The Gut and the Brain Work Together to Keep Us Healthy

Think of all the microbes in our gut - it’s pretty impressive the way our gut is thriving with life. There are trillions of microbes from at least a thousand different bacterial species living together to make up an incredible ecosystem - the gut microbiota. And those microbes aren’t just hanging out in the gut, waiting to be fed; they are actually very important for our physical and mental health! We live in symbiosis and we need each other equally.

The gut’s potential impact on physical health becomes obvious if you recognise that most of our immune cells originate in the digestive system, placing it as a key player in our immune defence arsenal. But the gut also has a (surprisingly) huge impact on our mental health. That’s because the gut and the brain are intimately connected. Anyone who’s ever been in a stressful situation has felt it: our mind takes hold of our body - we feel it in our heartbeat and we feel it in our gut.

This association has become clearer recently, as research on the gut-brain axis began to flourish. Growing evidence has been showing that our intestinal flora can release molecules that effectively and directly influence the brain’s chemistry. There are indications that they can even regulate the production of neurotransmitters, thereby influencing not only our overall health, but also our mood and behaviour. This awareness of the gut’s importance has shifted many concepts of health and disease.

The gut is so important that it even has a nervous system of its own – the enteric nervous system (ENS). The ENS is so massive (it probably has more neurons than the spinal cord!) that it’s often referred to as our “second brain”. The ENS controls the gut, but it also responds to its changes and to potentially threatening conditions, reporting them to the central nervous system (CNS). The brain and the gut communicate through neurons, but also through chemicals and hormones that travel in the blood to signal hunger, stress and danger, and to control immune responses, emotions, reactions to stress, feeding behaviour and digestion.

The vagus nerve is an important link in this brain-gut chain; it’s a highway for the bidirectional communication between the gut and the brain. A vast majority of the nerve fibres in the vagus nerve are devoted to keeping the brain posted on the state of our viscera. So, whenever there’s a significant change in the gut that can potentially affect the survival of our microbiota, the brain is promptly informed.

The CNS can influence the microbial composition of the gut by altering motility, secretion, and nutrient delivery in the gut. On the other hand, an adequate bacterial colonization of the gut is crucial to the proper functioning of our body, including our brain. The intestinal flora produces a myriad of molecules that can travel through the blood to pretty much everywhere to exert their actions, eliciting immunological and metabolic responses, and influencing mood and behaviour. And this is heavily dependent on our diet and feeding habits.

Studies in humans have linked a lack of diversity in the microbiota to a number of conditions that go way beyond gastrointestinal disorders. As an example, potential conditions include obesity, eating disorders, inflammatory diseases, chronic pain, anxiety, depression, stress, memory consolidation or cognitive decline. A more diverse diet seems to lead to a more diverse ecosystem in the gut and to a higher resistance to disease. This hints to the possibility that specific dietary interventions may potentially help improve health.

Stress is a great example of how easily the gut microbiota is disrupted. Stress has the power to actually change the intestinal microbial composition. Stress can induce the release of signalling molecules by neurons, immune cells, and endocrine cells that will act on the gut. And the opposite is also true – the microbes in our gut can strongly affect our stress levels and our response to stress. Experimental research has shown that the stress response is higher in animals whose gut has been deprived of germs, but when they’re supplied with specific bacteria, they become less stressed. This is huge because stress has a massive impact on our wellbeing.

An example of a condition that is closely associated with stress is the “leaky gut syndrome”, which includes symptoms such as bloating, cramps, food sensitivities, and visceral pain. Leaky gut is not really a pathology by itself - it’s more of a collection of symptoms that can be associated with other pathologies such as celiac disease, Crohn’s disease or inflammatory bowel disease. Its name comes from the increased intestinal permeability to which it is associated and that allows for substances to leak from the gut into the bloodstream. Although its causes are still somewhat unclear, both diet and chronic stress are considered to play a big role and are a target for therapeutic intervention.

In fact, stress is a risk factor for all sorts of diseases and a huge enemy of our brain’s health; it causes anxiety, depression, cognitive impairment and even accelerated aging. Given the effect that nutrition has on the gut microbiota, our diet may actually be a tool to manage stress. And the opposite is also true: managing stress will most likely help keep a healthy gut – it’s a positive feedback loop.

The idea that nutrition can be a means to obtain medical or health benefits has given rise to the term “nutraceuticals” (nutrition + pharmaceuticals), coined in 1989 by the US Foundation for Innovation in Medicine. The term refers to “any substance that is food or a part of food and provides medical or health benefits, including the prevention and treatment of disease”. This includes dietary supplements, functional foods, prebiotics, probiotics, and so on.

There are a number nutraceuticals that can potentially benefit a dietary stress management approach. Probiotics, which are beneficial bacteria and yeasts that can be found in some foods and supplements, have been showing promising effects in many neuropsychological conditions. By helping regulate the intestinal flora, they contribute to a balanced ecosystem in the gut, and to the consequent decrease in mood disorders, anxiety and stress. In that regard, bacteria from the genera Bifidobacterium and Lactobacillus have been the most extensively studied and their effects have been repeatedly validated.

However, in order to produce their effects, probiotics must be able to persist in the gastrointestinal tract by colonizing the gut. That requires, for example, surviving the acidic conditions created by gastric secretions and the myriad of digestive enzymes with which they run across on their way to the gut. Although it is hard to predict which probiotics will effectively colonize the gut and how long probiotic-induced health effects can last, research suggests that to obtain a sustained effect, a continued intake of probiotics is most likely needed.

There are also a number of herbal ingredients with recognized beneficial health effects, namely in what concerns anxiety and stress. L-theanine, an amino acid found in green tea, for example, is mostly known for its neuroprotective effects derived from its anti-oxidant actions. In experimental research, L-theanine was able to reverse stress-induced cognitive impairments. Clinical research has also indicated that L-theanine can reduce psychological and physiological stress responses.

Some adaptogens, which are believed to contribute to the stabilization of our body’s physiological processes, have shown interesting anti-stress properties in experimental research. Reishi, or Ganoderma lucidum, is a mushroom widely used for promoting health and longevity. Despite being scarce, there is experimental data showing that an aqueous extract from Reishi can reduce stress-induced anxiety in rats, and that supports its claimed antidepressant and anxiolytic effects.

Another widely used adaptogen is Ashwagandha, or Withania somnifera. Research on its effect is more abundant and has shown that it contains a vast number of biologically active molecules that have been associated with a number of benefits, such as anti-oxidant, anti-inflammatory, anti-tumor, anti-diabetic, anti-microbial, anti-stress, cardioprotective and neuroprotective effects. The nootropic effects (improvement of mental functions) of Ashwagandha are documented and include neuroprotective and memory enhancement actions. Clinical data has also shown that it has significant beneficial effects on anxiety and stress management.

Anything that relieves stress can potentially affect the gut. Although these are only a few examples of how you can manage stress through diet, and consequently keep a balanced gut, and although clinical evidence is still limited in some instances, experimental data is promising and indicates that it is possible to keep a balanced brain and gut through diet.

Thinking about how our diet can influence our gut’s life and, consequently, our own life, always takes me back to the phrase “you are what you eat”. It’s true – nutrition is key in health, and nutrition applied to disease prevention (or even treatment) is now a thriving research and medical field. In the end, a balanced and diverse diet is the key.

Credit

Sara Adaes is a biochemist with a PhD in neuroscience and with a keen enthusiasm for science communication.

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