Abstract This study investigated the effect of iron ions (Fe²⁺ and Fe³⁺) on lipid peroxidation in vitro in red blood cells from healthy individuals and patients with metabolic syndrome. It was found that Fe²⁺ increased malondialdehyde (MDA) levels in healthy donors but decreased it in patients with metabolic syndrome, demonstrating opposite effects. These results indicate different sensitivities of red blood cells to iron in metabolic disorders and highlight the importance of further studying the role of iron in the development of oxidative stress in this condition.