Science Says A Healthy Gut Is Crucial To A Healthy Immune System

While oftentimes probiotics are just short-term visitors to our gut, it doesn’t mean their beneficial impact isn’t felt by the immune system and the long-term microbial residents residing in our bodies.

The word probiotic translates to “for life.” The World Health Organization (WHO) defines them as “live microorganisms which, when administered in adequate amounts, confer a health benefit to the host”.[1]

As we’ll see, probiotics play an important role in optimizing our immune function and supporting the health of our gastrointestinal tract.

The Microbe-Immune System Connection

Our gut is home to some 500 to 1000 species of bacteria. In total, this conglomerate weighs approximately five pounds and outnumbers the totality of our own cells. These gut microbes are in constant communication with the part of our immune system located in the gut. In fact, 70-80% of the immune system resides in the gut, so it’s safe to say that the immune system is largely a bacterial communication system.

The immune system in our gut is known as the mucosal immune system. The mucosal immune system is comprised of a wide array of immune cells who are tasked to scrutinize the gut environment and stay in continual conversation with the resident microbes. If any foreign pathogenic invader is present, these immune cells will readily jump into action and eliminate the threat.

In an optimal situation, the mucosal immune system exists in a delicate balance between anti-inflammatory and pro-inflammatory activities. If it launches too weak of an immune response, invading pathogens can easily make their way through the GI tract and penetrate into intestinal cells, causing infection. If it mounts an overactive attack, this can result in excessive inflammation and cellular damage, resulting in things like allergies, inflammatory bowel diseases, and autoimmune conditions.

Probiotics can help finetune both the strength and extent of the immune response. As they make their way through the gastrointestinal tract, they communicate with our resident microbes and intestinal cells, and in the process revitalize our body’s immune defenses. By “teaching” our immune system an optimal level of sensitivity and responsiveness to foreign substances, they can prime our immunity against serious pathogenic assaults in the future.

Probiotics Prevent Pathogenic Bacteria from Colonizing the Intestines

Probiotics can block pathogenic bacteria from establishing residence in the gut in the first place. In essence, any foreign invader will be outcompeted for intestinal real estate if sufficient quantities of beneficial microbes are present on the intestinal turf known as the mucosal barrier.

This gooey mucus lines our intestinal cells and serves a two-fold function: it feeds beneficial bugs essential carbohydrates, and it gives our own cells a safe distance from the microbes. As it turns out, probiotics can help reinforce this barrier by helping our own cells secrete optimal quantities of this mucus.

In this way, our epithelial mucus-cell barrier is strengthened and regenerated, thereby feeding the residing beneficial microbes and preventing dysbiosis, or “leaky gut.” Leaky gut occurs when this mucus barrier is breached, spilling microbial contents well past the intestinal wall.

Probiotics Help Ward Off Infections

Probiotics can also assist our immune system in warding off infections in more direct ways. Probiotics can induce our intestinal cells to release molecules known as defensins. These antimicrobial secretions are used by our body to ward off infectious invaders like viruses, bacteria, and fungi.[2] In addition, many beneficial microbes produce lactic acid, a low pH metabolic byproduct that is known to be antipathogenic.

Many beneficial species also produce byproducts known as short-chain fatty acids or SCFAs. SCFAs help optimize the immune response by inducing a type of immune cell called T-regulatory cells. Treg cells are well-known to mediate inflammatory responses, and in good numbers, work to reduce the incidence of allergies, autoimmune diseases, and colitis.[3]

Overall, there is a large, emerging body of research in animals and humans that suggests probiotics can reduce both the severity and duration of infectious disease. One large study conducted in 2011 found that consumption of several Lactobacillus strains for 12-weeks significantly reduced common cold episodes compared to controls that didn't use the probiotics.[4] A number of other studies have found that probiotics can have a positive impact on individuals suffering from infectious diarrhea by reducing the severity and duration of symptoms.[5]

Final Thoughts

It’s well established that adequate microbial exposure in early life is tightly linked to healthy immune system development. What is increasingly being uncovered is that diversity within this population of bacteria is absolutely paramount to health. If diversity is compromised, say, from prolific antibiotic use, this can misdirect the immune system and create long-lasting health consequences.

Probiotic supplementation can support microbial diversity with extra “good strains” and optimize the immune system to maintain health and aid in the fight against infection.

References

• Food and Agriculture Organization and World Health Organization Expert Consultation. Evaluation of health and nutritional properties of powder milk and live lactic acid bacteria. Córdoba, Argentina: Food and Agriculture Organization of the United Nations and World Health Organization; 2001.
• Schlee M, Harder J, Koten B, et al. Probiotic lactobacilli and VSL#3 induce enterocyte beta-defensin 2. Clin Exp Immunol. 2008;151:528–535.
• Zhao, H. M., Huang, X. Y., Zuo, Z. Q., Pan, Q. H., Ao, M. Y., Zhou, F., Liu, H. N., Liu, Z. Y., & Liu, D. Y. (2013). Probiotics increase T regulatory cells and reduce severity of experimental colitis in mice. World journal of gastroenterology, 19(5), 742–749.
• Berggren A, Lazou Ahren I, Larsson N, Onning G. Randomized, double-blind and placebo-controlled study using new probiotic Lactobacilli for strengthening the body immune defense against viral infections. Eur J Nutr. 2011;50:203–210. [EJN]
• Allen, S. J., Martinez, E. G., Gregorio, G. V. & Dans, L. F. Probiotics for treating acute infectious diarrhoea. Cochrane Database of Systematic Reviews Issue 11. Art. No.: CD003048