A Comparative Study on the Phytochemical Composition, Antioxidant, and Anti‐Inflammatory Effects of Stem Lettuce ( <i>Lactuca sativa</i> var. <i>angustana</i> ) Depending on the Presence of the Peel

With increasing interest in food sustainability and waste valorization, plant-based by-products have been explored as sources of functional compounds. Stem lettuce (<i>Lactuca sativa</i> var. <i>angustana</i>), which is commonly consumed in East Asia, generates substantial peel waste that is largely underutilized. This study investigated the potential of stem lettuce peel as an upcycled functional food ingredient by evaluating its physicochemical characteristics, phytochemical profile, antioxidant activity, and anti-inflammatory properties. Two sample types were prepared: unpeeled stem lettuce (USL) and peeled stem lettuce (PSL). The physicochemical characteristics were analyzed using conventional methods, and the phytochemicals were identified and quantified using UHPLC-Triple TOF-ESI-MS/MS. Functional evaluations were conducted exclusively on USL and PSL samples, which were freeze-dried and extracted using 70% ethanol. Antioxidant activity was assessed using total polyphenol and flavonoid content, and DPPH, ABTS, and ferric reducing antioxidant power (FRAP) assays. Anti-inflammatory effects were tested in LPS-stimulated RAW264.7, using the WST-1 viability assay, nitric oxide (NO) production, and expression of inflammation-related genes. USL showed significantly higher antioxidant activity and phenolic content than PSL. In total, 29 phytochemicals were identified, including hydroxycinnamic acids, flavonoids, and sesquiterpene lactones such as lactucopicrin and lactucin, predominantly in peel-containing samples. USL markedly reduced NO production and inflammatory gene expression in a dose-dependent manner, whereas PSL had minimal effects. These findings suggest that the stem lettuce peel has strong potential as a sustainable and functional ingredient for future food applications.