The strongest signal that emerged from our GWAS is an association between the LCT gene and the abundance of Bifidobacteria in the human gut. This link has since been corroborated in multiple populations of European descent. The LCT genotype is predictive of lactase persistence; individuals genetically lactase-nonpersistent harbor higher levels of gut Bifidobacteria, suggesting a role for these bacteria in lactose degradation. Lactase persistence evolved independently in several places (Europe, North Africa, Eastern Siberia) but its impact on the microbiome outside of Europe and North America is unknown. What is known is that lactase-persistance does not map well onto lactose tolerance, which is microbially mediated. We are investigating how the relationship between host genotype, microbiome and phenotype differs around the world.