Large differences in COVID-19 death rates exist between countries and between regions of the same country. Some very low death rate countries such as Eastern Asia, Central Europe or the Balkans have a common feature of eating large quantities of fermented foods. Although biases exist when examining ecological studies, fermented vegetables or cabbage were associated with low death rates in European countries. SARS-CoV-2 binds to its receptor, the angiotensin converting enzyme 2 (ACE2). As a result of SARS-Cov-2 binding, ACE2 downregulation enhances the angiotensin II receptor type 1 (AT1R) axis associated with oxidative stress. This leads to insulin resistance, lung and endothelial damage, two severe outcomes of COVID-19. The nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is the most potent antioxidant in humans and can block the AT1R axis. Cabbage contains precursors of sulforaphane, the most active natural activator of Nrf2. Fermented vegetables contain many lactobacilli, which are also potent Nrf2 activators. It is proposed that fermented cabbage is a proof-of-concept of dietary manipulations that may enhance Nrf2-associated antioxidant effects helpful in mitigating COVID-19 severity.

To the Editor, The fermentation process, born as a preservation method in the Neolithic age, enabled humans to eat not-so-fresh food and to survive.1 Fermented foods are “foods or beverages made via controlled microbial growth (including lactic acid bacteria (LAB)) and enzymatic conversions of food components.” 2 Not all fermented foods contain live cultures, as some undergo further processing after fermentation: pasteurization, smoking, baking, or filtration. These processes kill or remove the live microorganisms in foods such as soy sauces, bread, most beers and wines as well as chocolate. Live cultures can be found in fermented vegetables and fermented milk (fermented sour milk, yoghurt, probiotics, …). The westernized diet is lacking many traditional fermented foods.3The gut microbiota has an inter-individual variability due to genetic predisposition and diet 3. Some foods like cabbage can be fermented by the gut microbiota. 4 The westernized diet has been associated with changes in the gut microbiome.5In this Rostrum, we consider loss of food fermentation either as a reduction of fermented food consumption in the diet or as a change in the microbiome leading to a reduction of fermentation of foods in the gut. This paper is based on the hypothesis that diet may partly explain differences in COVID-19 death rates within and between countries.6

To the Editor, A COVID-19 epidemic started in China and then disseminated to other Asian countries before becoming a pandemic. It appears that the pandemic has so far resulted in proportionately fewer deaths in China and most Eastern Asian countries. Many reasons can explain this picture.1 One of them is the type of diet in the low mortality countries. 2This paper is the sixth of a series attempting to understand the role of diet in the differences of COVID-19 death rates between and within countries with the aim to identify potential preventive measures against COVID-19. The concept paper 2 was followed by two ecological studies comparing death rates in European countries and the consumption of vegetables or fermented foods. 3,4 We then proposed that sulforaphane from cruciferous vegetables1 and lactobacilli from fermented foods (submitted) were possibly involved in the reduction of insulin resistance in COVID-19.It is noteworthy that fermented foods are largely used in Asia.5,6 It is therefore important to check whether some commonly eaten fermented foods in these countries may explain geographic differences in COVID-19. Kimchi will be used as a model of fermented cabbage.

To the Editor, Sulforaphane [1-isothiocyanato-4-(methylsulfinyl)butane] is a clinically relevant nutraceutical compound present in cruciferous vegetables (Brassicaceae). It is used for the prevention and treatment of chronic diseases and may be involved in ageing.1Along with other natural nutrients, sulforaphane has been suggested to have a therapeutic value for the treatment of the coronavirus disease 2019 (COVID-19).2 Sulforaphane is an isothiocyanate stored in its inactive form glucoraphanin.3 The enzyme myrosinase, found in plant tissue and in the gut microbiome, is involved in the conversion of glucoraphanin to its active form sulforaphane.4