Structural Characterization, Constipation-Relieving, and Hypolipidemic Activity of Polysaccharides from Fresh and Processed Dendrobium officinale

<i>Dendrobium officinale</i> (DO) is a traditional medicinal and edible plant whose polysaccharides help modulate gastrointestinal and metabolic functions. Fresh DO is commonly processed into "Fengdou" to prolong shelf life, but the effects of this processing on polysaccharide structure and bioactivity remain unclear. In this study, polysaccharides from fresh DO (FDOP) and Fengdou (DDOP) were isolated, purified, and comparatively characterized. Based on structural analyses, FDOP and DDOP have similar functional groups and O-acetylated pyranosyl structures in both polysaccharides, which are identified as mannose-glucose heteropolysaccharides. However, FDOP was characterized by a higher mannose-to-glucose ratio (79.77:19.57) and molecular weight (187.1 kDa), as well as a more structurally diversified →4-linked backbone. In contrast, DDOP contained more glucose (68.74:30.94) and exhibited a lower molecular weight (125.1 kDa) and simplified backbone. In zebrafish models, both polysaccharides were found to alleviate loperamide-induced constipation and reduce lipid accumulation. DDOP showed stronger constipation-relieving activity, whereas FDOP exerted more pronounced hypolipidaemic effects, which can be ascribed to the higher molecular weight, mannose enrichment, and more complex backbone structure. These findings provide a structural basis and theoretical support for developing DO-derived polysaccharides as functional food ingredients targeting constipation and dyslipidaemia.