This record contains the raw‑feature table and supporting files for the untargeted LC‑MS/MS metabolomics study entitled “Leucine Supplementation Modulates Lipid Metabolism and Metabolic Pathways in Type 1 Diabetic Mice: A Metabolomics Study”. Study design• 24 male C57BL/6J mice, four groups (Control, Control + Leu, T1DM, T1DM + Leu)• Plasma samples analysed in positive‑ion mode on a Waters ACQUITY UPLC coupled to a Xevo G2‑XS Q‑TOF.• Each biological sample was injected four consecutive times; pooled QC injections: 5 at the start, 1 every 8 study injections, and 3 at the end; blanks interleaved. Files included Table S1. Raw LC‑MSMS ion features.xlsx – exported peak‑area table from Progenesis QI, containing m/z, RT and abundances for every ion in each injection. Metadata_Table.csv – structured sample metadata: injection order, sample ID, mouse ID, group, sample type (Sample / QC / Blank). LCMS_Protocol.txt – full experimental protocol: sample preparation, chromatographic gradient, mass‑spectrometric settings, quality‑control strategy and data‑processing workflow.