The hygiene hypothesis proposes that the growing epidemic of atopic eczema, allergic rhinoconjunctivitis and asthma is related to reduced exposure to microbes at an early age as a result of environmental changes in the industrialized world. These include improved sanitation and living conditions, vaccinations and antimicrobial therapy, together with declining family size and changes in dietary intake. Recent scientific advances demonstrate that the hygiene hypothesis needs to be extended in three respects. Firstly, rigorous research in the field of probiotics points to the importance of the collective composition and the compositional development of the gut microbiota in consolidation of healthy immune responsiveness. Secondly, immunomodulatory and suppressive immune responses have been shown to complement the original immunological basis of the hygiene hypothesis, the so-called T helper 1/T helper 2 paradigm. Thirdly, host-microbe interaction appears to affect the risk of developing not only atopic disease but also other inflammatory Western lifestyle diseases, including obesity. The results of experimental studies suggest that deviations in gut microbiota composition predispose to excessive energy storage and obesity, and, more recently in humans, aberrant compositional development of the gut microbiota has been shown to precede overweight, inviting enormous possibilities to reach preventive and therapeutic applications in weight management.
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