Fig. 3.

Potential mechanisms implicated in the interplay between the gut microbiome, the faecal metabolome, and the blood metabolome. (1) Small dashed lines: metabolites are produced by the microbiota and then absorbed, resulting in associations between the microbiome and both the blood and faecal metabolites. (2) Large dashed lines: the microbiome affects the gut barrier integrity, resulting in alterations of metabolites absorption (i.e., the same metabolite is associated with a species/pathway in both blood and faeces, but the directions of effects are opposite). (3) Light continuous line: metabolites produced by the host, such as bile acids, affect microbial growth. (4) Bold continuous line: direct microbiome to host cell interactions that result in host systemic modulation (i.e., species are associated with blood metabolites but not with faecal metabolites)