Food Additives as Microbiome-Active Dietary Exposures: Mechanisms, Host Susceptibility, and an Operational Case for Microbiome-Inclusive Risk Assessment

Food additives are defining components of ultra-processed foods, yet microbiome-related effects remain largely outside routine safety assessment. This review evaluates evidence for six additive classes across four pathways—microbial community changes, altered microbial metabolites, mucus-epithelial barrier disruption, and immune modulation—and examines how these findings could inform microbiome-inclusive risk assessment. Mechanistic, translational, and epidemiological evidence was organized using a five-tier hierarchy: in vitro, ex vivo, animal, observational, and controlled human studies. Chronic-disease associations were graded across six domains by two raters using explicit criteria, with sensitivity analysis. Evidence was then translated into a tiered, decision-gated assessment protocol, illustrated with carboxymethylcellulose and back-tested against carrageenan, acesulfame K, and titanium dioxide, which return three different protocol outcomes. Non-nutritive sweeteners and emulsifiers have the strongest direct human and ex vivo evidence, including microbiome-dependent interindividual responses, although null and discordant trials remain. Gastrointestinal and metabolic outcomes received the highest support, cardiovascular associations were intermediate, and autoimmune, neurological, and psychiatric links remained preliminary. Metabolic and cardiovascular epidemiology relied largely on NutriNet-Santé, with modest hazard ratios (generally 1.03–1.15 per exposure increment) and indirect exposure estimation. Recent additive-mixture analyses from the same cohort were also considered. Food additives can act as context-dependent, microbiome-active exposures. Current evidence supports considering microbiome composition, barrier-integrity, and immune endpoints in additive safety evaluation, while causal links with most chronic-disease outcomes remain unproven.