It's good for your brain too with a large body of evidence now existing that suggests that diet is just as important to mental health as it is to physical health. In just a decade, the rapidly growing knowledge in this field has given rise to a new concept called “nutritional psychiatry” where before it was barely a blip on the health care radar. 

Mental health disorders, particularly depression, account for the highest burden of global disability. Depressive disorders affect almost 300 million people around the world and are associated with unemployment, poor physical health, impaired social functioning and, in its most severe forms, suicide. Counselling and medication gain most of the treatment attention for depression, but the promise of dietary changes in helping to improve the outlook of people with depression and other common conditions such as anxiety is gaining traction.

Fruit and Vegetables

First, let’s get some background here. A role for nutrition in mental health comes from the long-known association that a healthy diet is a common factor linked to a positive mental outlook. The problem with most of the observational research in this field though is that it is hard to tease out which one comes first. Are people who are experiencing depression or anxiety more likely to eat poorly because of their mood? Or does a poor diet worsen symptoms of depression and anxiety?

A major step forward in answering this question came from studies that looked at how diet and mental outlook are linked when they change over time in the same group of people – this is called a cohort study.

I’ll profile one such study in this area and it involved a group of more than 12,000 randomly selected people who had their diet, health, happiness, life satisfaction and well-being tracked over 4 years. And I’ll link to the study in the show notes https://ajph.aphapublications.org/doi/full/10.2105/AJPH.2016.303260

So, what sort of connection was seen? Happiness, life satisfaction and well-being all went up for every extra daily serve of fruits and vegetables a person ate. And this was after making allowance for people’s changing incomes and personal circumstances. The happiness health links reached a peak at eight servings of fruits and vegetables each day.

The research team took things one step further and compared the mental health improvements to what happens in life-changing situations. For someone going from eating no fruits and vegetables to eating eight serves a day, they could expect to experience an increase in life satisfaction equivalent to that seen when someone goes from unemployment to employment.

With an improvement in happiness levels, this will only reinforce the positive dietary change as people in a good mood are more likely to prefer nutritious foods and to focus on the long-term benefits of the healthy foods. So healthy eating improves mood which goes on to promote more healthy eating. And so the positive cycle goes on, making it easier to sustain the healthy eating habit.

There are likely many reasons to explain a link between eating more fruits and vegetables and well-being. Higher levels of antioxidants and other phytonutrients are one possibility. Then there is the role of fibre in supporting a healthy population of gut bacteria. Gut fermentation products can act directly on the brain, potentially altering mood and behaviour.

But while such research is certainly interesting, and it is hard to argue against the overall health benefits of eating more fruits and vegetables, such research is only observational – it can’t prove conclusively that positive diet changes are the cause of an improved outlook. Fortunately, just in the last few years, we’ve had some ground-breaking intervention studies published that really puts the focus onto just how much of an impact food can have in influencing mood.

SMILES

And in what I consider the most important nutritional intervention study ever conducted in the area of mental health for the quality of the research itself and the implications of the results to build this research build, a clear link has been established between diet changes and improved mental health. Called the SMILES study (which stands for Supporting the Modification of lifestyle in Lowered Emotional States), this was an Australian study led by Professor Felice Jacka from the Food and Mood Centre at Deakin University. And it used the most powerful type of study design: a randomised controlled trial. And I’ll link to the study in the show notes https://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-017-0791-y

Sixty-seven people with moderate to severe depression were involved in the 12-week trial. Those in the treatment group received seven 60-minute sessions of dietary counselling. The control group received a matching social support protocol, but got no specific dietary advice. Almost all the participants were receiving another active treatment for their depression be it psychotherapy, medications, or both.

The dietary counselling sessions were about getting people to eat more in line with dietary advice aligned with a modified form of the Australian Dietary Guidelines and Dietary Guidelines for Adults in Greece. So, there was a distinctly Mediterranean flavour to the advice. Wholegrains, fruits, vegetables, nuts, legumes, lean meats, chicken, and seafood were on the menu and highly refined starches, sugar and highly processed foods were shunned.

After 12 weeks, there was a statistically significant improvement in the rating scale of depression used in favour of the dietary treatment group. With almost a third of people in the dietary intervention achieving what was considered clinical remission from their depression. This was compared to just 8% who achieved this in the control group.

A range of secondary scores looking at mood and well-being were all pointing in a positive direction as well. Improvement in depression scores was also related to the degree of adherence to dietary advice. The benefit of diet on depression was independent of any changes in body weight, self-efficacy, smoking rates or physical activity.

A strength of the study was including a control group that received an equal level of social support from the research team. This is important as social interaction by itself can improve depression outlook and this could have been an influence in those receiving dietary advice from the research team.

The study was small in size, but the compliance and completion rates were very good. The fact that the dietary intervention group could make significant improvements to their diet quality suggests that dietary improvement is achievable for those with clinical depression despite the fatigue and lack of motivation that are prominent symptoms of this disorder.

How diet could be working here is not entirely understood, but several possibilities are likely. It could be related to a direct effect of improved nutrition providing more nutrients to the body. Equally, an anti-inflammatory effect of the diet could also be at play. And intriguingly, it could also be from a well-fed and more diverse gut microbiota. Science is now discovering just how much influence our gut microbes can play on our health, especially our mental health. The gut has a two-way communication link with the central nervous system – and we call this the gut-brain axis. I’ll talk a bit more about this later in the podcast.

And adding to the intervention work done in the area of nutrition and mental health, another study, this time in young adults with depression found that a switch to healthier eating can significantly reduce symptoms of depression.

Adolescence and young adulthood are periods where there is an increased risk of depression. These are also critical periods for establishing healthy eating habits which will carry over into adulthood. So how nutrition influences the mental health of younger adults was the topic of the study published in October of 2019. And I’ll link to the research in the show notes. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0222768

The study involved 101 adults aged between 17 to 35 years all who had moderate-to-high symptoms of depression. They were all considered to have a poor diet when assessed against Government healthy eating guidelines. 

The participants were randomly allocated into either a diet change group or a control group which meant following their regular diet. Any person taking anti-depressant medication or receiving psychological therapy was required to stay on that treatment plan during the study.

People in the diet change group were given advice on improving their diet along with a healthy food hamper and a small amount of money for buying groceries to help with purchasing healthier foods. The diet changes focussed on having more fruits, vegetables, wholegrains, nuts, fish, legumes, eggs, tofu and lean meats – all individualised to the person’s dietary preferences.

The control group received no advice on diet and were left to continue their normal habits for the 3 weeks the study ran. Symptoms of depression and anxiety along with overall mood and performance on learning and reasoning tasks were measured at the beginning and end of the 3 weeks.

Those in the diet change group were indeed successful in making positive changes to their diet. With these diet changes, significant improvements in mood, depression and anxiety were all seen. No such changes were seen in the control group.

Three months after the study finished, follow-up was made with 33 of the participants to see how they were tracking. One in five people of those in the diet change group were still keeping up with the healthy eating habits and for these people, their mood improvements stuck.

The clear downside to this study was that the control group did not get any general eating advice, financial support to buy food or check-in contact with the research team. Each of these factors potentially could have had an impact on mental health independent of diet changes. The results though are in line with the SMILES study I spoke about earlier which did control for these types of social factors and equally saw a marked improved in depression symptoms with positive diet changes.

This study adds more support for the benefits of positive dietary changes in improving mental health. Counselling, emotional support, and medications all have their role to play and now it seems that diet is another cornerstone to consider in dealing with depression.

Meta-Analysis

There is now a growing research interest in seeing how dietary changes can affect mood disorders with depression and anxiety as the main focus. There are now enough published intervention studies in the field that a meta-analysis can be done to combine all this research together. And this was published last year. I’ll link to the study in the show notes. https://insights.ovid.com/crossref?an=00006842-201904000-00007

Sixteen randomised-controlled trials were included in the analysis. Though I should flag that most of the people in the studies did not meet the criteria for the diagnosis of clinical depression. Even so, dietary intervention was effective in having a small, but meaningful effect in lowering depression symptoms. For anxiety, there was no improvement seen with dietary changes.

There was no one single diet that was studied - many different approaches were used across the interventions. Importantly, there were no differences in depression outcomes between dietary interventions primarily aimed at reducing body weight, improving nutrition or decreasing dietary fat intake. They all were effective.

Because most of the studies in the meta-analysis involved people who were not considered clinically depressed, then the results are more applicable to people with subclinical low mood. Because healthy dietary changes already align with public health dietary guidelines then dietary improvement could be an ideal option for people to adopt themselves as a self-management approach for reducing subclinical depressive symptoms.

If dietary changes are having a real effect on depression symptoms, then how it could be doing it is still unclear. Biochemical pathways involving inflammation, oxidative stress and mitochondrial dysfunction are all known to be disrupted in people with mental disorders so diet could be having a positive impact here. Then there is the growing realisation that gut microbes play an important part in stress responses, immune function and neurotransmission. 

Initial work in mice showed that changes in the gut microbiota can go on to lead to depressive-like symptoms. Supplementing the diets of the stressed mice can reverse the problems. So, what is the case in humans then? People with depression do have a different gut microbe fingerprint compared to healthy volunteers. And intriguingly, human clinical trials involving probiotic supplementation appear to demonstrate an improvement in psychological symptoms related to depression, anxiety and stress.

To keep your gut ecosystem thriving and having a positive influence on your brain and mood, aim to eat more foods high in different types of fibre. Fibre is nourishment for good gut bacteria, and you will find it an array of everyday foods such as most vegetables, wholegrains such as barley and oats, and legumes such as beans and lentils. Then add some fermented foods to your diet such as sauerkraut, kimchi, kefir and yoghurt which contain a good supply of healthy bacteria to help boost your gut bacterial community.

The simplest advice is to eat a wide variety of nutritious whole foods from all food groups, which would naturally contain different types of fibre and beneficial gut health nutrients. And from that, you can expect to reap not just the physical health benefits, but also mental health benefits.

Supplements

And just to round things out, I’ll touch on a couple of popular nutrients that often come up as having a link to mental health.

Fish oil is one such popular supplement often promoted for mental health. The types of omega-3 long-chain fatty acids found in fish oil are major structural components of the brain. These fatty acids also have an anti-inflammatory effect which is noteworthy as increased inflammation is associated with depression. And the fatty acids in fish oil have also been linked to changing neurotransmitter signalling. Most studies that have used omega-3s as an add-on therapy for people who are taking prescription antidepressants though show a limited benefit. A 2019 meta-analysis found that overall, omega-3s have some evidence for being effective for depression, but the findings are not unanimous because of variability between doses, ratios of the different types of fatty acids found in fish oil, and other study design issues.

For anxiety, a 2018 meta-analysis of studies that used omega-3 supplements to treat anxiety did find a benefit, but it was mixed with a stronger benefit seen in people with a specific clinical diagnosis of anxiety. Also, the doses typically needed were above 2,000mg per day of the omega-3 fatty acids which is about 7 standard fish oil capsules. I’ll link to both the studies I mentioned in the show notes. https://www.nature.com/articles/s41398-019-0515-5 and https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2702216

We’re used to hearing about how too much of the sun’s warm rays can be harmful to the skin. But did you know the right balance can have lots of mood-lifting benefits? Without enough sun exposure, your serotonin levels can dip. Low levels of serotonin are associated with a higher risk of depression and seasonal affective disorder or SAD. SAD is a form of depression triggered by the changing seasons. So spend a bit of time each day outdoors during the day to soak up a few rays. One recent meta-analysis of 4 randomised controlled trials of vitamin D supplementation in people with major depression did find that vitamin D supplementation favourably improved depression ratings, but because of the limited number of trials analysed and some inherent flaws in some of them, it is still early days to say that vitamin D supplements are an effective treatment or protective agent against depression. I’ll link to this study in the show notes https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6515787 

Summary

Diet and mental health is a rapidly growing research field. We can expect more research to come to light linking the benefits of healthy eating patterns with improved mental health. But really, the sorts of diets being studied for improving mental health are not that much different to what broad dietary guidelines have promoted for decades. You know, those guidelines that only a small fraction of the population follows yet seem to be continually blamed for our health problems? Now that’s mental.

So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.

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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.