Abstract: Age-related immune dysregulation and increases in inflammation, termed inflammaging, have been consistently implicated in most common age-related diseases, but the precise etiology of inter- individual differences in inflammaging are unknown. Changes in immunity and inflammation occur throughout the life course, but research on these processes among non-elderly populations has been limited. This is important because identifying sources of biological aging and inflammation before individuals reach older age may help identify points for intervention. The composition of the gut microbiota has been shown in animal models to have profound influence over, and interactions with, the immune system. Findings from germ-free mice suggest that commensal gut microbes are a key cause of inflammaging, but this hypothesis has not been well explored in humans. There are currently very few data examining how the microbiome relates to the fundamental aspects of aging biology, specifically inflammatory phenotypes and genomic markers of biological age. We propose to fill gaps in current microbiome research on aging, through the collection and analysis of oral and gut microbiome data in The National Longitudinal Study of Adolescent to Adult Health (Add Health), a nationally representative longitudinal cohort of adults with extensive social environment data and existing or ongoing analyses of genomic and phenotypic markers of inflammation and aging. The specific aims include the: 1) Collection of tongue and stool specimens with which to characterize the oral and gut microbiome in a nationally-representative sample (N ~10,155) of Add Health participants (mean age ~40); 2) Testing the association between the microbiome and biomarkers of aging and inflammation, and the creation of a novel “microbiome age clock”; 3) Examination of the relationships between life course exposures and microbiome species related to biomarkers of aging and inflammation as an adult; 4) Documentation and dissemination of data generated from this project. This proposal represents the first study to assess how the oral and gut microbiome are associated with biomarkers of DNA methylation aging and inflammation in a large US representative sample of midlife adults. Our study will significantly advance our understanding of the lifecourse exposures from gestation to adulthood that shape microbiome markers of inflammaging and DNA methylation aging. This is crucial because identifying microbiome markers of biological aging in adulthood will allow us to better identify signs of early aging via the microbiome. .