In Vivo and In Vitro Mechanisms of Equol Synthesis and Key Influencing Factors: A Critical Review.

Background: Equol exists in two enantiomers of S-equol and R-equol. The results of cell and animal experiments, as well as clinical trials, have supported its protective effects on menopausal symptoms, aging, and cardiovascular diseases, especially S-equol, which is a naturally occurring, non-racemic isomer produced by intestinal bacteria. However, the selective response of host microorganisms to soy isoflavones limits the exploitation of equol-producing bacterial resources. Additionally, factors such as low efficiency, byproduct generation, and environmental pollution hinder the further development and the application of traditional equol synthesis techniques. Methods: Therefore, in this review, we aimed to describe the forms and scope of equol, key influencing factors (e.g., hydrogen and dietary factors) of in vivo and in vitro equol synthesis, and potential molecular mechanisms of equol produced by microorganisms. Notably, the traditional synthesis technology has effectively improved the synthesis efficiency of equol (85-96%), but the substrates and microbial species (such as Escherichia coli) remain the key influencing factors. Results: This review suggests that breakthroughs based on synthetic biology and gene editing technology will support the efficient in vitro synthesis of equol. Conclusions: This review serves as a valuable reference for future research.