The human body is a host for thousands of different species of microorganisms making up the microbiome. From bacteria to viruses to fungi, our microbiome can be found in various locations in our body, such as our skin, mouth, nose, and most abundantly, our gut. 

Our gut, which includes our entire digestive tract, has over 100 trillion bacteria known as the gut microbiota. Scientists refer to the microbiota with its genetic material as a microbiome. Collectively, we carry about 2-3 kilograms of microbiota that vary in size and motion throughout our gut. Microbiota is most densely packed in the distal colon, making up over 60% of stool on a daily basis. Different areas of the digestive tract have varying pH levels to allow for a diverse selection of microbiota to thrive in the body.

Through the foods we eat and the environments we are exposed to, these microbes influence our mood, digestion, hormones, immune function, cognitive abilities, and overall well-being. Because of its key roles in everyday health, the microbiome is considered a supporting organ and has become increasingly researched in the past 20 years. 

The Human Microbiome Project, an initiative launched in 2007 to improve the understanding of microbiota, found that we are each born with a unique network of microbiota as determined by our DNA and birthing process. As we get exposed to different environments in our early years, and as we eat different foods, our microbiome changes accordingly.

GUT-BRAIN CONNECTION

You may have experienced the familiar "gut feeling" about a certain situation, making you uneasy, excited, or driven. This is because our gut acts as a second brain by communicating signals to impact the way we think, act, and feel; our gut instinct is not just metaphorical. These signals are part of the bidirectional communication of the gut-brain axis (GBA), which links the central nervous system (CNS) and the enteric nervous system (ENS) of the body. The GBA has neurons that communicate with the endocrine and immune systems to influence intestinal function. In total, there are over 100 million nerve cells that line our gastrointestinal tract. Our gut microbiota interacts with the GBA - both directly and indirectly - to impact our daily functions.

DIRECT SIGNALING

One way the gut microbiota interacts with the GBA directly is through the vagus nerve. When we eat, our gut neurons send a fast signal to our brain through the vagus nerve. This in turn activates several areas in the brain, such as the hypothalamus and limbic system responsible for regulating emotions.

More specifically, scientists have found neurons - now named neuropod cells - that have a particularly strong activation for sugars and are located throughout the intestinal walls. Once eaten, sugar causes neuropod cells to send a signal via the vagus nerve, which triggers the activation of reward centers in the brain and releases dopamine - a neurotransmitter involved in motivation, craving, and pleasure. A combination of sensing sugar in our mouth, along with activating the neuropod cells in our gut causes the sense of satisfaction we get from sugar. So next time you savor that cupcake and reach out to grab more, you can be sure to hold your neuropod cells responsible.

Interestingly, a recent study published in January of 2022 on Nature Neuroscience shows that blocking sweet taste receptors still resulted in an increase in sugar intake. This suggests that neuropod cells subconsciously drive us to seek more sugar even if we do not taste the sugar in our mouth.

INDIRECT SIGNALING

The gut microbiota indirectly interacts with the GBA by influencing metabolic processes and synthesizing neurotransmitters. For instance, dopamine can be synthesized by or from the microbiota Proteus vulgaris, among many others. This increases the baseline levels of dopamine and enhances mood. In fact, some studies show that close to 50% of dopamine is produced in the gastrointestinal tract. Similarly, scientists estimate that 90-95% of the body's serotonin is made in the gut as well, where it modulates ENS development, inflammation, sensation, and neurogenesis. These neurotransmitters have a significant impact on the way we act and feel throughout the day.

WHAT IS A HEALTHY MICROBIOME?

Scientists are actively trying to define what constitutes a healthy microbiome. Although much of the research in this area is still being produced, a vast amount of data highlights the negative effects of gut microbiota dysbiosis - imbalances in the composition and function of these intestinal microbes - on neurologic, respiratory, metabolic, and cardiovascular functions. Microbiota dysbiosis has been linked to an increased risk of chronic disorders such as Inflammatory Bowel Disease (IBD), Irritable Bowel Syndrome (IBS), metabolic disorders, atherosclerosis, and many more. Treatment options have now considered modulating the GBA to reduce symptoms of such disorders. These include vagus nerve stimulation (VNS) and meditation techniques, which showed an improvement in mood/anxiety disorders and in conditions associated with increased inflammation. 

However, researchers are still investigating the effects of having a diverse selection of microbiota in the gut to prevent disorders in the first place. A 2020 study on Scientific Reports showed a significant positive association between positive emotion and gut microbiome diversity, suggesting that more gut microbiota promotes mental health and enhances mood. Generally, research thus far has suggested that a more diverse microbiome is a healthier one.

MAINTAINING AND IMPROVING YOUR GUT MICROBIOME

Stress stimulates the sympathetic nervous system while inhibiting the vagus nerve. This has profound impacts on the gut microbiome, as it leads to gastrointestinal imbalances and inflammation. Stress can take many forms, including disturbed sleep. Ensuring that the body gets the recommended 7-9 hours of sleep every night is fundamental to gut health. 

It is becoming increasingly known that certain foods enhance the microbiome, while others do not. Current research emphasizes the benefits of eating lots of vegetables, legumes, beans, and fruit. These are high in fiber, which stimulates the growth of good gut bacteria. Fermented foods like yogurt and kombucha have also been shown to enhance the microbiome-immune system connection by increasing microbial diversity and decreasing markers of inflammation. This, along with whole grains and foods rich in antioxidants, enhances gut function and contributes to making you feel your best throughout the day.

But what about prebiotics and probiotics? While this remains an active area of research, prebiotics and probiotics have been shown to benefit the gut environment, especially after periods of stress, travel, or antibiotic treatment. It is recommended to monitor how you feel and find what works best for you when starting these supplements, as there has been a lot of individual variability. 

Among the many benefits that exercising regularly has on the body, it also promotes a healthy gut. The current Physical Activity Guidelines for Americans recommends that adults get at least 150 minutes of moderate intensity exercise each week. Even a simple 10 minute walk after a meal improves gut function.

Additionally, avoiding refined sugar, processed foods, excessive amounts of red meat, and antibiotics have all been linked to a better gut microbiome.

Through our microbiota, the foods we eat can have a significant impact on the way we feel. Whether we like it or not, we are living with ecosystems of microorganisms in our bodies and our genes, actions, and environments determine how much these organisms harm or benefit us. 

ADDITIONAL RESOURCES:

• The Vagus Nerve at the Interface of the Microbiota-Gut-Brain Axis
• The Simplified Guide to the Gut-Brain Axis – How the Gut and The Brain Talk to Each Other
• The gut microbiome
• The preference for sugar over sweetener depends on a gut sensor cell
• Serotonergic Mechanisms Regulating the GI Tract: Experimental Evidence and Therapeutic Relevance