Oxidative Stress Index, Antioxidant Activity, and Phenolic Composition of Commercial Spirulina and Undaria Extracts

Aim: This study aimed to compare the antioxidant activities, total oxidant/antioxidant status, and phenolic profiles of commercial Spirulina and Undaria extracts obtained with solvents of different polarity. Methods: Extracts were prepared using hexane, ethanol, and water. Antioxidant activities were assessed by DPPH, ABTS, CUPRAC, and metal chelation assays. Total phenolic (TPC), total flavonoid (TFC), and phenolic aldehyde contents were quantified spectrophotometrically. Phenolic identities and profiles were analyzed using LC–MS/MS in multiple reaction monitoring mode with authentic standards. Total antioxidant status (TAS), total oxidant status (TOS), and oxidative stress index (OSI) were measured using commercial kits. Results: Spirulina ethanol and hexane extracts exhibited the strongest radical-scavenging and chelating activities, with lower IC₅₀ values in DPPH and ABTS assays compared with Undaria. Spirulina consistently showed higher TAS and lower OSI, indicating a more favorable oxidative balance. Phenolic profiling revealed vanillin, vanillic acid, and gentisic acid as dominant in Spirulina, whereas Undaria was richer in high-molecular-weight phlorotannins but yielded lower TPC and TFC values. Solvent polarity strongly influenced outcomes: ethanol and hexane extracts provided higher phenolic content and stronger antioxidant activity than water. Conclusion: Spirulina extracts demonstrated superior antioxidant activity and phenolic diversity compared to Undaria, challenging the perception that brown algae always dominate in polyphenol content. By integrating classical assays with oxidative stress indices, this study highlights Spirulina’s potential as a more effective source of functional antioxidants.