By Meg Perdew
Disclaimer: the purpose of this article is to highlight 1) the potential relationship between dietary patterns and mental health and 2) the biological mechanisms by which our diet can impact mental health. However, this article is not suggesting that modifying the human diet will or can prevent or treat mental health conditions. Alternatively, we are summarizing the available scientific evidence detailing potential relationships between diet and mental health.
Recognition and diagnosis of depression and anxiety, among a myriad of other mental health disorders (i.e., cognitive and sleep disorders) has become more common in developed countries throughout the past decade. Current treatments for major depressive disorder address only approximately one-third of depressive disorders, demonstrating a need for alternative approaches to both prevent and slow their progression. Research has shown that dietary patterns and specific dietary components may be modifiable risk factors potentially impacting the pathophysiology of mental health disorders. However, throughout this post, we will be focusing on the possible associations between diet and depression.
Macro and Micro-Nutrients and Brain Health
Although each nutrient we consume plays an important role in our physiology, the nutrient requirement of a critical organ in our body that is often overlooked is the brain. Brain cells are enveloped by the blood-brain barrier, which is responsible for filtering nutritional components that have been hypothesized to play a role in brain health. Previous research has largely focused on the impact of fatty acids on mental health. For example, polyunsaturated fatty acids (PUFAs) docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) exert critical effects on brain cells. DHA is an important structural component of membrane phospholipids in the brain – which are responsible for neural membrane functioning; EPA is known as a precursor of anti-inflammatory cytokines (i.e., play an important role in immune function and health) and has been suggested to positively regulate metabolic and immune processes. Interestingly, research has shown that omega-3 fatty acids regulate serotoninergic and dopaminergic neuroendocrine communication (i.e., the brain regulates physiological processes in the body such as the formation and release of pituitary hormones).
Dietary protein and amino acids have been shown to positively impact sleep duration and quality via moderating neurotransmitter and neuromodulator expression; tryptophan is an essential amino acid that is a precursor for bioactive metabolites involved in sleep, including serotonin and melatonin. Remember the last time you ate a large thanksgiving turkey dinner? Often, after consuming a large amount of turkey people begin to feel tired – this is partially caused by the high concentration of tryptophan in the turkey. Further, non-essential amino acids glycine and L-ornithine may play important roles in improving sleep quality. For instance, glycine can reduce core body temperature as it exerts both an excitatory and inhibitory effect on neurotransmission, while L-ornithine is directly involved in the central nervous system, improving sleep by alleviating stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis, as well as mitigating the stress response controlled by the GABA receptor.
Furthermore, carbohydrates have the capacity to exert both positive and negative effects on the brain and mental health, which are partially dependent upon the ‘type’ of carbohydrate (complex vs. simple) consumed which affects their absorption rate. Different types of carbohydrates vary in their rates of absorption, which impacts how insulin responds and how blood glucose is released. In other words, the dietary glycemic load (i.e., a number that estimates how much a certain food will raise an individual’s blood glucose level following consumption) is associated with the magnitude of the inflammatory response. For example, when high-glycemic index foods such as white bread, sugary cereal or processed foods in general are consumed, insulin levels rise rapidly signaling the uptake of glucose into our cells through various cellular transporters (GLUT 4) – this process exists to help regulate blood glucose levels. Insulin is also responsible for stimulating the uptake of tryptophan, which is a precursor for serotonin that competes with other amino acids for transport across the blood-brain barrier. Thus, after consuming a carbohydrate-rich meal, the released insulin stimulates the uptake of larger competing amino acids in the plasma, which enables tryptophan to be transported across the blood-brain barrier in order to contribute to serotonin synthesis.
A prospective cohort study conducted by the Women’s Health Initiative demonstrated that a higher glycemic-index diet was associated with increasing odds of depression prevalence. Specifically, this study showed that added sugars (not total carbohydrates) were strongly associated with depression, while higher consumption of dietary fibre from vegetables and ‘non-juice’ fruit was associated with reduced levels of depression. This finding is supported by other research which confirms that a high intake of complex carbohydrates as fibre from legumes, whole grains and fruits and vegetables may have a positive impact on mental health. Although enzymes in the human gut are not capable of digesting complex carbohydrates, gut bacteria break down fibre, resulting in the release of short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate. These SCFAs possess anti-inflammatory effects, which are then transmitted to the brain by pathways related to the gut-brain axis.
It is well established that the composition and diversity of the human gut microbiota is impacted by the quality and type of food consumed in the diet (see post for further detail: “Dietary Fibre and the Gut Microbiome”. Specifically, higher fibre, plant-rich diets have been associated with greater diversity in the gut’s microbial population. Preliminary research has demonstrated a potential association between the gut microbiota composition and symptoms of depression as well as responses to chronic stress. Emerging research has shown that signaling occurs between the gut and the nervous system, involving neural, endocrine and inflammatory processes. The gut microbes can directly activate afferent neurons of the enteric nervous system to signal the human brain through the Vagus nerve; in fact, gut bacteria can impact mood, memory and cognition. The gut microbiota can also directly influence neurotransmitter metabolism via production of compounds such as SCFAs, bile acids, tryptophan metabolites, and GABA, in addition to monoamines including histamine, dopamine and serotonin. GABA and serotonin regulate neural signaling in the enteric nervous system and can also influence an individual’s behaviour and brain function. Additionally, the SCFA’s by-products produced via dietary fibre fermentation in the gut have been shown to prevent inflammation throughout the body, which plays a role in the development and severity of depression. It is also important to note that foods high in dietary fibre are often rich in B vitamins; vitamin B-12 status is associated with risk for depression and vitamin B-12 plays a crucial role in brain function, which may prevent the onset of certain mood disorders. Thus, preliminary evidence demonstrates that increasing the consumption of dietary fibre from plants (i.e., nuts, legumes, whole grains, fruit and vegetables) may act as a potential preventative mechanism for mental health disorders.
Dietary Intervention as a Preventative Strategy for Mental Health Decline
Nutrition has been established as a modifiable risk factor for depression among other mood disorders. In fact, dietary patterns have been examined as an independent predictor of mental health disorder risk. A meta-analysis of 16 randomized controlled trials (RCT) examined the impact of dietary intervention on symptoms of depression and anxiety and reported that across all studies aiming to reduce the intake of highly palatable processed foods and increase the consumption of nutrient dense foods high in fibre such as fruits and vegetables led to a positive effect on depressive symptoms (Firth et al., 2019). Similarly, the PREDIMED trial, a large RCT conducted in Europe with a sample of individuals with Type 2 diabetes mellitus tested the effects of a Mediterranean diet supplemented with mixed nuts; results from this study demonstrated that participants following the Mediterranean diet (i.e., increased consumption of fruits and vegetables, legumes, whole grains, fish, olive oil, reduced red meat or processed meat consumption) had a 41% reduction in the risk of developing depression compared to a standardized control group. A potential mechanism that could explain the positive impact of following a diet high in fruits, vegetables, and whole grains on symptoms of and risk for depression include the antioxidant and anti-inflammatory that these foods have on the body. The antioxidants present in these foods act as a protective mechanism against the negative impacts of oxidative stress which have been associated with depression. Further, some fruits and vegetables are rich in polyphenols, which provide antioxidant properties as well; specifically, polyphenols reduce inflammatory markers, which are often elevated among individuals with depression.
Overall, the field of ‘nutritional psychiatry’ is still in its infancy and researchers are aiming to uncover the biological mechanisms that could explain the impact of dietary patterns on the human brain and ultimately mental health. Nevertheless, the available preliminary evidence shows that following a high quality, nutrient-dense diet high in fibre from fruits, vegetables and other plants, not only has a positive impact on physiological health such as the host’s gut microbial diversity but may also play an important role in preventing and/or ameliorating symptoms of depression in addition to other mood disorders.
Firth, J., Marx, W., Dash, S., Carney, R., Teasdale, S. B., Solmi, M., Stubbs, B., Schuch, F., Carvalho, A. F., Jacka, F., & Sarris, J. (2019). The Effects of Dietary Improvement on Symptoms of Depression and Anxiety: A Meta-Analysis of Randomized Controlled Trials. 81, 265–280.
Gangwisch, J. E., Hale, L., Garcia, L., Malaspina, D., Opler, M. G., Payne, M. E., Rossom, R. C., & Lane, D. (2015). High glycemic index diet as a risk factor for depression: Analyses from the Women’s Health Initiative. 102, 454–463.
Godos, J., Currenti, W., Angelino, D., Mena, P., Castellano, S., Caraci, F., Galvano, F., Del Rio, D., Ferri, R., & Grosso, G. (2020). Diet and Mental Health: Review of the Recent Updates on Molecular Mechanisms. 9(346), 1–13.
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