Do we eat to live or live to eat?
For most of us, the answer may be both. We eat food every day to get sufficient energy that maintains our daily life. We have developed various eating habits, different food preferences and preferred food patterns since childhood, and they are constantly changing. People in different countries, even in different towns, have diverse and iconic recipes, such as fish and chips in the UK, pizzas in Italy, sushi in Japan and tacos in Mexico. People across the world also have some shared diet patterns, for example, Western people prefer coffee and Eastern people consider tea as part of their lifestyle.
There is an old saying in Chinese: “Mín yǐ shí wéi tiān”, which literally means “people regard food as heaven”. Eating healthily and deliciously is the goal we have pursued for thousands of years, not only having developed diverse food patterns in different areas but also seeing eating together as a social activity to solidify family relationships as well as friendships. People in Northern China even greet each other by saying “Have you eaten yet?” instead of “How are you?”.
Apart from the imprint of Chinese culture on me, fuelling my interest in the role that food plays in health, my previous research experiences in public health remind me that nutrition can be one of the most cost-effective interventions to promote health of vulnerable people at a population level. Thus, I chose to pursue my PhD degree (I’m currently in my 3rd year) in nutrition and psychiatry at the Institute of Psychiatry, Psychology and Neuroscience, to investigate the effects of dietary patterns and dietary polyphenols on adolescent mental health.
Other ITM authors have discussed important topics in the area of nutritional psychiatry, including the protective effect of omega-3 fatty acids on attention deficit hyperactivity disorder (ADHD), the association between dietary intake and neuroinflammatory biomarkers, and diet even being as a treatment for melancholy in the 18th century. However, in this blog, I will be discussing the beneficial impact of polyphenols on mental health, which are the most abundant bioactive molecules in the diet, especially found in berries, vegetables, coffee, and tea.
Mental health: an increasingly serious issue
Mental health is a growing public health issue, not just in the UK, but around the world. It is estimated that 1 in 6 people in UK experienced a common mental health problem in the past week, and mental health problems are one of the main causes of the overall disease burden worldwide. What’s more, this issue became worse as we faced the COVID pandemic and had to experience social isolation during last year.
For instance, a UK study showed that women, young people aged 18–29, those from more socially disadvantaged backgrounds and those with pre-existing mental health problems have worse mental health outcomes during the pandemic. It has been extremely difficult to overcome the fear of infection, the loneliness during the lockdown, the psychological stress, and the economic pressure.
As for me, I had to celebrate my birthday with my friends virtually during the 2nd lockdown in the UK, which was fine but not as exciting as usual. To deal with the stress during the lockdown, I tried cooking, baking and growing plants with my flatmates, to make life fulfilling and maintain our mental health.
As I have been a big fan of fruit since childhood, I also became more careful about eating a balanced diet during pandemic. Interestingly, existing evidence has shown that dietary intake can indeed impact our mental health, which means we can also boost our mental health through what we eat!
Better diet, better mental health
A large body of evidence supports the association between poor diet quality and greater risk of developing mental health problems. For instance, a well-known healthy diet is the Mediterranean diet, which is categorised by a high intake of olive oil, fish and whole grains, as well as fruits and vegetables. Adherence to the Mediterranean diet has been related to decreased risk of depression and better cognitive function in adults.
A systematic review has confirmed a positive association between higher unhealthy diet, characterised by a higher intake of foods with increased saturated fat, refined carbohydrates, and processed food products, and poorer mental health outcomes in children and adolescents. Furthermore, some research suggests that Mediterranean diet exerts anti-inflammatory and immunomodulating activities by decreasing some proinflammatory cytokines, thus reducing the risk of disorders related to oxidative stress, chronic inflammation, and the immune system, including mental disorders. This evidence all suggests that maintaining a healthy diet pattern is helpful to maintaining better mental health both for adults and children.
Polyphenols’ contribution to mental health
The anti-inflammatory effect of healthy diets, like the Mediterranean diet, has been proposed to be related to the presence of high levels of polyphenols, the most abundant bioactive molecules in the diet (intakes ∼1 g/day). The main polyphenol dietary sources are fruit and beverages, like fruit juice, wine, tea, coffee and beer, and, to a lesser extent, vegetables, dry legumes, and cereals. Researchers found that higher levels of polyphenols in the diet are associated with a positive effect on mood and cognition.
For example, individuals who frequently consume curry or drink green tea, which are rich in polyphenols (i.e., curcumin and catechin), are less likely to experience cognitive impairment, in comparison with those who rarely consume curry or drink green tea. Similarly, a positive association has been reported between higher dietary polyphenols and better language and verbal memory as well as less depressive symptoms.
In our recent preliminary results from 316 healthy people (aged between 8 and 80), we found that higher dietary intakes of some flavonoids (flavanols, flavanones, and flavonols) and hydroxybenzoic acids were significantly associated with better mood. Although it is not published yet, it inspired us to explore more of the mechanism between polyphenol intake and mood, as well as wider mental health outcomes. Currently, we are exploring the association between mood and polyphenol metabolites in urine and plasma, and our next step is to replicate these results in the association between polyphenols and depression, anxiety and externalising behaviours in adolescents.
Polyphenols: potentially mental resilience
Apart from the protective effect of high dietary polyphenols intake on mental health, recent studies indicate it can also enhance mental resilience, which will help us stay healthy in the face of adversity. For instance, increased dietary polyphenol intake and greater diversity in fruit and vegetable consumption are linked with enhanced psychological resilience. A study conducted in Australian university students found higher fruit and vegetable consumption per day, more consistently having breakfast, and less frequent intake of soft drinks and takeaway foods, were all significantly associated with both lower psychological distress and higher resilience.
These findings give us insight into the positive associations between adherence to a high polyphenol diet and psychological resilience, and suggest that by adopting a similar diet and keeping good eating habits, we can better handle psychological distress and even improve resilience.
Now, what can we do?
Despite us developing a better understanding of the health role that dietary polyphenols play, more studies are needed to confirm the biological mechanisms and public health implications. As consumers, we are now aware of the potential protective effects of polyphenols on mental health, so we can better align ourselves with a healthier diet pattern and select polyphenol-rich food intentionally in daily life. However, we should be careful when choosing polyphenol supplement before the publication of Dietary Reference Intakes (DRIs) for common and potentially harmful polyphenols.