COVID-19 and diabetes mellitus: potential metabolic associations.

The COVID-19 pandemic turned the SARS-CoV-2 into the main target of scientific research all around the world. Many advances have already been made, but there is still a long way to go to solve the molecular mechanisms related to the process of the SARS-CoV-2 infection, as well as the particularities of the disease, its course and the complex host-pathogen relationships. However, a lot has been theorized and associated with COVID-19, like the worst prognosis of the disease among individuals with some comorbidities, like diabetes mellitus. In this perspective, diabetic patients are repeatedly associated with more severe cases of COVID-19 when compared to non-diabetic patients. Even though ACE2 (angiotensin-converting enzyme 2) was recognized as the host cell receptor for both binding and entering of SARS-CoV-2 particles, it was also pointed out that this enzyme plays an important protective role against pulmonary damage. Therefore, paradoxically as it may seem, the low baseline level of this receptor in people with diabetes is directly linked to a more expressive loss of ACE2 protective effect, which could be one of the possible factors for the worst prognosis of COVID-19. Still, COVID-19 may also have a diabetogenic effect. From this point of view, the main topics that will be highlighted are (i) the mechanism of the viral entry, with special attention to the cellular receptor (ACE2) and the viral binding protein (spike), (ii) the relationship among the renin-angiotensin system, the infection process and the patients' prognosis, (iii) the glucose control and the medicines used in this event, and (iv) a brief analysis on diabetes triggered by COVID-19.