Prognostic Impact of New-Onset Type 2 Diabetes Mellitus After Acute Myocardial Infarction: Long-Term Mortality Compared with Pre-Existing and No Diabetes

<i>Background and Objectives</i>: Acute myocardial infarction (AMI) increases the risk of developing type 2 diabetes mellitus (T2DM), yet the long-term prognostic significance of new-onset T2DM after AMI remains unclear. We aimed to evaluate long-term mortality among AMI survivors who developed T2DM during follow-up compared with those who remained non-diabetic and those with pre-existing T2DM. <i>Materials and Methods</i>: We conducted a retrospective cohort study of consecutive AMI patients hospitalized between 2002 and 2017 at a large tertiary medical center. Patients were categorized into three groups: (1) new-onset T2DM after AMI (NOT2DM), 1-15 years post-discharge; (2) no diabetes (No DM), with no evidence of T2DM during the same period; (3) pre-existing T2DM (T2DM), diagnosed prior to or during the index AMI. Age- and sex-matching was performed. Primary outcome: all-cause mortality with up-to 10 years of follow-up. <i>Results</i>: A total of 4207 patients were included (1202 NOT2DM; 2404 No DM; 601 T2DM). Over a median follow-up of 2729 days, 1492 patients (35.5%) died. Mortality was highest in the NOT2DM group (44.9%) compared with No DM (31.2%) and T2DM (33.4%) (<i>p</i> < 0.001). After adjustment, NOT2DM remained strongly associated with increased mortality versus No DM (AdjHR 1.408; 95% CI 1.256-1.578) and T2DM (AdjHR 1.272; 95% CI 1.074-1.508) (<i>p</i> < 0.001 for each). The mortality impact was most pronounced in patients < 65 years. <i>Conclusions</i>: Development of T2DM after AMI identifies a high-risk subgroup with significantly worse long-term survival, especially among the young patients. These findings underscore the need for systematic metabolic surveillance and early preventive strategies in AMI survivors.