Gut health and COVID severity claim interesting but flawed

New research suggesting gut bacteria may play a part in how severe an infection with COVID-19 is, how the body’s immune system fights it, and why some people stay unwell long after recovering from COVID, is interesting but has methodological limitations, according to experts.

COVID-19 is a respiratory illness, but evidence suggests the composition of the gut microbiome (bacteria, fungi, yeast, etc) may play a role immune system response to the virus, according to a study published in the journal Gut.

University of Bristol epidemiology lecturer Kaitlin Wade warns the research has some shortcomings, including being observational, therefore not showing cause and effect, and of a small sample size with possible issues with the selection of patients.

Doctor Wade recommends more investigation be carried out before suggesting measures to improve gut health as a way to increase COVID-19 resilience.

“The key message here should be one of considerable caution. Although some evidence potentially linking COVID-19 and the gut microbiome has been found, much further and deeper research is required to understand this complex relationship and draw meaningful conclusions,” she says.

Gut bug link to COVID severity ‘far reaching’

Institute of Microbiology and Infection director Willem van Schaik also cautions that the data from the study does not support the ‘far-reaching’ claim that microbiome composition determines disease severity or the risk of ‘long COVID’.

Long-COVID is where patients experience autoinflammatory symptoms after recovery from the virus such as multisystem inflammatory syndrome and Kawasaki-like disease in children.

What the researchers observed

The University of Hong Kong researchers who wrote the paper analysed stool samples of COVID patients, with varying disease severity, and compared them with samples from uninfected people. They found the composition of the microbiome was ‘significantly altered’ in the COVID patients.

The presence or absence of several species of bacteria was associated with disease severity and with the concentration of inflammatory markers of immune response, found in blood samples.

Most striking is that the alterations in gut microbiome composition persisted in samples collected up to 30 days after the patients were cleared of COVID.

More evidence needed to confirm gut bug link

University of Manchester professor of immunology Daniel M Davis says it is possible these persistent changes could contribute to symptoms of ‘long-COVID’ but the evidence is circumstantial, at this stage.

“At the moment this idea is still speculative, but it demands further investigation, especially given the huge importance of understanding ‘long-COVID’”, Professor Davis says.

UCL Medical Microbiology emeritus professor Graham Rook says the hypothesis behind the study is reasonable and the researchers show that gut microbiota composition correlates with COVID-19 severity and inflammation markers, but the changes in microbiota could be a result of the disease rather than the cause of susceptibility to the disease.

Other factors, such as the stress of being hospitalised with COVID, may even change microbiome composition, Professor Rook explains.

He says the findings will encourage further research which may provide supporting evidence.

“My guess is that the hypothesis will be shown to be correct, and that it will prove to be one of the reasons for the disproportionate susceptibility of individuals of low socioeconomic status, who for numerous reasons, particularly bad diet, have suboptimal microbiotas. It would be exciting if transfer of microbiota from appropriate donors could be used to treat ‘long COVID’ but more definitive studies will be needed.”

The bottom line

Research linking gut microbiome composition and COVID-19 severity has methodological flaws which mean the data don’t fully support that this is a cause of susceptability, but further investigation is warranted to find see if evidence supporting the claim can be found.

For more on gut health and how to support your microbiome you might be interested in: Build better immunity: The secret weapon of a healthy gut and 10 ways to reset your gut health