Plant-Based Diets and COVID-19 Infection: Is There Really a Connection?
Could a predominantly plant-based diet could be linked to lower odds of COVID-19 infection? Experts have weighed in.
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A new study published in the journal BMJ Nutrition Prevention & Health claims that a predominantly plant-based diet is linked to 39% lower odds of COVID-19 infection.
The study, which involved a group of 702 adult volunteers recruited during the spring of 2022, found that the participants who reported eating a diet high in vegetables, legumes, nuts and low in dairy and meat had a lower incidence rate of contracting COVID-19 compared to their meat-eating peers. Additionally, the omnivore group were more likely to report moderate to severe infection than the plant-based group when they were infected.
However, other independent experts who were not involved in the study have criticised the authors’ conclusions – that a vegetarian diet may be considered to be preventative against COVID-19 – saying that the observational study had too small a sample size and does not prove causation.
Diet and COVID-19
The association between dietary factors and infectious disease is not a new one. Malnutrition is considered to be the primary cause of immunodeficiency worldwide, which can put people at a higher risk for contracting bacterial, fungal, viral and parasitic infections. Inversely, it is not uncommon for some infectious diseases to lead to weight loss, or in some severe cases, the development of anorexia or malnutrition.
It is perhaps not surprising then that dietary factors are also being closely monitored in relation to the coronavirus pandemic. Previously, population-level data have suggested that certain foods might be linked to incident COVID-19 infections, with other studies finding significant shifts in the gut microbiome during the course of the coronavirus pandemic.
In this new study, researchers from the University of São Paulo, Brazil, set out to study whether there might be any significant impacts on COVID-19 infection incidence, severity and duration in people with a more plant-based diet versus omnivores.
For this, they recruited 702 adults who were surveyed on their usual eating patterns as well as other lifestyle factors, medical history and COVID-19 vaccination status. Collectively, the study captured information from 424 people with an omnivorous diet and 278 people who identified as being either vegan, vegetarian or “flexitarians” who eat meat less than 3 times per week.
The study reported that in total, 330 people (47%) said they had experienced COVID-19 infection, of which 224 (32%) reported mild symptoms and 106 (15%) experienced moderate to severe symptoms.
The researchers found that the omnivores had higher rates of COVID-19 incidence compared to the more plant-based diet groups, with 52% of omnivores reporting an infection compared to 40% of their peers. The omnivorous group was also more likely to report moderate to severe infection, at 18% versus just over 11% for the plant-based group.
Are the study recommendations premature?
The researchers are upfront with the limitations of their research, writing in their paper that “as an observational study, we are unable to confirm a direct causal association between diet and COVID-risk or infer specific mechanisms.”
However, they also write that in light of their findings they would “recommend the practice of following plant-based diets or vegetarian dietary plans” as they believe this could influence the incidence of COVID-19 infections.
Others in the field are unconvinced. In comments to the Science Media Centre, Dr Gavin Stewart, senior lecturer in evidence synthesis at Newcastle University, said: “This work presents interesting data but the authors conclusions do not adequately reflect the uncertainty inherent in small observational studies that are not designed to assess causal relationships. The conclusion that plant-based diets have a preventative role in COVID-19 infection is premature and not warranted.”
In additional comments, Dr Duane Mellor, a registered dietitian and senior lecturer at Aston University’s Aston Medical School, drew attention to the relatively small sample size used in this study and the possibility for statistical errors.
“This study is observational and does not include confirmation of diagnosis, and although the researchers corrected for ethnicity, education and other factors associated with risk of COVID-19 infection, the small sample size might could have meant that any association could be a statistical error,” Mellor told the Science Media Centre. “It was also noticeable that there are data reporting errors, with the paper suggesting the incorrect percentage of participants reporting as white.”
Professor Kevin McConway, emeritus professor of applied statistics at the Open University, also notes the observational nature of the study in his comments to the Science Media Centre, reiterating that the results of such studies need considerable care in interpreting their results.
“The research paper does mention some possible ways in which eating a plant-based diet could act in the body to reduce infection chance, but the study itself provides no data on this. So a very considerable doubt remains,” McConway said.
Additionally, McConway highlights the choice by the authors to only report a relative measure of the associations they found.
“The press release says that “those following a predominantly plant-based or vegetarian/vegan diet were 39% less likely to become infected than the omnivores”, and that indeed corresponds to what is in the paper (after the adjustments for other factors),” he said.
“Translating this into absolute risks, the researchers found that, of a group of 100 people, like those in the study and on an omnivorous diet, 52 would have been infected during the research period. In another group of 100 people, who matched the first 100 in terms of the factors adjusted for, but were on a predominantly plant-based or vegetarian/vegan diet, an estimated 39 would have been infected, so considerably fewer. There is a margin of statistical error around that estimate, though – the number infected in the plant-based group could plausibly be anywhere between 33 and 47, so it’s possible that the difference in risk is quite a lot smaller or bigger than the simple estimate indicates,” McConway continued. “Also, I must emphasise that we still can’t be sure that it’s the difference in diet group that causes the difference in infection risk – it could still be due to some factor or factors that were not adjusted for.”
Reference: Acosta-Navarro JC, Dias LF, Gouveia LAG de, et al. Vegetarian and plant-based diets associated with lower incidence of COVID-19. BMJ Nutr Prev Health. 2024:e000629. doi: 10.1136/bmjnph-2023-000629