Food for Thought

Especially since COVID, many people are suffering with poor mental health. In Australia, 45% of people will have a mental illness at some point (1). Anxiety is the most common, affecting 14% of the population, followed by depression, which impacts six percent. Since COVID, there has been an 8-11% increase in incidence of serious mental health conditions. Women and young people (18-24 year olds) report higher levels of psychological distress. There are also more vulnerable populations including people who have a disability, people who are Indigenous, and people within the LGBTQA+ community.

Mental health has a huge impact on our overall health and wellness. But it’s complicated. Unlike medical science, psychological science is highly subjective (2). There is only one mental health disorder that includes an objective biomarker in its diagnosis criteria, and that’s a form of narcolepsy. All other disorders are based on how people feel, making mental health difficult to define and non-distinct. Traditionally, psychology focused on mental illness. However, today there is a new school of psychology called “positive psychology”. As its name suggests, positive psychology focuses on supporting a healthy mental state - and not just the absence of a mental disorder (3).

There is a biopsychosocial model of mental health - how our biology (e.g. diet, genetics & exercise), social factors (e.g. relationships, upbringing, media & stressors), and psychological factors (e.g. beliefs, emotions & coping capabilities) all interrelate to create mental health or wellness. This makes things even more complicated, and is depicted in the diagram below.

(Cited from https://delphis.org.uk/mental-health/the-biopsychosocial-model-of- mental-health/)

So how does nutrition play into our mental health? Let’s look first at studies that have evaluated the impact diet has on common mental health complaints - depression and anxiety. Is there even a link between food and mood? Let’s investigate!

The Mediterranean Diet & Depression

Several studies show very promising results indicating the protective effects of the Mediterranean diet on alleviating and reducing risk of depressive symptoms (4). The Mediterranean diet is defined by its high amounts of unprocessed plant foods, including fruits, vegetables, whole grains, legumes and nuts, seeds, as well as olive oil. It also contains modest amounts of fish, making it very low in saturated fats and higher in unsaturated fats. What is significantly reduced are red and processed meats, high fat dairy products, refined carbohydrates and sugars (ultra-processed foods) (5).

(Cited from Merra et al., 2021)

In a Swedish prospective cohort study, almost 50,000 women were followed for 20 years (6). Over this time, 1677 were diagnosed with depression. They found that those with a high adherence to the Mediterranean diet had an 18% reduced risk of depression compared to low adherence. This risk reduction increased to 24% for women over 50 years old (they assumed due to an accumulated benefit of this dietary pattern over time), and to 46% for severe depression. These results were quite remarkable!

The Mediterranean Diet & Anxiety

While there is less research on anxiety and diet, human studies have found that there is a positive association between hyper-caloric processed food intake and anxiety incidence (7). One study of 3172 people found that Mediterranean diet adherence was associated with reduced anxiety (8). The results indicated a 31% reduced anxiety risk between the highest and lowest adherence, with vegetable intake being most protective. Compared with the lowest vegetable consumption, those eating the most vegetables had a 43% reduced risk of anxiety.

All of these studies are adjusted - meaning they try to statistically account for “confounding” factors. They collect information on things like level of education, socioeconomic status, obesity and other variables that could be having the “real” effect on health outcomes. The link between mental health and diet was not lost! Turns out - food matters for the mind and we should therefore be carefully “minding” our food.

Now let’s have a look at several mechanisms that could explain these associations: the gut microbiota, inflammation & oxidative stress, and the hypothalamic-pituitary-adrenal (HPA) axis.

The Gut Microbiota-Brain Axis

There is a bi-directional link between the gut and the brain - and the mediator is the gut microbiota - the 10 trillion strong microbial ecosystem that happens to live in your colon. To learn more about this fascinating topic, check out my gut health blog here.

The gut-brain connection involves neural, endocrine (hormonal) and immune pathways (9). A key neurotransmitter, serotonin, which plays a pivotal role in mood is mostly (about 90%) produced by our gut microbes (10). The gut also metabolises hormones like sex hormones (e.g. oestrogen) and the key stress hormone, cortisol (11). An unhealthy gut microbial community leads to an impaired gut barrier, causing hyper-intestinal permeability otherwise known as “leaky gut”. This leads to inflammation throughout the body, including the brain (9).

A healthy gut microbiota contains high amounts of fibre-digesting or fermenting species (5). These produce substances called short chain fatty acids, which can pass the blood brain barrier and exert a positive effect on brain and mental health. As fibre only exists in whole plant foods, this is a key mechanism of why the Mediterranean diet is effective at improving mental health and reducing risk of depression and anxiety.

Probiotic interventions and faecal assessment studies illustrate a direct link between the gut microbiota and anxiety and depression (9). Probiotics are species of bacteria and other microbes that have been shown to have a positive health effect. Plant diversity has been shown to increase diversity of gut microbes and specifically increase the species associated with health. Turns out - this is the same for mental health! Those with depression and anxiety have lower diversity, and fewer fibre-consuming bacteria.

Inflammation & Oxidative Stress

Inflammation is an activation of the immune response. When this occurs chronically, it does not resolve, and damages tissues. This can occur in the brain and contribute to neuropsychiatric conditions like depression and anxiety. Inflammatory molecules can impact mood through neurotransmitter (brain chemical) metabolism, neuroendocrine function and functional brain activity (12). This can be activated by several factors, including stress, and diet. The Mediterranean diet is known for its anti-inflammatory effects, due to it being low in saturated fat and sugar, high in fibre and omega-3 fats, and abundant in vegetables and fruits (13).

Oxidative stress also leads to tissue damage, but through the production of “free radicals”. It is a process that occurs on an atomic levels and involves electrons - it goes deep! Oxidation is enhanced by various stressors, including hyper-caloric diets that are high in processed foods (12). Anti-oxidants reduce oxidation. They essentially “take one for the team” and sacrifice themselves to spare us! However, when oxidation is ramped up and antioxidants aren’t there to help curb the damage - we get oxidative stress.

Antioxidants are in whole plant foods. We also make antioxidants in the body, but we need need micronutrients to make them - which are in whole plant foods as well! Herbs and spices, by weight, have the highest concentration of antioxidants. The Mediterranean diet is extremely high in antioxidants!

Dysregulated Hypothalamic-Pituitary-Adrenal Axis

Those with depression and anxiety have much higher cortisol levels than controls, over 60% and 40-70% higher respectively (12, 14). Cortisol is the end product of the HPA axis, an hormonal negative feedback loop involving the brain and the adrenal glands, which sit on top of our kidneys. The brain sends chemicals to tell the adrenals to produce cortisol in certain circumstances, like when we are in an exam, when experiencing a conflict, and when we need to wake up in the morning. When this feedback loop is functioning properly, higher cortisol in the blood triggers the brain to stop or slow down the process that produces it (15).

But - this feedback loop can become dysfunctional, whereby the higher level of cortisol in the blood does not “turn off” or slow down the HPA-axis activity. This leads to cortisol staying high, and exerting its effects on the body e.g. higher heart rate, hypersensitivity, and higher blood sugar - physiological adaptations designed to help you deal with the stressful situation.

Early life trauma and also chronic stress can cause this HPA-axis dysregulation, but animal studies are suggesting that nutrition can also play a role (12). It has been shown that HPA-axis dysregulation can occur as a consequence of insulin resistance (12). I have covered this fascinating topic extensively in a four part series you can check out here.

The Mediterranean diet may also be beneficial for mental health because it improves insulin sensitivity - i.e. it reduces insulin resistance (16). The specific nutrients that have been researched to positively impact HPA-axis dysregulation are probiotics (i.e. good gut bacteria that we can foster through dietary fibre), omega-3 fats we can get through fish but also flax, chia and hemp seeds, and vitamin c - which is abundant in fruits and vegetables like citrus fruits, capsicum, and cauliflower. Foods that cause insulin sensitivity are those high in saturated fats like meats, high fat dairy and coconut oil, as well as refined carbohydrates like sweets.

While we definitely need more studies on mental health and diet and lifestyle, it is clear that a Mediterranean diet that is loaded with vegetables, fruits, whole grains, legumes, nuts and seeds, and low in meats, high fat dairy and sugar is going to be very helpful in supporting mental health. Diet is only one piece of the puzzle, and so taking an holistic approach by also considering exercise, mind-body practices like yoga, and fostering social and community engagement is key. And, of course, if you are struggling with mental health, please talk to someone about it or seek out professional help.

Written by Jessica Zabow
Clinical Nutritionist (BHSc)
& Yoga Teacher (RYT500)

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References:

1. Australian Institute of Health and Welfare. (2022). Prevalence, impact and burden of mental illness. Retrieved from https://www.aihw.gov.au/getmedia/8f4d5b03-469e-4565-afff-6bc4a3db0e33/Prevalence-impact-and-burden-of-mental-illness.pdf.aspx

2. Clark, L.A., Cuthbert, B., Lewis-Fernandez, R., Narrow, W.E., & Reed, G.M. (2017). Three approaches to understanding and classifying mental disorder: ICD-11, DSM-5, and the National Institute of Mental Health's research domain criteria (RDoC). Psychological Science in the Public Interest, 18(2), 72-145. https://doi.org/10.1177/1529100617727266

3. Westerhof, G.J., & Keyes, C.L.M. (2010). Mental illness and mental health: The two continua model across the lifespan. Journal of Adult Development, 17(2), 110-119. https://doi.org/10.1007/s10804-009-9082-y

4. Ventriglio, A., Sancassiani, F., Contu, M.P., Latorre, M., Di Slavatore, M., Fornaro, M., & Bhugra, D. (2020). Mediterranean diet and its benefits on health and mental health: A literature review. Clinical Practice & Epidemiology in Mental Health, 16(1), 156-164. https://doi.org/10.2174/1745017902016010156

5. Merra, G., Noce, A., Marrone, G., Cintosi, M., Tarsitano, M.G., Capacci, A., & De Lorenzo, A. (2021). Influence of Mediterranean diet on human gut microbiota. Nutrients, 13(1), 1-12. https://doi.org/10.3390/nu13010007

6. Yin, W., Lof, M., Chen, R., Hultman, C.M., Fang, F., & Sandin, S. (2021). Mediterranean diet and depression: A population-based cohort study. International Journal of Behavioural Nutrition and Physical Activity, 18(1), 1-10. https://doi.org/10.1186/s12966-021-01227-3

7. Jacques, A., Chaaya, N., Beecher, K., Ali, S.A., Belmer, A., & Bartlett, S. (2019). The impact of sugar consumption on stress driven, emotional and addictive behaviours. Neuroscience and Behavioural Reviews, 103(1), 178-199. https://doi.org/10.1016/j.neubiorev.2019.05.021

8. Sadeghi, O., Keshteli, A.H., Afshar, H., Esmaillzadeh, A., & Adibi, P. (2021). Adherence to Mediterranean dietary pattern is inversely associated with depression, anxiety and psychological distress. Nutritional Neuroscience, 24(4), 1-12. https://doi.org/10.1080/1028415X.2019.1620425

9. Jarbrink-Sehgal, E., & Andreasson, A. (2020). The gut microbiota and mental health in adults. Current Opinion in Neurobiology, 62(1), 102-114. https://doi.org/10.1016/j.conb.2020.01.016

10. Barandouzi, Z.A., Lee, J., del Carmen Rosas, M., Chen, J., Henderson, W.A., Starkweather, A.R., & Cong, X.S. (2022). Associations of neurotransmitters and the gut microbiome with emotional distress in mixed type of irritable bowel syndrome. Scientific Reports, 12(1), 1-11. https://doi.org/10.1038/s41598-022-05756-0

11. Madison, A., & Kiecolt-Glaser, J.K. (2019). Stress, depression, diet and the gut microbiota: Human-bacteria interactions at the core of the psychoneuroimmunology and nutrition. Current Opinions in Behavioural Sciences, 28(1), 105-110. https://doi.org/10.1016/j.cobeha.2019.01.011

12. Marx, W., Lane, M., Hockey, M., Aslam, H., Berk, M., Walder, K., Borsini, A., Firth, J., Pariante, C.M., Berding, K., Cryan, J.F., Clarke, G., … Jacka, F.N. (2020). Diet and depression: Exploring the biological mechanisms of action. Molecular Psychiatry, 26(1), 134-150. https://doi.org/10.1038/s41380-020-00925-x

13. Yin, W., Lof, M., Chen, R., Hultman, C.M., Fang, F., & Sandin, S. (2021). Mediterranean diet and depression: A population-based cohort study. International Journal of Behavioural Nutrition and Physical Activity, 18(1), 1-10. https://doi.org/10.1186/s12966-021-01227-3

14. Lenze, E.J., Mantella, R.C., Shi, P., Goate, A.M., Nowotny, P., Butters, M.A., Andreescu, C., Thompson, & Rollman, B.L. (2011). Elevated cortisol in older adults with generalised anxiety disorder is reduced by treatment: A placebo-controlled evaluation of escitalopram. American Journal of Geriatric Psychiatry, 19(5), 482-490. https://doi.org/10.1097/JGP.0b013e3181ec806c

15. Heck, A.L., & Handa, R.J. (2019). Sex differences in the hypothalamic-pituitary-adrenal axis' response to stress: An important role for gonadal hormones. Neuropsychopharmacology, 44(1), 45-58. https://doi.org/10.1038/s41386-018-0167-9

16. Mirabelli, M., Chiefari, E., Arcidiacono, B., Corigliano, D.M., Brunetti, F.S., Maggisano, V., Russo, D., Foti, D.P., & Brunetti, A. (2020). Mediterranean diet nutrients to turn the tide against insulin resistance and related diseases. Nutrients, 12(4), 1-37. https://doi.org/10.3390/nu12041066