Probiotics are not just for digestive health. The gastrointestinal tract is where our body is able to determine which parts of the foods we eat should be allowed passage into our body, and which should not be allowed entry. We want to absorb nutrients from the food we eat, but we do not want to allow any infectious bacteria or viruses entry into the body through the GI tract. If an apple was contaminated with a pathogen, for example, the gastrointestinal tract would act to prevent the entry of this pathogen into the body, ideally sparing us from infection.

It makes sense, then, that the digestive tract can have an important role in immunity.

The gut microbiome, probiotics and immunity

The gut microbiome refers to the microorganisms (microbiota) living in our intestines. Gut microbiota play a role in gut health and management of several gastrointestinal disorders. The composition of the gut microbiome is affected by dietary intake and can be changed by dietary carbohydrates, fat intake, prebiotics, and probiotics.

Probiotics are defined as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host”. Usually this benefit is exerted in the gastrointestinal tract. It is key to remember that the health benefits of probiotics are strain specific.

Probiotics exert health effects by nonspecific, species-specific, and strain-specific mechanisms. The nonspecific mechanisms vary widely among strains, species, or even genera of commonly used probiotic supplements. These mechanisms include inhibition of the growth of pathogenic microorganisms in the gastrointestinal tract, production of bioactive metabolites (e.g., short-chain fatty acids), and reduction of luminal pH in the colon. Strain-specific mechanisms can include vitamin synthesis, gut barrier reinforcement, bile salt metabolism, enzymatic activity, and toxin neutralization. Through all of these mechanisms, probiotics might have wide-ranging impacts on human health and disease. (National Institutes of Health Fact Sheet)

How do probiotics impact immunity?

Immune health is one of the more commonly studied health outcomes of probiotics. When it comes to researching immunity, studies often measure the frequency of the common cold, or upper respiratory tract infections (URTIs) and the duration and severity of cold symptoms among study participants.

A 2015 Cochrane Review titled Probiotics for Preventing Acute Upper Respiratory Tract Infections, which included 13 randomized controlled trials, found that probiotics were significantly better than placebo for reducing the number and duration of upper respiratory tract infections. This means that probiotics are likely working with the immune system to have a protective effect against the pathogens that cause URTIs.

The authors of the review suggest probiotics can influence immunity in the following ways:

1. Probiotics have been shown to help protect against infection by improving the strength of the intestinal barrier between the inside of the intestine and our body. This reduces the ability of infectious microbes to enter our body via our gastrointestinal tract.
2. Some probiotics can produce proteins or acids that inhibit the growth of pathogens in the GI tract
3. Some probiotics, or the products they produce, can interact with the immune cells of the human body to influence their effectiveness (LPI, Cochrane). For example, some probiotics can increase the production of cytokines in the intestine. These cytokines act as chemical messengers to regulate immune responses.

It is important to remember that the benefit will depend on the specific strain of probiotic being used in a study. For example, this study found that consuming Lactobacillus gasseri PA 16/8, Bifidobacterium longum SP 07/3, and B. bifidum MF 20/5 together for at least 3 months significantly shortened common cold episodes by almost 2 days and reduced severity of symptoms. This study found that providing BC30™ to kids for 12 weeks significantly reduced the frequency of cold symptoms. Any claims made against a probiotic would need to be substantiated by studies using that strain.

Probiotics and COVID-19

Probiotics are even being explored for COVID-19 to prevent secondary bacterial infection. However, the authors note “currently, there is no direct clinical evidence that the modulation of gut microbiota plays the therapeutic role in the treatment of COVID‐19, but we speculate that targeting gut microbiota may be a new therapeutic option or at least an adjuvant therapeutic choice.” This will be an interesting area to keep an eye on as the science progresses.

Choosing a probiotic

Resources from the International Scientific Association for Probiotics and Prebiotics

The probiotic marketplace can be confusing for consumers. See here for some basic information on choosing a probiotic and reading a probiotic label. Some basic principles to guide your search:

• There is no one strain or one dose that is best. Sometimes lower dose products or products with fewer strains have the best evidence.
• Any health benefit claim made should be substantiated with a human trial. But the types of claims allowed in the USA on foods and dietary supplements are restricted by law. Contact the manufacturer to get information on what studies have been conducted, or consult Clinical Guide for Probiotic Products Available in the United States.
• One of the biggest challenges in the probiotic market is keeping the probiotic strain alive. Responsible manufactures go to great lengths to be sure their probiotics retain viability and deliver an efficacious dose through the end of the product’s shelf life. Unfortunately, not all products on the market are responsibly formulated. Consumers should buy products from companies they trust.