The connection between the gut microbiome and various diseases has been attracting attention. The gut-brain axis has long been acknowledged as a close relationship between the gut and central nervous system (CNS). Moreover, the gut is closely related to the immune system, and both the gut environment and the immune system are implicated in the pathogenesis of immune-mediated neurological diseases such as multiple sclerosis as well as neurodegenerative diseases such as Parkinson’s disease, both of which have been associated with dysbiosis. Although the exact mechanisms by which the gut environment regulates CNS inflammation remain unclear, metabolites or components produced by gut bacteria affect not only the immune system but also glial cells and contribute to disease pathology.

The human gut microbiota changes dynamically from birth through infancy and stabilizes by the age of three. The gut microbiota formed during infancy plays a major role in human health and disease, including allergic diseases, and its disruption (dysbiosis) can affect health status not only in the short term but also in the long term. Although dysbiosis has been reported to precede the onset of typical allergic diseases in children such as atopic dermatitis, asthma, and food allergies, the involvement of specific bacterial genera is not clear. Several studies have shown that butyrate, a metabolite of intestinal bacteria, is involved in the remission of allergic diseases, but the involvement of short-chain fatty acids in the onset is not yet clear. The formation and development of the gut microbiota is greatly influenced by mode of birth, the presence or absence of antibiotic use, and nutritional practices during infancy. In the prevention of allergic diseases by probiotics and other means, postnatal interventions are not effective, but the effectiveness of interventions during pregnancy has been reported. Analysis of the effects of foods and nutrients on the intestinal microbiota is particularly desirable in the future.