Although host-related factors are important for the formation of gut microbiota, environmental factors cannot be ignored

The gut microbiome is essential to human health. However, little is known about the influence of the environment versus host-related factors (e.g. genetic background, sex, age, and body mass) in the formation of human intestinal microflora. Here, we present evidence in support of the importance of host-related factors in the establishment and maintenance of individual gut assemblages. We collected fecal samples (n = 249) from 44 Korean naval trainees and 39 healthy people living in Korea over eight weeks and sequenced the bacterial 16S rRNA genes. The following hypotheses were tested: 1) microbiome function is linked to its diversity, community structure, and genetic host-related factors, and 2) preexisting host-related factors have a more significant effect on gut microbiome formation and composition than environmental factors. For each individual, the difference between the initial gut microbiota and that after eight weeks was negligible even though the 44 naval trainees lived in the same area and received the same diet, the same amount of exercise, and the same amount of physical stress during the study. This suggests that host-related factors, rather than environmental factors, is a key determinant of individual gut microflora. Moreover, eight weeks of physical training and experiencing the same environmental conditions resulted in an increase in the species Bifidobacterium, Faecalibacterium, and Roseburia in most trainees, suggesting a healthier intestinal environment. IMPORTANCEIn order to understand the role of human gut microbiome, it is important to know how individuals gut microbiota are formed. In this study, we tested the host-related factors versus environmental factors to affect gut microbiome and found that the former have a more association. However, we also found that the controlled environment give an effect on the gut microflora as well. This study provides preliminary evidence that differences in the formation and diversity of gut microbiota within a population could be determined by host-related factors rather than environmental factors.