There are a lot of strange and unusual things about SARS-CoV-2, the coronavirus responsible for the COVID-19 pandemic. One of them is the sudden loss of one’s sense of smell, known as anosmia, a symptom which now helps doctors distinguish COVID-19 from the flu without the need for a diagnostic test. A new study suggests the bizarre symptom may be caused by alterations in the glucose metabolism inside certain brain regions involved in the processing of smell.

Scientists previously described how coronaviruses that cause other respiratory illnesses, such as SARS or the common cold, also account for 10-15% cases of anosmia. It is therefore not that surprising that COVID-19 would also cause anosmia in infected patients.

In early April, researchers at the Department of Neurobiology at Harvard Medical School published a study suggesting that cells and stem cells found in the olfactory epithelium express two key genes, ACE2 and TMPRSS2, that allow SARS-CoV-2 to infect cells. The olfactory epithelium is a specialized tissue found in the nasal cavity that houses olfactory sensory neurons and a variety of supporting cells.

Researchers in Belgium, however, believe that COVID anosmia is more likely triggered due to changes in key areas of the brain involved with processing odors.

In their new study, the scientists at the Functional Brain Mapping Laboratory at Belgium’s ULB scanned the brains of 12 COVID-19 patients with anosmia symptoms, as well as 26 healthy individuals who acted as controls.

For seven of the patients, loss of smell was the main symptom of the disease. Five out of the 12 COVID-19 patients recovered their smell within 10 weeks after they recovered from the infection. But the other seven still had difficulties perceiving smells up to 16 weeks after the infection was cleared from their bodies.

Compared to the healthy participants, the COVID-19 patients had a higher glucose metabolism in the olfactory cleft, the olfactory bulbs, and adjacent neural tissue — three brain regions involved in smell.

This isn’t the last word on the matter, though. The study was reported in the pre-print website medRxiv, and therefore has not been peer-reviewed. The small sample size involving only 12 patients also limits the study’s findings. For instance, this study doesn’t answer why some COVID-19 patients lose their sense of smell while others don’t.

Nevertheless, this study can serve as a pilot for subsequent research that might probe the mechanisms of COVID-19 anosmia. In doing so, perhaps scientists can find a way to help patients with months-long anosmia recover faster.