With the advent of the hygiene hypothesis, probiotics have provided an avenue of hope in curbing the allergic epidemic. The initial enthusiasm has been tempered by recognition of the inherent complexities of this approach. This review examines the current clinical evidence and practical issues in using probiotics and related products, for the prevention and treatment of allergic disease. So far, probiotics have shown more promise, albeit limited, in the primary prevention of allergic disease rather than in the treatment of established disease. These effects have largely been limited to the prevention of early childhood conditions such as eczema, with no consistent effects on other allergic outcomes. There is emerging evidence that clinical effects may be strain specific, but again these findings have been inconsistent. While there have been several meta-analyses to examine probiotics in both the prevention and the treatment of allergic disease, these have been hampered by significant heterogeneity between studies, including wide variations in the strains used, the methods and timing of administration and the age and assessment of allergic outcomes. In any case, these have also become outdated by a series of new studies published in the last year. Although it is not yet clear exactly how the growing number of new studies will modify the results of meta-analyses, it is likely that these will add yet further heterogeneity that will continue to make interpretation of pooled data difficult. At this stage, the effects of prebiotics, synbiotics and postbiotics are even less clear. Thus, while there is little doubt that microbiota modulate immune development and can prevent the allergic phenotype, the optimal way of achieving this is far from clear. Given the current level of evidence, it is not appropriate to recommend prebiotics/probiotics/synbiotics or postbiotics as a part of standard therapy or for the prevention of any allergic conditions. Further studies are needed to address the growing speculation that supplementation with a single probiotic strain may be oversimplistic and that approaches that have a more global effect on colonization may be warranted.
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