This was a single-center prospective observational cohort study conducted in Poland in the context of routine metabolic outpatient care. The study assessed selected metabolic, biochemical, oxidative-stress, inflammatory, and body-composition parameters in individuals with overweight or obesity undergoing standard lifestyle-based body weight reduction management. Participants with BMI \> 25 kg/m² were followed before and after body weight reduction, while individuals with normal body weight served as a comparison group. The weight reduction management included individualized dietary recommendations, physical-activity guidance, health education, regular follow-up visits, fasting blood sampling, and body-composition assessment. The aim of the study was to evaluate whether body weight reduction is associated with changes in metabolic and biochemical parameters, with particular attention to glucose and lipid metabolism, oxidative stress, inflammatory markers, adipokines, cytokines, and body-composition changes.