People often talk about a healthy intestinal flora, but until now it has not been clear what a 'healthy' gut microbiome actually means. Researchers at the University Medical Center Groningen have now been able to properly determine the composition of a healthy gut microbiome through large-scale research in more than 8,200 Lifelines participants. The detailed analysis, reported today in Nature, allowed the UMCG team to not only compile a deep catalog of the microbiome but also to indicate which gut microbes are associated with disease and which with health. Their study also shows that people living together in a household share a more similar composition and that sharing a living environment and lifestyle has a greater influence than our genetics. Interestingly, the composition of the microbiome is not only determined by current living conditions but also by past circumstances such as the urbanicity of one’s childhood home.