Microbiota & Seasonal Allergies:
Understanding the Connection and Supporting Gut Health
Joseph Katzinger, ND Salugenecists Director of Health Science
The Evolution of the Hygiene Hypothesis First introduced in 1989, the “hygiene hypothesis” proposed that exposure to specific microbes, perhaps during crucial windows of development, educates the immune system in a way that modifies the risk for allergic, inflammatory, and autoimmune diseases. Initially, this was proposed in response to the observation that the position of a child in a family (their place in the birth order) was associated with the risk for hay fever in a large national sample from the UK. It was suggested that younger children were more likely to be exposed to microbes (“unhygienic contact”) from their older siblings, thereby conferring some type of protection to the younger children. This idea has gained support and modifications over the years, such as adding the “old friends hypothesis,” the idea that exposure to harmless microorganisms (bacteria, helminths, etc.) that have co-evolved with humans helps the maturation of Treg cells, as deficiencies in these cells lead to either excessive T helper 1 (Th1) or Th2 responses.
The Epithelial Barrier Hypothesis A more recent theory, the epithelial barrier hypothesis, builds on these earlier theories and adds an additional component that may help to explain some of the theoretical gaps; briefly, damage to lining of the GI tract promotes a poorly regulated immune system. This theory posits an initial insult to the epithelial barrier that leads to a vicious cycle, characterized by colonization of the gut with pathogens, loss of healthy commensal organisms, and altered immune function and inflammation in response to these changes in the microbiome. The authors also provide examples of many of the agents capable of damaging the epithelial barrier, ranging from cigarette smoke and air pollution to plastics.
The Connection Between Gut Microbiota and Allergic Rhinitis
There is mounting evidence for a link between allergic rhinitis and the composition of the microbiota, especially early in life, as well as modulation of gut microbiota as a possible intervention. For example, in a 2022 review, several studies describe an altered microbiome among study participants with allergic rhinitis compared to healthy controls. In an analysis published in the American Journal of Rhinology & Allergy, study participants with allergic rhinitis had significantly lower diversity of their gut microbiota, and specific shifts in relative abundance, including elevated amounts of Bacteroidetes and reduced levels
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of Actinobacteria and Proteobacteria (compared to controls). A second analysis published in the International Archives of Allergy and Immunology also found substantially reduced diversity among participants with allergic rhinitis compared to controls, as well as increased Bacteroidetes and reduced levels of Firmicutes, among other differences. Although the precise differences observed between controls and those with allergic rhinitis are not the same from study to study, they consistently have significant differences.
Mechanisms of Microbial Influence on
Immune Function A number of mechanisms help to explain the effect these differences have on immune tolerance and activation. For example, healthy commensal bacteria produce short-chain fatty acids (SCFAs) which modulate the inflammatory process; this includes inhibiting the production of inflammatory compounds in response to LPS, also known as endotoxin, a component of Gram- negative bacteria which generally activates the immune response. But commensal bacteria that produce SCFAs can inhibit LPS- and nitric oxide while inducing the production of anti-inflammatory mediators, such as IL-10.
Clinical Benefits of Probiotic Supplementation The promotion of anti-inflammatory mediators may have clinical benefit. For example, supplementation of a combination of Bifidobacterium longum and Lactobacillus plantarum was associated with higher IL-10 levels after 4 weeks (they remained stable while in the placebo group, IL-10 level dropped), and significant improvement in symptoms along participants with allergic rhinitis, including reductions in rhinorrhea, nasal congestion, watery eyes, and sleep complaints.
That probiotic supplementation may shift the abundance of microbiota to a more favorable profile and improve allergic symptoms has been documented in large meta-analyses as well. For example, in a systematic review of 30 randomized and controlled trials with nearly 3,000 total patients, symptoms and quality of life among those receiving probiotics were significantly improved compared to placebo. A second systematic review and meta-analysis had similar conclusions; probiotics effectively reduced symptoms and improved quality of life among people with allergic rhinitis,
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