If our goal is to understand the interplay between the gut microbiome and the brain, it helps to first understand the factors that influence the overall health of the gut. Throughout our life numerous factors influence the gut-biome's health; and this can include genetics, health status, environment, diet, lifestyle and even mode of birth.
Some of these factors can be more influential than others because they are more readily modifiable. An individuals' diet can fall under this category, although it can be argued that not everyone can easily modify their diets, especially when we're talking about access to healthy foods.
Here are five factors influencing gut-biome health:
Environment
Antibiotics/Drugs - Antibiotics are known to destroy both pathological (bad) and beneficial (good) microbes indiscriminately. Additionally, growth of unwanted microbes can be encouraged due to a lack of competition. Many other drugs (antipsychotics and anti-inflammatory meds, like steroids) also degrade the biome. Each alteration impacts our gut motility and influences digestion.
Heavy Metals- Our gut bacteria also play an important role in metabolizing heavy metals that are absorbed through our skin, the foods we eat, and the air we breathe. For example, our gut bacteria are able to transform inorganic arsenic into a less toxic version and alternatively, our gut (if altered) can generate a more toxic version of mercury.
Pesticides- Several studies have demonstrated an association between gut biome toxicity and pesticide exposure. The existing defense is that some pesticides used in agriculture are safe to humans because their targeted pathways do not exist in the human body; but this fails to consider how our gut microbes react and/or are altered by the exposure.
Food Additives - Artificial sweeteners, emulsifiers, and preservatives are added in food products with pre-approved safety limits. They are considered safe because they are poorly metabolized by the human body. However, similar to agricultural pesticide use, our gut bacteria's health was not considered when these limits were established even though it is the bacteria that are actively involved in the metabolism of these additives.
Smoking - This shouldn't surprise anyone, but smoking changes both the biome (the intestinal mucosa) and the microbial population, thus impairing immune response.
Age
Age is an interesting factor when it comes to influencing the biome as it is ever-changing. Changes that come with age can potentially reduce the diversity of a biome.
Microbiome development begins in utero and mode of delivery even determines which types of bacteria will dominate the gut initially. For example, cesarean deliveries transfer bacteria from mom's skin versus the gut bacteria introduced during vaginal deliveries.
There are also age-related changes in nutrient intake that can potentially alter the gut biome. Advanced age is often associated with altered nutrient intake due to changes in eating habits. Eating habits change with declining dentition (ability to chew foods appropriately and without pain), decreased salivary function (prepping our body for digestion), altered digestion, and decreased or increased intestinal transit time.
Age-related sensory changes, including taste and smell, may cause certain foods to become unappealing.
Diet
Diet is arguably the most influential factor.
The food in our diet is broken down into carbs, protein, and lipids which can then be further metabolized (broken down) by the gut biome.
Changes in diet, even for a short duration, can alter the gut biome. For example, research shows drastic changes in the diversity of the gut biome when an individual consuming a plant based diet switches to an animal based (and vice versa), even for just 24 hours.
Diets rich in fat and/or protein (often characterized as a western diet) show reductions in butyrate-producing bacteria. Butyrate itself is a short-chain fatty acid that provides fuel for the cells of our gut lining.
Diets higher in fermentable fibers and probiotics can actually restore levels of the butyrate bacteria and they can also increase levels of bifidobacteria (a bacteria connected to immunity).
Gut microbes also help to make a number of vitamins (B12, B6, B5, B3, vitamin D, and vitamin K).
Polyphenols are a group of compounds found in grapes, apples, pears, cherries, berries, tea, coffee, red wine, legumes and chocolate. Polyphenols aid in digestion and inflammation, and they are also known for their brain neuron protecting qualities. It is gut microbes that ferment these compounds for us making it possible for us to access the benefits.
Lifestyle
If there was an order to changeable influencing factors, lifestyle would be second to diet.
Research suggests that exercise can 1- enhance the number of beneficial bacteria and 2- enrich the bacterial diversity. Low intensity exercise can influence the gastrointestinal tract (GIT) by reducing transient stool time. This means there is less contact time between pathogens and the GI mucus layer which in turn reduces the risk of colon cancer, diverticulosis, and inflammatory bowel disease.
It must also be stated that research shows an association between food restriction and exercise (especially intense exercise practices), as decreasing the good bacteria and increasing the bad bacteria that cause gut disorders.
Sleep quality is positively associated with bacteria diversity. We mostly understand that sleep is regulated by our homeostatic and circadian processes, but research is now showing sleep is also connected to our gut biome diversity.
Stress can also alter the composition and function of the gut biome. Luckily, acute stress (normal life experiences) is not shown to impact the gut biome long-term. However, over time, persistent stressors can disturb the biome equilibrium. It's important to note how childhood adversity (and early life stressors may extend to adulthood). These stressful life events are strongly linked with gut biome alterations which are further associated with depressive and anxiety based disorders (which we'll talk more about in the next post).
Genetics
Microbial communities within the gut are dynamic (ever changing) and they differ from one person to another. Research indicates that the gut microbiota can be similar among family members, but that individual variations are observed. This affirms that we are not locked into a certain level of health purely based on our genetics. I would go so far as to say out of the other factors mentioned, genetics is the least influential while still being important to discuss when it comes to gut biome health.
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Gut - Biome Series References
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12) Sandhu, K. V., Sherwin, E., Schellekens, H., Stanton, C., Dinan, T. G., & Cryan, J. F. (2017). Feeding the microbiota-gut-brain axis: Diet, microbiome, and neuropsychiatry. Translational Research, 179, 223–244. https://doi.org/10.1016/j.trsl.2016.10.002
13) Tu, P., Chi, L., Bodnar, W., Zhang, Z., Gao, B., Bian, X., Stewart, J., Fry, R., & Lu, K. (2020). Gut microbiome toxicity: Connecting the environment and gut microbiome-associated diseases. Toxics, 8(1), 19. https://doi.org/10.3390/toxics8010019
14) Wastyk, H. C., Fragiadakis, G. K., Perelman, D., Dahan, D., Merrill, B. D., Yu, F. B., Topf, M., Gonzalez, C. G., Van Treuren, W., Han, S., Robinson, J. L., Elias, J. E., Sonnenburg, E. D., Gardner, C. D., & Sonnenburg, J. L. (2021). Gutmicrobiota-targeted diets modulate human immune status. Cell, 184(16). https://doi.org/10.1016/j.cell.2021.06.01
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