Written by: Adriana Souto Pereira Nuncio
Edited by: Esther Melamed
This body organ has more neurons than the entire spinal cord and is not part of the central nervous system (CNS). What are we talking about? Yes, that is right, it is your gut, the body’s second brain!
The concept of the gut-brain axis, a bidirectional connection between the brain and the gastrointestinal tract, is being actively studied, especially in the neuroscience field. Neurological disorders, such as depression, anxiety, stress, Parkinson’s disease, Alzheimer’s disease, autism spectrum disorder and multiple sclerosis (MS) are believed to influence and to be influenced by the gut-brain axis.
Experiments, mostly in animal models, have demonstrated that neurological conditions may be associated with a different composition of gut microbiota compared to healthy individuals, which may influence neurological symptoms or disease progression (Maiuolo et al., 2021). Mechanisms such as systemic inflammation (which weakens gut integrity), vagus nerve stimulation (which affects the parasympathetic system), bacterial translocation, and metabolite production (leading to over-stimulation of immune cells) all may contribute to dysbiosis changes in the gut microbiota (meaning that the gut microbiota are more pathogenic).
Currently, there is no consensus in the scientific community on what defines “healthy gut” microbiota, although some striking features are high taxonomic diversity, high microbial gene richness and stable core/commensal microbiota. Composed of several microorganisms, including a wide variety of bacteria, yeast, and viruses, the gut microbiota depends on our childhood experiences (birth gestational time, type of delivery, methods of milk feeding, weaning period) as well as external factors such as diet, exercise and antibiotic/other medication use.
Each person has a unique gut microbiota profile that plays many specific functions in host nutrient metabolism, maintenance of metabolic and structural integrity of the gut mucosal barrier, immunomodulation, and protection against pathogens. While early gut colonization plays a critical role in shaping our gut microbiota, diet has perhaps the greatest influence on microbial composition throughout life. In fact, diet modulates the mucous barrier via alterations in gut microbiota which can provide resilience against different disorders and improves human health (Larroya et al., 2021). The old saying, ‘You are what you eat,’ may have more meaning than we previously thought, and a regular consumption of specific groups of foods can be beneficial for the gut-brain axis.
Here are some of the most important gut-brain axis dietary factors:
Omega-3 polyunsaturated fatty acids (ω3-PUFAs)
Fish – such as salmon, mackerel, lake trout, herring, sardines, and albacore tuna – are a rich source of ω3-PUFAs. Our brain has high levels of ω3-PUFAs, especially docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which are indispensable for neuronal membrane synthesis. Evidence from animal models has suggested that higher intake and blood levels of omega-3 fatty acids may help to reduce blood–brain barrier disruption associated with aging, reducing the risk of age-related cognitive decline and Alzheimer’s disease (Barnes et al., 2021). In addition, docosahexaenoyl ethanolamide (DHEA), a lipid compound derived from DHA found in fish oil, has been shown to have anti-inflammatory effects on microglia and astrocytes, and at certain concentrations, can significantly decrease T-cell responses and suppress the production of monocyte chemotactic protein-1 (MCP-1), a cytokine that plays a pivotal role in chronic inflammation associated with autoimmune diseases, including MS (Yilmaz et al., 2019; Boghozian et al., 2017).
Probiotic-rich foods
Probiotic-rich foods – such as kefir, yogurt, pickled vegetables, tempeh, kombucha tea, kimchi, miso, and sauerkraut – are known to restore the natural balance of the gut microbiota. Strains of lactic acid bacteria (LAB) belonging to the genera Lactobacillus and Bifidobacterium are commonly used/found in probiotics foods and their metabolites are potent agonists of HCA3 receptor, which is highly expressed on immune cells and can mediate anti-inflammatory responses (Peters et al., 2019). Therefore, a diet rich in probiotic-rich foods not only can enhance the diversity of gut microbes but also lead to lower levels of neuroinflammation.
Prebiotic-rich foods
Prebiotics hold potent anti-inflammatory and anti-oxidative properties. Just like probiotics, eating prebiotic-rich foods – such as asparagus, bananas, greens, onions, garlic, soybeans, and artichokes – can also improve immunity function and brain health. Although very similar in name, prebiotics and probiotics are not the same. Prebiotics are nondigestible nutrients that act as a food source for the healthy bacteria (probiotics) in our gut. Fructo-oligosaccharides (FOS), galacto-oligosaccharides (GOS), and trans-galacto-oligosaccharides (TOS) are the most common prebiotics and have been widely used to stimulate the growth of Lactobacillus and Bifidobacterium (Davani-davari et al., 2019).
High-fiber foods
People tend to associate a high-fiber diet with a healthy digestive system and lower cholesterol levels, but eating high-fiber foods can also directly affect our brain. High-fiber foods – such as broccoli, nuts, oats, beans, seeds, and whole-grain bread – can reduce both intestinal and brain inflammation, preventing or delaying age-related cognitive decline and Alzheimer’s disease. When gut bacteria ferment dietary fiber, they produce short-chain fatty acids (SCFAs), which are energy sources for intestinal epithelial cells and can modulate host immunity. One of these SCFAs, butyrate, acts as a signaling agent in the gut-brain axis, increasing transcripts for brain-derived neurotrophic factor (BDNF), an important molecule for neuronal integrity (Silva et al., 2020). Animal studies have shown that butyrate, at a certain dose, has anti-inflammatory properties on microglia and can improve memory in mice (Matt et al., 2018).
Polyphenol-rich foods
Polyphenols are a class of compounds naturally found primarily in plant-based foods, including fruit, vegetables, grains and beans. Coffee, green tea, red wine, dark chocolate and cocoa are important polyphenol-rich foods and have received much attention during the past years because of their potent antioxidant and anti-inflammatory activities. Polyphenol-rich foods are known to protect neurons against injury induced by neurotoxins, an ability to suppress neuroinflammation, and to promote memory, learning, and cognitive function (Naomi et al., 2023). Therefore, the regular consumption of polyphenol-rich foods throughout life holds a potential to limit neurodegeneration and prevent age-related cognitive decline.
Tryptophan-rich foods
Adequate consumption of tryptophan-rich foods – such as chicken, turkey, red meat, pork, tofu, fish, beans, milk, nuts, seeds, oatmeal, and eggs – have a beneficial impact on neuronal pathways. Tryptophan is one of the nine essential amino acids supplied by diet and the sole precursor of peripherally and centrally produced serotonin. Commonly referred to as the “happy chemical”, serotonin is a key neurotransmitter involved in the modulation of emotional control, food intake, sleep, and pain processing. During inflammation, serotonin acts as a chemotactic agent, increasing proinflammatory cytokine secretion (interleukins IL-1, IL-6, NFκB) and enhancing phagocytosis (Kanova and Kohout, 2021).
The emerging role of the gut microbiota in neurologic diseases has opened an exciting window of opportunity for the treatment of neurologic disorders. Therefore, in order to treat the brain, it seems we must “trust our gut”.
References
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