Probiotics for the prevention of allergy: A systematic review and meta-analysis of randomized controlled trials

Allergic diseases are increasingly common, and it is estimated that up to 20% of the US population experiences atopic dermatitis, food allergy, asthma, allergic rhinitis, or conjunctivitis. The decrease in infectious diseases in developed countries has been associated with the risk for allergies, leading to the hygiene hypothesis for the rise of allergic disease. In order to inform World Allergy Organization guidelines, Cuello et al have examined the available data on the use of probiotics for the prevention of allergy (J Allergy Clin Immunol 2015; 136(4):952-961).

The composition of the gastrointestinal microbiota promotes potentially antiallergenic processes: T_H1-type immunity; generation of transforming growth factor (TGF), which has an essential role in suppressing T_H2-induced allergic inflammation and induction of oral tolerance; and IgA production, an essential component of mucosal immune defense. Alterations in these microbiota, the early and most massive source of microbial exposure, may underlie the allergy epidemic. As such, the use of probiotic supplementation could promote an adequate microbiota balance, which could in turn prevent the development of allergies.

The authors systematically reviewed randomized trials assessing the effects of any probiotic administered to pregnant or breastfeeding mothers and/or infants. Infants ingest the supplements as an oral preparation or within formula, and mothers take them while they are pregnant or breastfeeding. The 29 studies that fulfilled the specified inclusion criteria showed probiotic supplementation decreases the risk of eczema, including atopic eczema in infants. There was no evidence that probiotics prevent the development of other allergies.

The authors state the limitations of their findings stem from the limitations of the available body of evidence on this topic. Their confidence that one would observe effects on eczema in real life is low, due to the paucity of direct evidence, high likelihood of bias in primary studies, and great variability in the probiotics that were included. They call for future trials focused on the most common probiotics that measure and report effects in the prevention of all allergic diseases, as well as potential adverse effects, reducing the overall risk of bias.