People infected with COVID-19 experience a variety of symptoms and degrees of severity, the most commonly reported ones, including high fever and difficulty breathing. However, autopsies and other studies have also shown that the infection can affect the liver, kidney, heart, spleen, and even the gastrointestinal tract. A significant proportion of patients hospitalized with breathing problems also experience diarrhea, nausea, and vomiting, suggesting that when the virus becomes involved in the GI tract, it increases the severity of the disease.
Published in a review this week in mBioMicrobiologist Heenam Stanley Kim, Ph.D. from the Human-Microbial Interactions Laboratory at the University of Korea in Seoul, examined new evidence suggesting poor gut health adversely affects COVID-19 prognosis. Based on his analysis, Kim suggested that bowel dysfunction – and the associated leaky gut – could worsen the severity of the infection by allowing the virus to access the surface of the digestive tract and internal organs. These organs are prone to infection because they have widespread ACE2 – a protein target of SARS-CoV-2 – on the surface.
"There appears to be a clear link between the altered gut microbiome and severe COVID-19," Kim said.
Studies have shown that people with underlying conditions like high blood pressure, diabetes, and obesity are at higher risk for severe COVID-19. The risk also increases with age, with older adults being the most susceptible to the most serious complications and likelihood of hospitalization. However, both factors – advanced age and chronic illness – have a known association with altered gut microbiota. This imbalance can compromise the integrity of the gut barrier, Kim said, which can give pathogens and pathobionts easier access to cells in the gut lining.
So far, the link between gut health and COVID-19 prognosis has not been empirically proven, Kim noted. Some researchers have argued that unhealthy gut microbiomes could be a reason why some people have such serious infections.
What studies have been done suggests a complicated relationship. For example, a study of symptomatic COVID-19 patients in Singapore found that about half had detectable levels of coronavirus in fecal tests – but only about half of those patients had GI symptoms. This study suggests that SARS-CoV-2, even if it reaches the GI tract, may not cause problems. Kim also noted that a person's gut health at the time of infection can be critical to symptom development.
Many recent studies have found decreased bacterial diversity in gut samples from COVID-19 patients compared to samples from healthy people. The disease has also been linked to the depletion of beneficial bacterial species – and the accumulation of pathogenic ones. A similar imbalance has been linked to influenza A infection, although the two viruses differ in how they alter the overall microbial composition.
The depleted bacterial species associated with COVID-19 infection include some families responsible for the production of butyrate, a short-chain fatty acid that is critical to gut health by strengthening intestinal barrier function.
Kim said he began analyzing the studies after finding that affluent countries with good medical infrastructure – including the US and western European states – were hit hardest by the virus. The "western diet" common in these countries is low in fiber and "a low-fiber diet is a major cause of altered gut microbiome," he said, "and such gut microbiome dysbiosis leads to chronic disease."
The pathogenesis of COVID-19 is not yet fully understood. If future studies show that gut health affects the COVID-19 prognosis, doctors and researchers should use that association to develop better strategies for preventing and treating the disease. Eating more fiber, he said, can lower a person's risk for serious illness. A fecal microbiota transplant could be a treatment worth considering for patients with the worst cases of COVID-19.
However, the gut health problem goes beyond COVID-19, he said. Once the pandemic is over, the world will still face chronic diseases and other problems associated with poor gut health.
"The whole world is suffering from this COVID-19 pandemic," said Kim, "but people don't know that the pandemic of damaged gut microbiomes is now far more severe."