Tissue-Specific Profiles of Free and Bound Phenolics and Their In Vitro Antioxidant and Glycemic-Regulation-Related Activities in Fingered Citron (Citrus medica L. var. sarcodactylis (Siebold ex Hoola van Nooten) Swingle)

Background/Objectives: Fingered citron (Citrus medica L. var. sarcodactylis (Siebold ex Hoola van Nooten) Swingle) is rich in phenolic constituents, yet systematic comparisons of free and bound phenolics across tissues and origins remain limited. This study compared the peel, pulp, and blend (whole fruit) of fingered citron from five Chinese regions: Zhejiang (ZJ), Yunnan (YN), Sichuan (SC), Guangdong (GD), and Guangxi (GX). Methods: Phenolic compositions were determined by colorimetric assays and HPLC. Antioxidant activity was assessed by ORAC and PSC, and hypoglycemic-related activity by α-glucosidase and α-amylase inhibition and glucose consumption in an insulin-resistant HepG2 (IR-HepG2) cell model. Results: Phenolic distribution followed the order peel > blend > pulp, and free > bound. HPLC identified 11 free and 5 bound phenolics, predominantly hesperidin, quercetin, and 5,7-dimethoxycoumarin. GX peel exhibited the highest free phenolic content (106.34 ± 0.23 mg GAE/100 g FW) and superior ORAC (30.56 ± 0.50 μmol TE/g FW), strongly correlating with total phenolics (r = 0.98, p < 0.01). Free phenolics showed stronger α-glucosidase and α-amylase inhibition, whereas bound phenolics produced higher glucose consumption in the IR-HepG2 cell model. The GX blend bound fraction showed the highest glucose consumption (5.48 ± 0.98 mmol/L). Conclusions: Under fresh-weight-based conditions, phenolic composition and in vitro bioactivities differed by fruit part, region, and phenolic fraction. Peel, especially GX peel, tended to show higher phenolic levels and stronger antioxidant-related performance, whereas the GX blend bound fraction showed the highest glucose-consumption-promoting activity.