By Alisha Aminuddin
In 1825, the French gastronome Jean Anthelme Brillat-Savarin published a book which inspired the age-old phrase, ‘you are what you eat’, meaning that if you wish to live healthily, you must first eat healthily. Since then, the idea of eating a balanced diet to feel your physical best has been instilled in us from a young age, but what we’re often not told is that what you eat also has an effect on your mental health. This brings me to consider the research done on the effects of the health of the gut microbiome on mental health, alongside the direct effects of food on your brain. This has led to exciting new research into possible treatments which use food to manage anxiety and depression, possibly as an alternative or supplementary to antidepressants.
Understanding how the brain and the gut are linked
By exploring the effect of food at a molecular level, we can look at the effects of our digestive health on our brain chemistry and how it is affected by the food we eat. Interest in this field was first sparked by a landmark study published in 20041, establishing the ‘bidirectional communication between the gut microbiota and central nervous system’,2 called the ‘gut-brain axis’ (GBA).
The composition of a person’s gut microbiome is mostly dependent on two factors: genetics and diet. Firstly, genetics can influence the presence of certain species of bacteria (particularly Firmicutes spp. and Verrucomicrobia spp.)3, and is an intrinsic factor which is fixed. On the other hand, diet usually influences which one of the three enterotypes are present in the gut. An enterotype is a way of classifying the living organisms in the gut microbiome, based on which species is dominant in the bacteriological composition of a person’s gut microbiota .4 For example, diets high in carbohydrates are associated with microbiomes with a large number of in Prevotella spp., diets high in protein and animal fats are linked with high prevalence of Bacteroides spp., and diets high in resistant starch are linked with high prevalence of Ruminococcus spp.5 Interestingly, whilst dietary changes can lead to a shift in enterotype in a time span of as little as 24 hours2 (as the gut bacterial composition changes), a person’s enterotype is usually quite stable, and will revert back to its original state if the dietary change lasts for less than a month.5
When the microbiome is subjected to changes in diet, antibiotics or stress, its composition changes, leading to a dysbiotic state. This leads to an increase in intestinal permeability, enabling stray molecules from the gut, such as bacteria, bacterial metabolites and food substrate molecules, to travel in the bloodstream.2 Furthermore, conditions such as IBD, Crohn’s disease or an infection, can trigger an inflammatory response in the gut. This causes the release of certain neurotransmitters and pro-inflammatory cytokines, which causes changes in our mental health and induces stress on the microbiome in a multitude of ways.
Firstly, the release of these neurotransmitters and cytokines, as well as an increase in intestinal permeability due to dysbiosis, also allows stray molecules from the gut to travel in the bloodstream. Secondly, an elevated level of cytokines (specifically TNF-α and MCP) can lead to an increased permeability of the blood-brain barrier.6 This can amplify the effects of any stray molecules from the gut. Thirdly, an elevated level of the cytokines TNF-α and IL-6 were also shown by a study to be directly correlated with an increase in symptoms of anxiety and depression, through an unknown mechanism. Healthy participants with no previous history of depression were given endotoxin infusions in order to trigger the release of cytokines, and were consequently found to present with classical depressive symptoms .7 However, participants injected with TNF-α did not present with depressive symptoms,8 suggesting that they were caused by TNF-α and IL-6 produced due to toxin-induced inflammation. Lastly, the pro-inflammatory cytokines released have a larger systemic effect on the endocrine system, as they also play a role in stimulating the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus is stimulated to release corticotropin-releasing factor, which stimulates the release of a cascade of hormones, resulting in the release of cortisol, a known stress hormone. Individuals with elevated levels of cortisol are also more likely to have anxiety and/or depression.9
What treatments have now been tested as a result of this research?
In order to suppress pro-inflammatory cytokines linked to ill function of the gut-brain axis, scientists have now been looking at probiotics as a supplementary or alternative treatment to antidepressants. Studies done with patients suffering from chronic inflammation and irritable bowel syndrome have shown promising results, where the use of probiotics has correlated to the decreased production10 of TNF-α and improved intestinal barrier integrity.11 This would hopefully reduce the inflammatory response, and thus reduce the prevalence of symptoms of anxiety and depression.
In one study, patients suffering from chronic stress were given a milk-based probiotic drink containing the Bifidobacteria species for three weeks. Patients were then asked to rate their mood every day on 6 scales: energetic/tired, composed/anxious, elated/depressed, clear-headed/muddled, confident/unsure, and agreeable/angry. Overall, patients rated themselves to be happier by the end of the study, with patients in the bottom third of the elated/depressed scale demonstrating the greatest mood improvement with treatment.2, 12
In a different study, healthy volunteers (with no previous history of depressive symptoms) were given either probiotics or an antidepressant (Diazepam, commonly used to treat anxiety) for 30 days. Interestingly, the volunteers who were given probiotics showed similar improvements in cortisol levels and self-reported psychological effects to the volunteers given Diazepam. Similar studies using Citalopram and Diazepam showed that probiotics were also able to reduce depressive symptoms.13
Overall, the use of probiotics has shown great promise in improving symptoms of anxiety and depression, however it is important to note that these discoveries made by recent studies are still unable to prove a causal relationship between improved mental health and a healthier gut-microbiome. This is because its mechanism of action is still unknown, likely due to the number of factors which may be related; our mental health, gut health and brain function are all tightly linked by the gut-brain axis and hypothalamic-pituitary-adrenal axis.
How does the food we eat directly affect our mental health?
On a more general level, the food we eat can have an almost immediate effect on our mood and function. For example, when blood sugar is low, it can present as tiredness, irritability or depressive symptoms. This can be prevented by eating regularly, as well as eating foods that release energy slowly (such as whole grain foods) in order to keep blood sugar steady. Furthermore, adding more protein into your diet may help improve your mood, as the amino acids contained in proteins are necessary to produce neurotransmitters, which play a large role in mood regulation and healthy bodily function. Staying hydrated is also important, as dehydration causes difficulties in thinking clearly and concentrating. However, hydrating with beverages high in caffeine (such as coffee and energy drinks) may worsen symptoms of anxiety and depression, as it is a stimulant. Therefore, it might be useful to switch to decaf versions of beverages, or timing when caffeine is consumed so that it does not interfere with sleep, as insufficient sleep may also exacerbate symptoms of anxiety and depression.14
What have we taken away as a result of this research?
In summary, scientists have discovered that there are a myriad of ways in which the body’s inflammatory response to food and changing conditions of the gut microbiome are linked to an increased prevalence of symptoms of anxiety and depression. In order to reduce this inflammatory response and increase the number of “good” gut bacteria, aiming to reduce symptoms of anxiety and depression, scientists have turned to probiotics, which are a relatively cheap treatment. Whilst the relationship between probiotics and reduced symptoms has yet to be shown as causative rather than correlative, the results of recent studies have shown that consumption of probiotics can lead to similar improvements in mental health as with antidepressants, and so could be used as a supplementary or alternative treatment. Whereas antidepressants may lead to side effects that may actually exacerbate anxiety and/or depression,15 the side effects of probiotics are mostly harmless and are usually limited to gas, bloating, or diarrhoea.16
In my opinion, probiotics are no substitute for antidepressants, as the benefits of antidepressants largely outweigh the side effects, and for some, are able to completely treat the symptoms of anxiety and depression. However, in cases where antidepressants or alternative therapies like CBT (cognitive behavioural therapy) may be inaccessible due to long waiting lists, or have previously failed to work, I believe that probiotics, in products such as kefir, sauerkraut, kimchi, tempeh and some yoghurts, are relatively safe to try. Otherwise, they can help increase the number of “good” gut bacteria, improving overall digestive function.
Disclaimer: Do not look to this article for medical advice, and please do consult with a mental health professional if you feel that you are struggling with your mental health. Please see the free online resources linked below for some avenues of support.
References
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