We all know that a healthy, balanced diet dominated by unprocessed foods is a recipe for gut and overall health, but scientists have now found just how one nutrient – fiber – can trigger a microbiotic chain reaction that actually shields the body from influenza and other viruses.
While the researchers from the Chinese University of Hong Kong (CUHK) advise against giving up your annual flu shot, natural fiber from the likes of vegetables, pulses and grains will act like a 'booster' to ward off infection and limit symptoms if you do manage to get hit with the bug.
Natural fiber encourages the production of probiotic bacteria in the gut, which specialize in ramping up the production of short-chain fatty acids (SCFAs). These SCFAs may be produced in the gut, but they then venture out on a circulatory system road trip to reach peripheral tissues, bolstering the body's immune response.
“In addition to vaccination, the public can strengthen their immunity through other means," said Francis KL Chan, a professor of Medicine and Therapeutics at CUHK. “Studies have shown that short-chain fatty acids produced by probiotic bacteria in the gut can boost immunity against influenza and other viral infections.”
Conducting in vitro (cell-based), in vivo (animal) and ex vivo (human tissue) research, the team assessed how effective SCFAs were in fighting off a number of viruses, including SARS-CoV-2, hepatitis B virus (HBV), respiratory syncytial virus (RSV), influenza A virus (IAV) and rotavirus.
What the researchers found was that a boosted SCFA count benefited T-cell (immune) metabolism and reduced the severity of IAV infection. In RSV and rhinovirus, the SCFAs showed a marked reduction in the viral load detected in infected lungs. And in SARS-CoV-2, SCFAs helped block viral entry and replication in vitro and enhanced antiviral immunity.
However, news was not so positive for chikungunya virus and latent viruses like HIV-1 and HSV-1, where it actually exacerbated inflammation in some infections.
But the results surrounding the flu are certainly worth talking about. The team found that dietary fibers – especially non-digestible carbohydrates like cellulose and resistant starch – cook up a fermentation storm once they hit the gut microbiome. Here, SCFAs made up mostly of acetate, propionate and butyrate are produced in the colon and then dispersed around the body.
The SCFAs were then shown to boost immune cell function in several ways. They activated G-protein-coupled receptors (GPCRs) that go on to influence inflammatory responses. They also stimulated T-cells and regulated cytokine production, subduing inflammation triggered by RSV and IAV.
Fiber also saw a spike in the population of beneficial SCFA-producing bacteria in the gut, notably Faecalibacterium prausnitzii and Ruminococcus spp.
If you're not interested in the mechanisms but how it can help protect you against flu, you'll probably want to know what level of fiber intake reaps these immune system rewards. While the scientists don't quantify it, they do say that synthetic fiber does not garner the same results. Overall, male adults should be aiming for 30–38 grams of fiber a day, and 21–25 g for females. Kids are recommended close to the female minimum, with around 19-25 g each day.
The fermentable fiber – which has the best return on SCFA production – is found in fruits like apples, bananas and berries, vegetables such as onions and leeks, legumes like lentils and chickpeas, as well as grains and nuts and seeds.
While upping your fiber in flu season won't make you impervious, the researchers suggest that it could help make the difference between severe infection and mild symptoms – as far as RSV and influenza A are concerned.
“SCFAs are beneficial metabolites of the gut microbiota with immune-modulating properties," added Tun Hein Min, Associate Professor of the Jockey Club School of Public Health and Primary Care at CU Medicine, who led the study. "Produced by gut microbes through the fermentation of dietary fibre, SCFAs enhance the ability of respiratory tract cells to resist viruses, reduce viral loads, enhance the efficacy of vaccines, and minimise serious complications."
"Studies also showed that influenza patients are deficient in SCFA-producing gut bacteria," he added. "In other words, gut microbiota plays an important role in strengthening the immune system against influenza."
The study was published in the journal Trends in Microbiology.
Source: The Chinese University of Hong Kong