The present inventors have found that the content ratios of isoalloLCA, 3-oxoLCA, alloLCA, and 3-oxoalloLCA in the feces of centenarians are higher than those of younger ones, and have also identified gut microbiomes peculiar to centenarians involved in the production of these bile acids. Furthermore, it has been found that these bile acids, enzymes involved in the production thereof, and bacteria producing the enzymes reduce the risk of infection with pathogens, prostate cancer, and the like, and are involved in longevity.