Whey Protein Peptides Self-Assembled Nanoparticles with Intrinsic Cholesterol Esterase Inhibition Enhance Stigmasterol Bioaccessibility and Hypocholesterolemic Effects

Background: Stigmasterol (St) holds promise as a natural cholesterol-lowering agent, yet its poor aqueous solubility and low bioaccessibility severely constrain its application in functional foods. Methods: In this study, we exploited whey protein peptides (WPP) with intrinsic cholesterol esterase (CEase) inhibitory activity to construct St-loaded self-assembled nanoparticles. Subsequently, we investigated its physicochemical properties and formation mechanism and evaluated its hypocholesterolemic activity through animal experiments. Results: Driven by non-covalent hydrophobic and hydrogen-bonding interactions, the optimized St-loaded WPP nanoparticles (St@WPP) ((234.47 ± 1.36) nm) achieved a high encapsulation efficiency (EE) of 74.73% ± 2.19%. This nano-encapsulation markedly improved St dispersibility and storage stability, and significantly elevated its bioaccessibility from 9.00% ± 0.14% to 19.41% ± 0.69% after simulated gastrointestinal digestion. St@WPP administration effectively ameliorated dyslipidemia, as evidenced by reduced serum total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C), which may be associated with reduced intestinal cholesterol availability and increased fecal cholesterol content. Moreover, St@WPP alleviated hepatic steatosis, attenuated systemic inflammation, and rebalanced gut microbiota architecture. Conclusions: Collectively, the WPP shell functions as a delivery carrier that improves the water solubility and bioaccessibility of St, thereby broadening its potential for application in low-fat or aqueous-based food systems. The resulting St@WPP exhibits notable cholesterol-lowering activity, positioning it as a promising functional food ingredient with potential benefits for cholesterol management.