In accordance with a new study based on cell cultures, natto – a fermented soy bean dish served for breakfast in Japan – may hold an answer to the world’s current problem: COVID-19.

Natto was previously found to be a diet staple in those who were classed as least likely to die from stroke or cardiac arrest, which is why it has been long thought to contribute to longer healthier lives across Japan – home to more than a quarter of the world’s population of over 65s.

Now, research published in Biochemical and Biophysical Research Communications has found that extract made from the sticky, strong smelling natto may inhibit the ability of the virus that causes COVID-19 to infect cells. 

“Traditionally, Japanese people have assumed that natto is beneficial for their health,” said paper author Tetsuya Mizutani, director of the Center for Infectious Disease Epidemiology and Prevention Research at the Tokyo University of Agriculture and Technology (CEPiR-TUAT). “In recent years, research studies have revealed scientific evidence for this belief. In this study, we investigated natto’s antiviral effects on SARS-CoV-2, the virus that causes COVID-19, and bovine herpesvirus 1 (BHV-1), which causes respiratory disease in cattle.”

Natto is made by fermenting soybeans with Bacillus subtilis – bacteria that is found in plant and soil. The researchers prepared two natto extracts, one with heat and one without and applied them to sets of lab-cultured cells from cattle and humans. One being infected with SARS-CoV-2, while the other was infected with BHV-1. 

When treated with the natto extract made without heat, both SARS-CoV-2 and BHV-1 lost the ability to infect cells. However, neither virus appeared to be affected by the heat-treated natto extract. 

“We found what appears to be a protease or proteases – proteins that metabolize other proteins – in the natto extract directly digests the receptor binding domain on the spike protein in SARS-CoV-2,” said Mizutani. However, Mizutani also explained that the protease appears to break down in the heat which means it loses the ability to digest proteins, letting the virus remain infectious.

The spike protein sits on the virus’s surface and binds to a receptor on host cells. With an inactive spike protein, SARS-CoV-2 cannot infect healthy cells, a similar effect was found on BHV-1. 

“We also confirmed that the natto extract has the same digestive effects on the receptor binding domain proteins of the SARS-CoV-2 mutated strains, such as the Alpha variant,” Mizutani said. 

While the results are promising, Mizutani has warned that further studies are required to identify the exact molecular mechanisms at work. Further stressing that the research does not provide proof of reduction of viral infection by consuming natto as food.

once the researchers have fully identified the components and their functions are verified, there is an aim to advance their work to clinical studies in animal models.