As we have been discussing so far in this series of articles, chronic diseases have spread in incidence and prevalence across the world and unfortunately continue to do so. One of the primary underlying reasons for this is the resultant impact of chronic, systemic inflammation.  Within our intestine, or gut, this inflammation wreaks havoc on the symbiosis that we share with our bacteria.  Whereas we are approximately 30 trillion cells, our bacteria make up as many as 40 million cells!  Even more amazing is that our bacteria, or our microbiota, contain 150 times more genes than that of our own.  Thus, by cell count and genetic information we are more bacterial than we are human, which may come as a surprise to many people.  We are first given bacteria in utero by our mother and then even more bacteria are transferred as we actually come through the birth canal to take our first breath.  If we are delivered by Cesarean section, we are given less bacteria than by natural birth, but still receive some bacteria.  Our microbiota quickly evolves in childhood, and it is greatly affected by our environment, e.g., our mother’s milk, food, drugs, in particular antibiotics, and other environmental exposures.  If we are reared in a healthy environment, then we will grow up with a rich, diverse microbiota that helps to ensure we avoid acquiring non-communicable diseases such as allergy, asthma, and type 1 diabetes or cardiovascular disease and type 2 diabetes later in life.  If we are fed poorly and/or are exposed to too many toxins, drugs, or other stressors, then non-communicable diseases may be the consequence.

Likewise, the gut is now recognized as the “other brain,” as so many of our neurotransmitters (hormones that are key for proper brain function) are formed by the cells in the small intestine. For example, most of the serotonin and dopamine, two neurotransmitters that are crucial to our mood and normal brain function, are produced in much greater quantities in the gut than they are in the brain.  Thus, regulating the health of the gut dramatically impact the health of the brain.  Taking it one step further, any shift in the microbiota will impact the overall status of the gut, which then has implications for the status of the brain.

Given the evolving understanding of the links between the microbiota and the gut and the gut and the brain, how likely is it that taking a probiotic can impact the microbiota-gut-brain axis? While the probiotics niche of the dietary supplement industry is currently growing, the idea that taking a probiotic to influence the balance in the microbiota-gut-brain axis is relatively recent.  Most of the research behind the current marketing claims of probiotics relates more to how a particular species or strain may influence symptoms related to irritable bowel syndrome or related phenomena, such as diarrhea, constipation, gas, or bloating.  While certain probiotic species or strains may be effective for irritable bowel syndrome symptoms, the degree of benefit and the most effective type and number of species and strain are lacking conclusive findings at this time.  We are still limited in our knowledge of the microbiota, how complete it really is, the timing of its development, and what factors influence that development.  This level of information is necessary to determine a probiotic may modulate the microbiota-gut-brain axis.

While researchers have classically focused on utilizing probiotics for gut health, the evidence is growing, at least among animal studies, that probiotics can be used to modulate emotional responses and structural functions of the brain. For example, germ-free mice show changes in their responsiveness to stress and anxiety and brain chemistry compared to normal mice.  Probiotics have also been used in adult rodents to modify pain and emotional behaviors and brain biochemistry.  In addition, much is being learned about how the microbiota and probiotics ultimately affect the brain through specific physiological mechanisms.  These mechanisms include molecules such as amino acid metabolites and short chain fatty acids, immune system functions, and vagal nerve activation, which ultimately impact our affective and sensory perceptions.

Because of these discoveries, it has been suggested that the microbiota might affect human behavior and that changing the makeup of the flora could likewise play a role in brain health and functioning in humans. However, at this point the data are not very strong in humans.  Nonetheless, a few studies have recently been published demonstrating some efficacy of probiotics on the brain or related emotional outcomes.  For example, a four week study in healthy women showed that a probiotic formula (containing Bifidobacterium animalis subsp Lactis, Streptococcus thermophiles, Lactobacillus bulgaricus, and Lactococcus lactis subsp Lactis) compared to no treatment positively affected the parts of the brain that control central processing of emotion and sensation.  The probiotic contained 1.25 × 10¹⁰ colony-forming units per cup of B lactis and 1.2 × 10⁹ colony-forming units per cup of S thermophilus and L bulgaricus.  Another recent study showed that Lactobacillus helveticus and Bifidobacterium longum had a positive psychological effect and a reduction in serum cortisol in adults.  Besides these, the data among humans are few and far between at this point, and the existing data do not offer conclusive recommendations.  Thus, how much a probiotic can modulate the microbiota-gut-brain axis is a question that likely cannot be answered with absolute certainty any time soon.  Nonetheless, in the meantime, it would appear that the use of a probiotic can positively impact the microbiota, which theoretically should improve gut-brain health in the process.

One of the main limitations to recommending a probiotic for any condition relates to the type of bacteria, i.e., the species and/or the strain. Similarly, you would not take folic acid for a vitamin D deficiency and expect your vitamin D level to improve.  Likewise, the research in probiotics has a particular need to clarify which bacterial strain has an effect, if any, on an outcome or symptom.  Based on the current state of science, it would be false and misleading to say that one bacteria’s effect on gut or brain health means that all types of bacteria can do the same thing.  This is being shown in the literature related to irritable bowel symptoms, but is not even remotely close to being explored in the microbiota-gut-brain axis at this point in time.  Given the findings so far, the data may eventually show that a probiotic impacts the brain and our perceptions of the world around us.  Like with all dietary supplements, taking a high-quality product that has been backed by research supported by the manufacturer is the best recommendation to make regarding taking a daily probiotic.  A probiotic could likely be incorporated into your daily supplement regimen with little risk and as the literature evolves more clarification will hopefully be achieved regarding how much, what strain, and for how much time it would be required to demonstrate sustained effects on the microbiota-gut-brain axis by a probiotic.  Meanwhile, a diverse, predominantly whole-food, plant-based diet will have a great impact on the health and status of your microbiota, which will not only keep your gut functioning properly, but have a dramatic impact on your brain and the rest of your organ systems as well.