Metabolite and biomass flow through the levels.

(A) Cascading nature of nutrient flow across trophic levels: nutrient intake to the gut (the leftmost turquoise bar) is gradually converted into microbial biomass (red bars in each level) and metabolic byproducts (turquoise bars in each level). Some fraction of these byproducts (blue bars in each level) cannot be consumed by the microbiome and hence remains further unprocessed until it leaves an individual as their fecal metabolome. The metabolic byproducts of each level (turquoise bars) serve as the nutrient intake for microbes in the next level. The process ends at level 4 where all byproducts remain unconsumed thereby enter the fecal metabolome. (B) Normalized contribution of of the nutrient intake to microbial biomass (red) and fecal metabolome (blue) split across levels 2 to 4. Dashed lines show that consumable metabolites generated at a previous level serve as metabolic inputs to the next level.