Boron Bioavailability Revisited: From Plasma-Accessible Species to Microbiota-Accessible Complexes—Implications for Nutritional Essentiality

Boron (B) remains one of the least understood trace elements in human nutrition. Traditionally regarded as non-essential, its biological role has been reevaluated in light of emerging microbiome research. We provide a narrative synthesis of mechanistic, preclinical, and clinical studies to assess whether the colonic actions of B meet accepted criteria for nutritional essentiality. This review revisits B bioavailability through a dual-pathway framework distinguishing plasma-accessible boron (PAB)—small, fully absorbable species with transient systemic effects—from microbiota-accessible boron complexes (MABCs)—indigestible conjugates that reach the colon intact. Evidence indicates that PAB exerts short-term metabolic modulation, whereas MABCs act as prebiotic cofactors that stabilize microbial quorum sensing (autoinducer-2–borate; AI-2B), reinforce the colonic mucus barrier through borate–diol crosslinking, and support host–microbiota symbiosis. Deficiency or low intake of MABCs leads to dysbiosis, barrier fragility, and low-grade inflammation along gut–organ axes—effects reversible by MABC-rich diets. Analytical and clinical tools are proposed to discriminate between PAB and MABC pathways, including fecal B/speciation, AI-2B assays, and mucus-penetration markers. Recognizing B’s essentiality as a microbiota-dependent nutrient reframes its nutritional assessment, guiding future dietary guidelines and prebiotic design toward the microbiome–mucus interface.