Covid-19 has been found to disproportionately affect people with types 1 and 2 diabetes, but earlier this month it was announced in a letter to the New England Journal of Medicine (NEJM) that the novel coronavirus might cause diabetes to develop in patients who were previously healthy. If proven to be a direct result of Covid, this wouldn’t be the first time a viral illness has been linked to the new onset of diabetes in recovered patients.

Diabetes is the result of islet cells in the pancreas no longer being able to carry out their function, resulting in dangerously high or dangerously low blood sugar as the pancreas can’t respond to changes in blood glucose levels. Type 1 diabetes is an autoimmune disease as the islet cells are destroyed by the body’s own immune system. In Type 2, the islet cells wear out from overuse as they’re required to produce vast amounts of insulin in order for desensitized target organs such as the liver and muscle to respond to the insulin.

It was already known pre-Covid that viral infections could trigger a new onset of diabetes as the illness destroys islet cells in the pancreas. The phenomenon has been seen in patients with mumps and enterovirus infections. During the 2002-2004 SARS outbreak, another coronavirus, there were a number of infections in previously healthy patients who developed SARS pneumonia and were later found to have acute diabetes. The majority of new onset diabetes cases were resolved within a few years but for 10 percent of affected patients, the diabetes proved to be permanent.

The authors of the NEJM letter only cite one case study where SARS-CoV-2, the coronavirus pathogen that causes Covid-19, resulted in a young man developing diabetes after contracting the disease. The patient presented with developed severe diabetes after falling ill causing ketoacidosis, which is brought on by very high blood sugar. The symptoms of ketoacidosis include nausea, headache, confusion, shortness of breath, and loss of consciousness.

SARS and SARS-CoV-2 share a characteristic spike protein which enables them to attach to cells, especially those that are abundant in lung, kidney and islet cells in the pancreas. It’s been suggested that these coronaviruses may cause diabetes by disrupting normal cell function when they attach to cells in the pancreas. This could either directly inhibit blood glucose maintenance or trigger an immune response as the invasion makes the area become inflamed, leading to the body attacking its own cells.

As a widespread disease that has the potential to become a seasonal illness like influenza, understanding the potential complications of Covid-19 is important to assuring effective treatment for future infected patients and therapies for those who have already recovered. As such, it's important to pursue research into potential complications such as diabetes, as suggested by NEJM letter, but experts warn that care must be taken not to make sweeping generalizations before adequate data is obtained to draw firm causal links.

“There is no robust data yet to indicate that Covid-19 causes new diabetes or worsens existing diabetes. Some data however suggests there could be a possible link so researchers are seeking to explore this further," said Dr Riyaz Patel, Associate Professor of Cardiology & Consultant Cardiologist, UCLH, who was not involved in the study. "The researchers are experts in diabetes and propose to study this in more detail by collecting data at scale internationally. If there is a direct link then that will have important implications on how we treat Covid patients during and after the acute illness."

Data on new instances of diabetes in Covid patients is currently limited, but authors of the NEJM letter have established a register to facilitate further data capture and research into Covid-related diabetes cases. If a link is confirmed between Covid and diabetes, this will become an important area of research in developing treatments as the widespread incidence of the disease means such a complex complication could greatly impact the future quality of life for many recovered patients.