An empirical pattern recurs across the dietary intervention literature: committed dietary patterns—sustained ketosis (<35 g carbohydrate/day with verified β-hydroxybutyrate ≥0.5 mM) and Mediterranean diet—each improve inflammatory markers and, under verified conditions, produce favorable or non-atherogenic lipid profiles. Intermediate carbohydrate restriction (50–150 g/day, or vacillating compliance without sustained ketosis) may not achieve either. Simultaneously, strict ketogenic diets produce dramatic gut microbiome restructuring, including near-elimination of Bifidobacterium adolescentis and expansion of Akkermansia muciniphila. This paper proposes that microbiome-mediated bile acid signaling is the mechanistic link connecting these observations. The microbiome generates the majority of bile acid chemical diversity through deconjugation, dehydroxylation, and epimerization of host-synthesized primary species, while the host simultaneously produces counter-regulatory bile acid conjugates. Dietary patterns that produce stable microbiome configurations therefore also produce stable bile acid signaling environments that coordinate, through multiple receptors including FXR, TGR5, S1PR2, VDR, and RORγt, both lipid metabolic and immune outcomes across organ compartments. This coordination is distributed across tissues and receptors with sometimes opposing outputs, not tightly coupled through a single molecular effector. The hypothesis must account for established findings that constrain it: FXR’s metabolic and anti-inflammatory programs use mutually exclusive post-translational modifications within single cells; the FXR agonist obeticholic acid improved hepatic inflammation while worsening atherogenic lipid profiles in Phase III trials; individual bile acid species exert cell-type-dependent effects on the same receptor; and the most potent bile acid immune tolerance pathways bypass FXR and TGR5 entirely. Moreover, bile acid–mediated immune tolerance may simultaneously suppress beneficial anti-tumor immunity in certain tissue contexts. Despite these constraints, the framework generates testable predictions and a staged, affordable experimental program is proposed. Take-home message: Bile acids are not passive fat-absorption facilitators but a multi-receptor signaling network through which committed dietary patterns may simultaneously coordinate lipid metabolism and immune tolerance—explaining why these outcomes co-vary under dietary intervention and why intermediate restriction fails at both.