S-Equol as a Gut-Derived Phytoestrogen Targeting Estrogen Receptor β: A Promising Bioactive Nutrient for Bone Health in Aging Women and Men: A Narrative Review

<b>Background/Objectives</b>: Osteoporosis is highly prevalent and contributes substantially to morbidity and mortality, yet long-term concerns about pharmacologic therapies leave a major treatment gap. Soy isoflavones have been investigated as safer alternatives, but results across trials are inconsistent. A key unresolved issue is the equol-producer phenotype, the gut microbial ability to convert daidzein to S-equol, the most bioactive isoflavone metabolite, which may explain much of this variability. This narrative review synthesizes mechanistic, translational, and clinical evidence to clarify the potential skeletal relevance of S-equol. <b>Methods</b>: Literature was identified through PubMed and Scopus searches (January 2000-October 2025) for experimental, mechanistic, and clinical studies examining S-equol, estrogen receptor β (ERβ), and bone metabolism, with emphasis on equol-producing status, bone strength and bone microarchitecture. <b>Results</b>: S-equol acts as a high-affinity ERβ agonist with antioxidant and anti-inflammatory properties but lacks the carcinogenic or thrombotic risks linked to ERα activation. In estrogen-deficient rodent models, S-equol improves trabecular bone volume by 10-20%, increases trabecular number, and enhances biomechanical strength. These findings align with preclinical evidence demonstrating that S-equol preserves trabecular microarchitecture, enhances bone strength, and reduces bone turnover. A limited number of human trials show reductions in bone resorption by 20% at a daily dose of 10 mg S-equol. In contrast, trials of soy isoflavones in humans have produced inconsistent findings, partly because of substantial variability in equol-producer phenotype among participants and the reliance on dual-energy X-ray absorptiometry, which cannot distinguish trabecular from cortical compartments. Advanced bone imaging and microbiome-informed approaches enable the precise evaluation of S-equol's skeletal effects on trabecular bone and cortical bone, separately. <b>Conclusions</b>: S-equol represents a promising model for "precision nutrition," where microbiome, hormonal, and host factors converge with potential to prevent age-related bone fragility. Rigorous trials that integrate microbiome phenotyping and advanced imaging are needed to validate this approach, translate mechanistic promise into clinical benefit, and better define safety.