The common cold is a frequent viral infection of the upper respiratory tract, most commonly caused by rhinoviruses. Since no cure exists, the focus is often on prevention and symptom reduction. Probiotics are live microorganisms that, when consumed in adequate amounts, confer a health benefit to the host. Interest has grown in whether manipulating the gut microbiome can influence the immune system and offer a defense against respiratory illnesses.
How Probiotics Influence Respiratory Immunity
The mechanism linking gut bacteria to respiratory health is known as the gut-immune axis, which centers on the Gut-Associated Lymphoid Tissue (GALT). GALT represents a significant portion of the body’s immune cells residing in the digestive tract. Probiotics interact directly with these immune cells, including dendritic cells, initiating a systemic immune response.
One outcome of this interaction is the increased production of secretory immunoglobulin A (IgA) antibodies. IgA is a protective antibody found on mucosal surfaces, such as those lining the respiratory tract, where it helps neutralize pathogens before they can cause infection.
Probiotics also promote the development of T-regulatory cells (Tregs), which are specialized immune cells that help control and suppress excessive inflammation. By stimulating Tregs, probiotics can reduce systemic inflammation, potentially lessening the severity of symptoms throughout the body.
Clinical Evidence for Reducing Cold Severity and Duration
Clinical trials and systematic reviews have shown a modest but positive effect on common cold outcomes. Evidence suggests that prophylactic use of probiotics can help reduce the incidence of upper respiratory tract infections (URTIs). One meta-analysis estimated that probiotics may reduce the number of people experiencing at least one URTI by about 24%.
The evidence is compelling regarding the duration and severity of cold symptoms. A systematic review found that probiotics can reduce the mean duration of an acute URTI episode by approximately 1.22 days in both healthy children and adults.
Probiotic supplementation has also been linked to a reduction in the severity of symptoms. People taking probiotics experienced less severe coughing, fever, and rhinorrhea (runny nose) compared to those taking a placebo. Furthermore, a reduction in the need for antibiotics to treat URTIs has been observed, suggesting fewer secondary bacterial infections or complications.
Identifying Effective Probiotic Strains and Dosage
The benefits of probiotics are highly strain-specific, meaning not all products offer the same immune support. Efficacious strains often belong to the Lactobacillus and Bifidobacterium genera. When evaluating a product, consumers should look for the full genus, species, and strain designation listed on the label.
Documented Efficacious Strains
Examples of strains with documented benefits for respiratory health include:
- Lactobacillus acidophilus NCFM
- Bifidobacterium lactis Bi-07
- Lactiplantibacillus plantarum HEAL9
- Lacticaseibacillus paracasei 8700:2
Dosage is also a factor, and the general recommendation for immune-modulating effects is in the billions of Colony Forming Units (CFUs). Many successful clinical trials used daily doses ranging from 1 to 100 billion CFUs per day. Probiotics are considered safe for healthy individuals, but because they are often regulated as food, the actual content and potency can vary between commercial products.