New insight into the hygiene hypothesis: family-size affects gut microbiota

Published Online: July 29, 2013

Numerous studies showed that number of siblings are a protective factor for allergy. Over the years, many potential causal factors, underlying this so-called “sibling effect,” have been presented ranging from in utero programming to childhood infections. In a recent article published in The Journal of Allergy & Clinical Immunology (JACI), Penders et al demonstrate, for the first time, that the bacterial composition found in the human digestive system might be determined by this sibling effect. Within a randomized placebo-controlled trial on the prevention of atopic dermatitis (AD) by oral supplementation of a bacterial lysate in infancy, the authors examined the establishment of the gut microbiota and its role in the development of AD. Next to birth mode and breastfeeding, birth order appeared to have a strong impact on the infant bacterial composition. In particular, colonization rates of clostridia, a common bacterial group or bacterial genus, at 5 and 13 weeks postpartum, decreased with increasing numbers of older siblings. Colonization with clostridia at these ages was also associated with an increased risk of subsequently developing AD. The authors demonstrated that the bacterial composition is likely to be one of the underlying mechanisms explaining the birth order effect causing allergies.These results strengthen the evidence for a causal pathway and provide new leads for the primary prevention of allergies by modulating the gut bacteria.


The Journal of Allergy and Clinical Immunology (JACI) is an official scientific journal of the AAAAI, and is the most-cited journal in the field of allergy and clinical immunology.

AAAAI - American Academy of Allergy Asthma & Immunology