Current Landscape of the Interrelationship Between Periodontitis, Type 2 Diabetes Mellitus, and COVID-19

The inflammatory response plays a central role in the pathophysiology of various chronic diseases such as periodontitis, type 2 diabetes mellitus (T2DM), and coronavirus disease 2019 (COVID-19), whose coexistence is associated with an increase in clinical complications and a more severe and serious course of these diseases. Current evidence on the interrelationship between periodontitis, T2DM, and COVID-19 remains insufficient, highlighting the need for further research to elucidate these associations. The main aim of this narrative review is to provide the current landscape of the most relevant aspects of the interrelationship between periodontitis, T2DM, and COVID-19. This narrative review was carried out through a specialized, exhaustive, and structured search of published studies indexed in the electronic databases PubMed and LILACS, for the inclusion of studies in English and Spanish, respectively, without date restriction. A search strategy was performed using the Boolean operators AND, OR, and NOT, with the following DeCS/MeSH terms: "periodontal disease", "periodontitis", "type 2 diabetes mellitus", "SARS-CoV-2", and "COVID-19". A variety of articles were included, focusing on the most relevant aspects of the interrelationship between periodontitis, T2DM, and COVID-19. Findings suggest that inflammation is a unifying mechanism, which leads to the severity of these conditions through four shared axes: (1) a clinicopathological axis involving systemic manifestations; (2) an axis associated with metabolic alterations linked to glycemic dysregulation; (3) an axis related to enzyme overexpression linked to altered angiotensin-converting enzyme (ACE)-2 expression and glucose metabolism; and (4) an inflammatory axis. These synergistic interactions can cause these three diseases to mutually enhance each other, creating a vicious cycle, worsening the patient's health.