Upcycling of hull from a traditional black soybean (Rhynchosia nulubilis): NRF2–HO-1-mediated antioxidant and anti-adipogenic effects

Abstract Rhynchosia nulubilis (Seomoktae), a traditional black soybean widely consumed in Korea, is valued for its nutritional and cultural significance. However, its hull, which is rich in polyphenols and anthocyanins, is often discarded during processing, representing an underutilized bioresource. In this study, we investigated the potential of R. nulubilis hull extract (RNHE) as an upcycled functional ingredient and evaluated its antioxidant and anti-adipogenic effects in 3T3-L1 adipocytes. RNHE exhibited high levels of polyphenols (598.67 mg GAE/g) and flavonoids (156.09 mg CE/g), along with strong radical scavenging activities. The extract showed no cytotoxicity up to 200 µg/mL and significantly suppressed lipid accumulation in a dose-dependent manner, accompanied by downregulation of adipogenic markers (PPARγ, C/EBPα, and FABP4). Stage-specific analysis revealed that RNHE exerted stronger inhibitory effects during the early phase of adipocyte differentiation, associated with reduced expression of C/EBPβ and C/EBPδ. Furthermore, RNHE decreased intracellular reactive oxygen species levels and enhanced antioxidant defense systems. Mechanistic investigation demonstrated that RNHE activated the NRF2–HO-1 signaling pathway, suggesting that modulation of redox balance plays a key role in its anti-adipogenic effects. These findings highlight the potential of R. nulubilis hull as a value-added functional ingredient and support its utilization within a sustainable food system through upcycling of traditional food by-products .