The outbreak of coronavirus disease 2019 (COVID-19) became a long pandemic that lasted over 3 years. Although WHO declared that COVID-19 public health emergency ended on May 5, 2023 (1), there are still new variants of the SARS-CoV-2 prevailing and, in January 2024, there were more than 30,000 new patients weekly hospitalized in the U.S. (2). As of May 26, 2024, the cumulative cases of patients who contracted the virus surpassed 775 million with over 7 million deaths worldwide (3). Several factors may influence the infection, the severity of symptoms, and the recovery process, for example, genetic factors, age, and pre-existing conditions (4–9). Moreover, there were geographical differences in the number of cases around the world. It may have been the result of government strategies, such as herd immunity, and/or genetic differences (10, 11), however there was also the possibility of differences in the food consumed by different countries and the effects of phytochemical compounds included in them contributing to prevention of virus infection and/or improving the recovery process (12).

Indeed, multiple studies showed the effects of phytochemicals in plant-based diets and beverages on COVID-19 [e.g., tea (13, 14), plant-based diet (15, 16), the effects of eating carnation and citrus fruits (17)]. In silico and review studies have suggested that there are phytochemicals which may bind to the proteins of SARS-CoV-2 responsible for COVID-19 (18, 19). Such binding may contribute to suppress the infection if the phytochemicals bind to the spike protein of the virus (12, 20). Binding of the phytochemicals to the enzymes involved in the replication of the virus (12) may suppress the replication of the virus and may help recovery from COVID-19. Foods that contain large amounts of phytochemical compounds with bioactive properties may resemble the effects of drugs used for treatment of COVID-19 (21). These studies suggest that daily consumption of foods and beverages that contain these phytochemicals may prevent/suppress the contraction of COVID-19 or facilitate faster recovery from it (21–25). Some examples of these foods and beverages are green vegetables such as broccoli/cabbage, citrus fruits such as lemon and orange, red fruits such as strawberries and blueberries, and coffee and black tea.

In addition, some patients continue to have severe symptoms for long term after the acute phase of COVID-19 (post-acute sequelae of COVID-19 (PASC), or Long-COVID), and one of the reasons causing PASC is due to the remaining virus and particles of the virus in their tissues (26). Daily consumption of foods/beverages may help in their recovery by binding to the S-protein or the enzymes related to replication of these remaining virus or particles of the virus.

There are many phytochemicals with anti-inflammatory effects as well (19, 27). As inflammation is one of the key symptoms of COVID-19 and PASC, phytochemicals with anti-inflammatory effects may have beneficial influences on the recovery from COVID-19 and PASC (28, 29). Many of the phytochemicals included in cuisines possess anti-inflammatory and antioxidant effects [for example zingerone of ginger (30, 31) and curcumin of turmeric (30, 32)]. There are also phytochemicals that have anti-inflammatory effects as well as binding affinity to SARS-CoV-2 that causes COVID-19 [for example carvacrol in oregano and thyme (20, 33), cinnamaldehyde in cinnamon (33), geraniol in lemongrass, lavender, rose and others (33), curcumin in turmeric (33–35) and others (36)]. Depending on the traditional cuisines in each country, there may be differences in the type and amount of these phytochemicals consumed, and that may have contributed the differences in the infection, severity of the symptoms, and recovery from COVID-19. These phytochemicals are also included in beverages, such as teas from tea plants (14, 24) and herbal teas. A recent study on healthy identical twins has shown that those who were assigned to consume vegan diet had significantly improved cholesterol concentration, fasting insulin level, and body weight loss compared to those who were assigned to eat omnivorous food, indicating the effects of phytochemicals on several health conditions even among genetically identical twins (37). The health benefits of fermented plant-based foods have also been shown (38–40), which not only affect gut microbiota by the probiotic lactic acid bacteria in them, but also can improve the health status of the person directly by the phytochemicals with anti-inflammatory, antioxidant, and other bioactive properties included in them. Herbal medicines (41), supplements (42), and essential oils (43) may also contribute to prevent and improve the recovery from COVID-19 due to the phytochemicals with bioactive properties included in them.

The goal of the present study was to determine if there were associations between the self-reported consumption of phytochemicals and COVID-19 infection rates and recovery.

Our survey suggests that the participants who did not get COVID-19 ate leaf vegetables, root vegetables, other vegetables, and fermented foods/beverages, and used herbs and spices daily at high percentages. In addition, the participants who contracted COVID-19 and answered that they ate vegetables, fermented foods/beverages, and used herbs and spices daily reported fewer days to recover from COVID-19. The key findings from our survey were as follows: (1) Higher percentage of people who did not get COVID-19 daily ate various types of vegetables and fermented foods/beverages. (2) In India and Japan, the people who did not get tested and who tested positive showed significantly shorter time to recover from COVID-19. (3) The people who tested positive or did not get tested and recovered rather quickly were significantly more daily eating/drinking these vegetables and fermented foods/beverages. (4) Fruits are known to have beneficial effects on health, but consumption of fruits was not different between those who contracted and did not contract COVID-19. (5) Teas are also known to have beneficial effects on health and on COVID-19, and people who did not get tested and who did not get COVID-19 were found to have a higher daily intake of tea. (6) People who tested positive reported more severe symptoms compared to people who did not get tested. (7) There were significant differences in the number of days to recover among countries in both the group that did not get tested and the group that tested positive.

Tea and coffee were found to be the most preferred beverages to drink daily, although there were some country-dependent differences about which one is more preferred. In our survey, we found that tea was significantly more consumed daily by the people who did not get COVID-19 and those who were not tested than by the participants who tested positive to COVID-19. Although it is difficult to conclude from survey data whether it had some effects or not, there were significant differences in the severity of the symptoms between people who tested positive and those who did not get tested. Previous studies have shown that (−) epigallocatechin gallate (EGCG), theasinensin A (TSA), and galloylated theaflavins such as theaflavin 3,3′-di-O-gallate (TFDG) included in teas have anti-SARS-CoV-2 activities (13, 14, 24, 45, 46). The differences in the processing methods generate large differences in the concentration of chemical composition in green tea, yellow tea, white tea, oolong tea, and black tea (47). TSA and TFDG are catechin derivatives that increase by enzymatic oxidation during the processing of the tea leaves, and thus EGCG is decreased in black tea (47, 48). The studies showing that EGCG, TSA, and TFDG have anti-SARS-CoV-2 effects indicate that these changes in the amount included by differences in the processing methods do not affect the potential positive effects of tea. Importantly, it should be noted that there are studies showing that milk casein blocks the anti-SARS-CoV-2 effects of tea because of the binding of them to casein (49). These studies suggest the possible effects of teas on preventing and suppressing severity of COVID-19, but also indicate that depending on how the tea is prepared (for example, milk tea), the effects can be blocked.

Although in our survey the results on coffee did not show its effects on facilitating recovery from COVID-19 or suppressing infection, a study using a large amount of data (n > 37 K) in the UK Biobank on dietary habits and COVID-19 infection rate has shown a significant negative correlation between the drinking habit of coffee and the infection rate, i.e., people drinking coffee were less infected to COVID-19 (50). An in vitro study has also shown that coffee, caffeine, or diluted human serum samples collected from healthy adults who drank coffee suppressed infection of SARS-CoV-2 (25). It could be that the lack of tendency in our project is due to the smaller data size, and it is also possible that the differences are due to the international nature of our study, or possible interactions with other beverages or foods or the way of preparation like in case of teas.

An interesting result in this study is that the people who did not get COVID-19 showed general trend to consume leaf, root and other vegetables, mushrooms, fermented foods/beverages, beans and peas, and herbs and spices daily and that the people who tested positive and those who did not get tested and consumed them daily recovered faster. This tendency of inverse correlation between intake of vegetables and COVID-19 was also found in previous COVID-19 literature. Tadbir Vajargah et al. (51) found that the severity of COVID-19 symptoms of 250 hospitalized patients negatively correlated with the amount of consumption of vegetables as well as fruits and dietary fiber (51). They also recovered faster, leading to shorter hospitalization and convalescence periods. A meta-analysis study, which was conducted before the COVID-19-pandemic, has also shown that higher intakes of fruits and vegetables reduce proinflammatory cytokines, and improved the immune system profiles (52). In a survey-based study focusing on six European countries, researchers found that people who were on plant-based diet or pescatarian diet (plant-based diet with fish) had milder symptoms of COVID-19 (16). The results of our study are in line with these studies and suggest the importance of vegetable intake and also the importance of studies on the key phytochemicals or combinations of phytochemicals that contributed to these differences.

In the case of fermented foods and beverages, there are additional factors that are necessary to take into consideration, i.e., the roles of bacteria. There are also animal-based fermented foods/beverages, which may have different effects compared to those that are plant-based. Muhialdin et al. (40) summarized the antiviral activities of probiotic bacteria in fermented foods. They suggested that the antiviral effects of fermented foods could be exerted directly through the viral neutralization by the bioactive compounds and indirectly through improving the innate immune system (40). It should be noted that the fermented foods/beverages category was one of the two most commonly reported categories in India and Japan; the two countries that showed significantly faster recovery from COVID-19. This suggests the importance of further studies on fermented foods/beverages.

Considering the well-known beneficial effects of fruits on health, we hypothesized that we would find higher daily consumption of fruits, but, interestingly, with the exception of stone fruits, daily fruit consumption was rather low compared to vegetables and fermented foods. In fact, there are studies that found the negative association of fruits consumption and COVID-19 (51). One study identified foods that contain chemical compounds with bioactive properties that resemble clinically approved drugs for COVID-19, and they identified 52 phytochemical compounds with anti-SARS-CoV-2 effects (21). They reported that flavonoids, coumarins, stilbenes, indoles, and phenolic acids are the major groups of phytochemicals with anti-COVID-19 effects, and examples of the phytochemicals in these groups are quercetin, kaempferol, myricetin of flavonols, luteolin and apigenin of flavones, procyanidin B2 of flavanols, naringin of flavanones, daidzein, genistein, legumelin of isoflavonoids, trans-resveratrol of stilbenes, 3-indole-carbinol of indoles, and gallic acid of phenolic acids. Many of these phytochemicals with bioactive properties against SARS-CoV-2 are present in fruits (berries, citrus fruits, pome fruits, stone fruits, and tropical fruits). It could be that, similarly to coffee, there may be some differences in the images of fruits among countries (for example, fruits are called “mizu-gashi” in old Japanese language, which means “watery sweets/desserts”). Japanese participants reported very high daily intake of all types of vegetables and fermented foods/drinks in comparison to other countries, but daily consumption of fruits was rather low. Daily consumption of citrus fruits and tropical fruits was high in India and Iran, and Iran showed higher daily consumption of pome fruits than other countries. These differences could be an example of cultural differences in eating habits among countries.

We were unable to control individual differences in the amount of each serving size of food and beverages. There are also differences in the amount of chemical ingredients even within the same plant, depending on its location and seasonal differences, and in the way the food/beverage is prepared. The nutrients also differed depending on whether they were cooked or raw. Another issue to take into consideration is the possibility that the results of this survey could be telling that the people who consume various vegetables daily are more cautious of their overall health and may modify other activities of daily living other than their eating habits, such as self-distancing, wearing masks, and sanitizing their hands and other things. As such, there are also limits in the study using surveys.

In conclusion, our results suggest that daily consumption of phytochemical compounds included in the vegetables may have contributed to preventing the onset of COVID-19 as well as to accelerating the recovery from COVID-19. The food and beverage categories that were found to be eaten daily more by the people who did not get COVID-19 are the suitable candidates to conduct studies in detail with more controlled experimental conditions, studies using animal model systems, and at the level of phytochemical compounds to determine mechanisms of action in future.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The survey contained a question that asked consent to participate and we obtained consent from the participants. Ethical approval was obtained as an exempt study from Indiana University Human Research Protection Program (HRPP) in the U.S.A. (protocol #14915), the Office of Regulatory Research Compliance of Howard University (IRB-2022-0380), the Jikei University School of Medicine Ethics Committee in Japan (#34–003), the Bioethics Committee at the A.N. Severtsov Institute of Ecology & Evolution of Russian Academy of Sciences (no.2022-63-NC), and by the Second Affiliated Hospital of Xi’an Jiaotong University, Medical Ethics Committee in China (#2022023). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

SK: Writing – review & editing, Writing – original draft, Visualization, Supervision, Resources, Project administration, Methodology, Formal analysis, Data curation, Conceptualization. PJ: Investigation, Funding acquisition, Writing – review & editing, Supervision, Project administration, Conceptualization. VS: Visualization, Validation, Writing – review & editing, Supervision, Project administration, Conceptualization. TH: Methodology, Writing – review & editing, Supervision, Project administration, Conceptualization. PA: Writing – review & editing, Formal analysis, Data curation. RKa: Writing – review & editing, Validation, Supervision, Project administration, Methodology, Investigation, Formal analysis, Data curation. RKu: Writing – review & editing, Project administration, Investigation, Formal analysis, Data curation, Conceptualization. RA: Writing – review & editing, Supervision, Methodology, Investigation, Formal analysis. SB: Writing – review & editing, Methodology, Investigation, Conceptualization. OC: Writing – review & editing, Methodology, Investigation. CM-C: Writing – review & editing, Project administration, Methodology, Investigation, Data curation. JC: Writing – review & editing, Methodology, Investigation. KC: Writing – review & editing, Formal analysis, Data curation. SD: Writing – review & editing, Validation, Supervision, Methodology, Investigation. PR: Writing – review & editing, Project administration, Methodology, Investigation. MG: Writing – review & editing, Methodology, Investigation, Data curation. MK: Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation. TL: Writing – review & editing, Methodology, Investigation, Data curation. EM: Writing – review & editing, Methodology, Investigation. ZN: Writing – review & editing, Methodology, Investigation, Data curation. HN: Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation. MÖ: Writing – review & editing, Investigation. SP: Writing – review & editing, Methodology, Investigation. EÖ: Writing – review & editing, Methodology, Investigation. DS: Writing – review & editing, Methodology, Investigation. FT-H: Writing – review & editing, Methodology, Investigation. RU: Writing – review & editing, Methodology, Investigation. VV: Writing – review & editing, Project administration, Methodology, Investigation, data curation.