By David Bender
Since the pandemic broke it has become obvious that if you want to receive wide publicity and research funding one sure-fire way is to include “SARS-CoV-2”, “COVID-19” or “coronavirus” among the key words. The outcome of most research is negative, nevertheless, with these flags your ideas will spread on social media, and even in the reports of reputable journalists. In some cases (e.g., the promotion of the supposed curative properties of hydroxychloroquine) this can cause harm, but in others, for example, the promotion of something useless, but harmless, there are no adverse effects. This may be said of that idea that fermented foods protect you against COVID-19.
There has been an explosion of knowledge about the different species making up what is now known as the gut microbiome. Until about a decade ago, identification of intestinal flora was largely by culture of faeces – a difficult and often malodorous process – and many micro-organisms resisted culture. Nowadays, genetic sequencing techniques allow rapid identification of large numbers of bacterial species.
The human gastro-intestinal tract contains some 1014 microorganisms, belonging to more than 2,000 species, so that the gut microbiome contains 150- to 500-fold more genes than human DNA. There is a very considerable variation between individuals in the species diversity of their gut microbiome. To some extent this is genetic (identical twins have more similar microbiomes than fraternal twins), and to a great extent it depends on diet.
The concept of encouraging a healthy intestinal microflora with lactic acid bacteria has a long history
For some years now it has been known that a diet rich in dietary fibre, oligosaccharides and starch that is resistant to digestion in the small intestine provides a substrate for growth of (mainly beneficial) bacteria, which produce, among other products, short chain fatty acids that are a preferred fuel for the cells that line the intestinal wall and may offer protection against the development of colorectal cancer. The presence of flourishing communities of lactic acid-producing and other beneficial bacteria inhibits the growth of pathogenic organisms. We also know that there is considerable interaction between intestinal bacterial metabolites, immune system cells in the intestinal wall (and elsewhere) and indeed the central nervous system. The term prebiotic has been coined for the various oligosaccharides that promote intestinal bacterial growth.
The concept of encouraging a healthy intestinal microflora with lactic acid bacteria has a long history. At the turn of the 20th century, Elie Metchnikoff, who shared the 1908 Nobel prize with Paul Ehrlich for their work on cell mediated and humoral immunity, attributed the longevity of Bulgarian peasants to their consumption of yogurt. His ideas led to the establishment of Danone, originally a Spanish company, making and marketing yogurt in 1919. In 1930, Minoru Shirota in Japan identified what he called Lactobacillus casei Shirota (now L. paracasei Shirota) in miso, a fermented soy bean paste. In 1935 he began marketing Yakult, a drink made with milk fermented with this organism, and went on to establish the Tokyo-based Yakult Honsha company in 1955.
More recently there has been interest in other fermented foods as sources of potential probiotics (the potentially beneficial micro-organisms themselves). All yogurt that has not been pasteurised after fermentation will be a source of lactobacilli, not just those produced by Danone or Yakult, although these provide many times more bacteria than ordinary yogurt. For those who want something more exotic than yogurt, kefir has become popular. It is milk fermented with L. kefiranofaciens and the yeast Saccharomyces turicensis, as well as other bacteria and fungi. Of course, any cheese that has not been pasteurised after manufacture will also be a rich source of probiotics – and especially in the case of soft cheeses, the riper the cheese, the greater its probiotic content – both bacteria and fungi.
Sauerkraut is European fermented cabbage, and a source of lactobacilli and other organisms. Again, for those who want something more exotic, there is spicy Korean kimchi: fermented cabbage, carrot, daikon radish, and spring onions with ginger and chili. Kombucha is made by fermenting sweetened tea with yeast and bacteria to produce a sparkling, slightly alcoholic and sharp tasting beverage.
Oddly enough, those who promote the benefits of sourdough bread as a fermented food ignore “ordinary” bread, which is also the result of yeast fermentation
Other fermented foods, for example sourdough bread, made using wild yeasts and lactic acid bacteria, are sometimes grouped in with those that contain probiotics. Fermentation may well change the proteins and other ingredients of the bread, but no micro-organisms will survive baking. Oddly enough, those who promote the benefits of sourdough bread as a fermented food ignore “ordinary” bread, which is also the result of yeast fermentation, albeit using standardised strains of yeast.
Similarly, beer, cider and wine are all the result of fermentation, but no living micro-organisms survive by the time we drink it. (It is noteworthy that in the past, when cholera was transmitted via contaminated drinking water, beer made using the same water was safe because the alcohol killed the pathogens.)
So, now we come to connection between COVID-19 and fermented foods. The tenuous reasoning might be as follows:
- SARS-CoV-2 enters cells by interaction of its spike protein with angiotensin converting enzyme II (ACE-II) on cell surfaces.
- ACE-II is found on the surface of many cells, including intestinal enterocytes.
- SARS-CoV-2 RNA is found in faeces.
- Different species in the gut microbiome may affect cell surface ACE-II.
- Different species in the gut microbiome may therefore affect SARS-CoV-2.
- Fermented foods containing live micro-organisms may modify the composition of the gut microbiome – this is mainly a transient effect; the introduced organisms rarely colonise the gut permanently.
One reviewer (1) sums up the potential (and the lack of evidence – the italics are mine): “…nutritional strategies to promote immunity against SARS-CoV-2, are being discussed. Certain fermented foods and probiotics may deliver viable microbes with the potential to promote gut immunity. Prebiotics, on their side, may enhance gut immunity by selectively stimulating certain resident microbes in the gut. Different levels of evidence support the use of fermented foods, probiotics and prebiotics to promote gut and lungs immunity. Without being a promise of efficacy against COVID-19, incorporating them into the diet may help to low down [sic] gut inflammation and to enhance mucosal immunity, to possibly better face the infection by contributing to diminishing the severity or the duration of infection episodes.”
The notes from a recent British Nutrition Foundation virtual event on nutrition and COVID-19 (2) say “Emerging research shows that gut health may be compromised by COVID-19 infection and, that gut microbiome status can influence health outcome in patients with COVID-19. Probiotics and prebiotics are purported to have a potential role in supporting the gut microbiome to help reduce severity of clinical outcomes of COVID-19. However more research is needed.” Again the italics are mine.
David A Bender, Emeritus Professor of Nutritional Biochemistry, University College London
- Antunes AEC et al. Potential contribution of beneficial microbes to face the COVID-19 pandemic. Food Res Int. 2020 Oct; 136: 109577. Published online 2020 Jul 24. doi: 10.1016/j.foodres.2020.109577. PMID: 32846611
- British Nutrition Foundation event: BNF Virtual Event: Nutrition and COVID-19. Held online 19 December 2020. Information and compilation of talks available at https://www.nutrition.org.uk/healthyliving/covid19/nutritioncovid19event.html