Jun Kim Ph.D.
Worldwide sensitization rates to common allergens among school children are approaching 40%-50%. The prevalence of diseases such as allergic rhinitis, asthma, and atopic dermatitis have been increasing especially in the developed world. For example from 1980 to 1994 the prevalence of asthma increased by 75%, while in the developing world, the lower prevalence of allergic disease has not changed significantly during the same period. The disorders have genetic basis and are heritable, but the rapid increase suggest that there are external factors from the environment. The hygiene hypothesis posits that increased human longevity and allergy prevalence are consequences of lower rate of infections during childhood. Although mechanisms remain unclear, the hygiene hypothesis is generally considered strong.
The association between the gut microbiota and allergy has been focused in multiple studies. For example, associations have been found between allergic diseases and differences in the gut microflora among children in countries with a low and high prevalence of allergies. Much evidence suggest that establishment of the gut microbiota plays an important role in directing immune system development. Such findings led to the microbial hypothesis, which states that exposure to microbes affect the development of the immune system and allergic diseases, to explain the hygiene hypothesis. Given the immunological basis of allergic diseases and probiotic effects on immune system, probiotics have been investigated for their beneficial effect in preventing allergic diseases.
Changes in the gut microbiota can modulate immune response in distal organs, and studies suggest probiotics can alleviate allergic rhinitis[6,7]. Probiotics prevented the pollen-induced infiltration of white blood cells into the nasal mucosa and altered immune response in allergic rhinitis[6–8]. A study of young children (6–24 months) showed that orally taken Lactobacillus rhamnosus mildly decreased allergic sensitization. Another study showed that Lactobacillus casei Shirota modulated immune response and alleviated the severity of symptoms in adult patients. However, there are other studies showing few or no clinical benefits of probiotics. Allergic rhinitis may be subdivided into different kinds and new studies that take this into consideration may provide clearer results.
Some studies report beneficial effect of probiotics for atopic dermatitis. For children, it was especially effective when both prenatal and postnatal probiotics were used. Also, the preventive effect was greater in those with a family history of allergic diseases[13–18]. Meta-analyses suggest that there is convincing evidence for probiotics preventing the development of atopic dermatitis in high-risk infants but with varying degrees among the different disease subtypes and treatments[19–21]. At this point a preventive effect of probiotics is not confirmed but the results seem to show at least some benefits.
In a study with infants with atopic dermatitis, probiotics prevented asthma-like symptoms. Another study showed that the clinical severity of asthma and allergic rhinitis decreased in the probiotic-treated patients compared to the controls, suggesting that probiotic supplementation may have clinical benefits for children with allergic airway diseases. Although the possibility of using probiotics to treat asthma has been promising with animal models, no significant effect has been shown in human trials[24,25]. For example, in a study with 1223 mothers with infants at high risk for allergy, prenatal and postnatal use of probiotics did not have a preventive effect on asthma, although some allergic diseases occurred less in cesarean-delivered children receiving probiotics. Similar results were shown in two other trials with no significant difference in terms of wheezing and prevalence[15,21]. In general, the results are affected by variables such as method of delivery, supplementation periods, and follow-up periods, which makes well-controlled trials very challenging.
The hygiene hypothesis and the microbial hypothesis suggest that the link between the immune system and the gut microbiota affects the development of allergic diseases. Currently, the evidence is not strong enough to conclude that there is an absolute benefit. There are many contradicting results with confounding variables. However, collectively there has been a significant progress in understanding the mechanism behind how the gut microbiota can affect the human body and allergic diseases, and this can lead to new trials with more effective probiotics. Some of the proposed mechanisms will be discussed in the next article.
Disclaimer: The above article is sponsored by Thyrve, the world’s first Gut Health Program that incorporates microbiome testing and personalized probiotics to ensure a healthier gut, happier life, and a brighter future.
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