Although COVID-19 is often viewed as a respiratory disease because of how it's transmitted, its effects reach far beyond the lungs.
Experts believe the coronavirus compromises blood vessels throughout the body, causing a mixed bag of symptoms ranging from loss of taste and smell to chest pain to diarrhea.
Now, researchers are also finding links between the illness and the gut — specifically, different kinds of gut bacteria.
In a review published in mBio this week, Korean microbiologist Heenam Stanley Kim proposed a hypothesis linking gut dysfunction and severe COVID-19 infection.
"There seems to be a clear connection between the altered gut microbiome and severe COVID-19," Kim said in a press release.
While the link has not been specifically investigated, some small-scale studies have found that patients with COVID-19 have less diverse bacteria in their guts compared to healthy people. A close look at fecal samples from 15 hospitalized COVID-19 patients in Hong Kong found an increase in harmful bacteria and a decrease in friendly microbes compared to healthy gut ecosystems.
An imbalance in gut bacteria can make it easier for intruders like the coronavirus to infiltrate the intestinal lining, Kim wrote.
More research is needed to explore this potential connection, but it's possible that gastrointestinal problems such as "leaky gut" syndrome may allow the coronavirus entry into cells in the digestive tract. This occurs when the intestinal lining is disrupted, creating tiny gaps through which interlopers such as coronavirus can penetrate into surrounding organs and tissue.
A less diverse population of gut microbes could be a risk factor
Some of the people most vulnerable to coronavirus complications — the elderly and people with chronic medical conditions — are more likely to have altered gut microbiota, with fewer varieties of beneficial bacteria.
Conditions such as diabetes, heart disease, kidney disease, and obesity are linked to reductions in good bacteria in the gut. That's in part because antibiotics used to manage these conditions can kill even the beneficial microbes inside the body.
Aging, too, can reduce the diversity of our microbiome, which research shows may make us more vulnerable to cognitive decline. Evidence suggests people who live the longest, healthiest lives tend to maintain a stable, diverse microbiome as they age.
The gut microbiome can vary widely based on factors such as genetics, lifestyle habits, and even culture, as diet plays a major role in gut health. Eating patterns that are rich in fiber, healthy fats, and whole foods, such as the Mediterranean diet are closely linked to a healthier microbiome. In contrast, research shows that diets high in processed foods and refined sugars, such as the typical Western diet, are associated with less microbial diversity, and worse health outcomes.
GI symptoms like diarrhea may be an early indicator of acute COVID-19 infection
Gastrointestinal symptoms have been linked to COVID-19 since early in the pandemic, with initial reports suggesting they afflict about one in ten patients. Although nausea, cramps, and diarrhea were less well-known than the telltale cough and fever, ongoing research has confirmed that these symptoms are important indicators of the disease.
Diarrhea could even be a sign of more severe COVID-19 infection, according to an August 2020 study out of the University of Southern California. In a dataset of nearly 56,000 COVID-19 patients, every patient who experienced diarrhea as an early symptom eventually came down with pneumonia or respiratory failure.
However, the presence of the coronavirus in the gastrointestinal tract isn't always associated with symptoms like diarrhea and vomiting. A study of symptomatic COVID-19 patients in Singapore found the virus in about half of the patients' poop, but only half of those patients — roughly 25% of those studied — had gastrointestinal symptoms.
We might one day treat COVID-19 with poop transplants and designer probiotics
So far, there's been relatively little attention to the connection between coronavirus and the gut, in part because it's primarily a respiratory virus, but also because there's still so much we don't know about the microbiome, Kim said.
More research is needed on COVID-19 and the GI tract, but we already know that certain illnesses can be treated via the microbiome with a fecal microbiota transfer (FMT), or poop transplant.
"If FMT is confirmed effective as a treatment for COVID-19, I think that hospitals around the world will readily adopt it, particularly for the patients belonging to the high-risk group," Kim said.
Another viable treatment in the future might be probiotics, food or supplements that contain beneficial bacteria. Conventional probiotics today wouldn't be helpful for treating COVID-19, Kim clarified — we would need to develop a very specific type of bacteria to use, so don't expect to see them available anytime soon.
"Certain butyrate-producing bacteria can be selected and developed as next-generation probiotics. However, this development will take some time, and new therapeutic bacteria have to pass regulatory hurdles," he said.
In the meantime, there's already an easy way to improve your gut microbiome. Eating more dietary fiber (in the form of leafy greens, veggies, whole grains, and the like) has been consistently shown to improve microbial diversity. While it's too early to know for sure, this might be a preventative strategy against coronavirus, according to Kim.
To get the benefits for a healthy gut microbiome, you'd need to consume between 25 to 30 grams of dietary fiber a day consistently for a long period of time (the specific duration varies by person), he said.
