Fig. 4

Linear regression analyses relating Prevotella to Bacteroides ratio with age at relocation to the mainland USA, among individuals who relocated to the USA before age 26. At the time they were studied, all cohort members were 25 years or older. Thus, by limiting to those who relocated to the mainland USA between birth and age 25, we could make a comparable analysis of age at relocation within groups defined by attained age. For example, only the oldest age groups of 55–64 years old (blue) or 65+ years old (magenta) could have contained an individual who had migrated at age 50 years old. However, any of the groups defined by attained age could have contained an individual who had migrated during childhood. As expected, after exclusion of those who relocated after age 26, there was no correlation between age at relocation and current age (Additional file 1: Figure S4). Within each of the groups defined by attained age, we observed a trend of increasing Prevotella to Bacteroides ratio with older age at relocation to the mainland USA