What are the structural and bioactive differences between fresh/dried Rehmanniae Radix or Rehmanniae Radix preparata oligosaccharides and polysaccharides? A review of bioavailable possessions

Rehmannia glutinosa (RG), officially recognized as a medicinal and edible substance in China since 2024, exhibits significant potential for pharmaceutical and functional food applications. Its processed forms—Fresh Rehmanniae Radix (FRR, Xiandihuang), Dried Rehmanniae Radix (DRR, Shengdihuang), and Rehmanniae Radix Preparata (RRP, Shudihuang)—display distinct oligosaccharide (RGO) and polysaccharide (RGP) profiles that directly influence their bioactivities. While FRR/DRR are rich in stachyose-type RGOs with hypoglycemic and hepatoprotective effects, RRP is rich in polysaccharides with immunomodulatory, anti-aging, and anti-inflammatory properties. Despite their therapeutic promise, systematic comparisons of these saccharides across processing methods remain scarce. This review comprehensively analyzes extraction techniques, structural features, bioactivities, and structure-activity relationships of RGOs/RGPs, highlighting their mechanisms in metabolic regulation, gut microbiota modulation, and multi-organ protection, while bridging traditional applications with modern scientific validation. Notably, RGPs demonstrate enhanced bioavailability and therapeutic efficacy when formulated into liposomes or nano-adjuvants. This study systematically examines processing-induced structural transformations, bioactivities and safety profiles to establish theoretical foundations for advancing RGOs/RGPs development and proposes novel research directions. The findings provide critical guidance for developing RG-based functional ingredients targeting diabetes management, immune modulation, and preventive healthcare, with expected applications in pharmaceuticals, health foods, and nutritional supplements.