Covid research: Study claims plant-based diet linked to 39% lower odds of virus infection. Experts disagree

Indian experts argue that the study is based on healthy user bias, with no real focus on other factors that could result in contracting Covid.

BySumit Jha

Published Jan 13, 2024 | 8:05 AMUpdatedJan 13, 2024 | 11:12 AM

Study reveals that adopting a predominantly plant-based or vegetarian diet is associated with a 39% lower likelihood of contracting Covid infection. (Creative Commons)

Can your chance of getting Covid be determined by your eating habits? The choice between a vegetarian and non-vegetarian diet could potentially influence the likelihood of infection, says a recently published study which, experts say, has some questionable findings.

Research published in the open-access journal BMJ Nutrition Prevention & Health concludes that adopting a predominantly plant-based or vegetarian diet is associated with a 39 percent lower likelihood of contracting Covid.

Based on these findings, the researchers suggest that adopting a diet rich in vegetables, legumes, and nuts, while reducing the intake of dairy products and meat, may contribute to a reduced risk of contracting the virus.

“Plant-based and mainly vegetarian diets were associated with a lower incidence of Covid-19 infection. These dietary patterns may be considered protective against Covid-19 infections,” said the study.

“This research adds to the existing evidence, suggesting that diet may have a role in susceptibility to Covid infection,” commented Shane McAuliffe, Senior Visiting Academic Associate, NNEdPro Global Institute for Food, Nutrition and Health, which co-owns BMJ Nutrition Prevention & Health, in a statement.

“But this remains an area of research that warrants more rigorous and high-quality investigation before any firm conclusions can be drawn about whether particular dietary patterns increase or decrease the risk of Covid infection,” he added.

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Experts sceptical

While an appropriately planned vegetarian diet is healthy and nutritionally adequate, and can offer a reduced risk of chronic diseases of high morbidity and mortality — ischaemic heart disease, type 2 diabetes, hypertension, certain types of cancer, and obesity — there is a need to consider socio-economic factors when evaluating dietary preferences.

“Vegetarianism in some countries is a habit of people who are more wealthy and health-conscious, and therefore, have better baseline health parameters and educational background. This makes them less likely to get exposed to high-risk situations such as contracting Covid,” Dr Rajeev Jayadevan, Co-Chairman of the National Indian Medical Association Covid Task Force, tells South First.

He points out that, for instance, these people could take more science-based precautions every day and could be doing jobs that do not involve coming into contact with too many people, and live in less crowded, well ventilated environments.

“Note that none of these factors are the direct result of being vegetarian,” Dr Jayadevan points out.

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Study limitations

Criticising the study, Dr Jayadevan, argues, “This is an example of a study that comes to wrong conclusions based on a weak study design. It could be listed as a teaching example of how not to interpret a study. In science, it is said that correlation does not mean causation. Just because two factors A and B are seen together, it does not mean that A caused B.”

Speaking to South First, he uses the analogy of observing rich people wearing Rolex watches:

If rich people are observed wearing a Rolex watch, purchasing a Rolex watch does not make a person rich. In other words, the Rolex watch is not the reason for the person being rich.

“This is again an online questionnaire-based study where the participants had to respond based on their memory of the food taken. Not a reliable study,” Shashikant Iyengar, certified metabolic health coach and low-carb practitioner, tells South First.

He adds that, overall, this study is not a well-designed one and no conclusion can be reached based on this.

Iyengar notes, “Whole-food, plant-based vegetarians are usually ‘healthy eaters with proper supplementation’, while so-called omnivorous meat-eaters eat refined carbs and sugary drinks. This phenomenon is called as healthy user bias. Non-Vegetarians can also be smokers and drinkers, and be generally unhealthy.”

He further adds that a large-scale, well-defined rigorous study is needed to confirm the conclusion of the study.

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Just an ‘observational study’

Dr Jayadevan tells South First that this is a small observational study of only 702 people in Brazil, who were followed up over a period of time to see how many of them reported a diagnosis of Covid. “On superficial reading, it seems plausible and interesting,” he adds.

However, when we look closely at the two groups, it can be seen that there are important baseline differences between them, aside from their dietary preference, that could explain the apparent difference in Covid outcomes.

The vegetarian group was better educated, physically more active and less obese, and had fewer pre-existing health conditions than the non-vegetarian group.

Also, when a relatively small number of people are studied, there will inevitably be differences in outcomes between the two groups.

“They looked at just 278 people who were vegetarian, this is less accurate when compared to looking at 2,00,000 people who are vegetarian. If we toss a coin 10 times, we may get heads only twice (20 percent), even though statistically, we expect it to be five times (50 percent). But when we toss it 10,000 times, we will get a result that is very close to 50 percent, which is closer to the truth. That’s the effect of sample size in studies,” explains Dr Jayadevan.

In the end, if a person decides to become vegetarian or non-vegetarian, neither choice is going to make them less or more likely to contract Covid.

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The study findings

The researchers from Brazil, the country with the second-highest Covid death toll worldwide, suggest that lower infection rates among individuals who consume a predominantly plant-based diet may be attributed to the abundance of nutrients in these diets.

“Due to the high intake of some key nutrients and phytochemicals in groups following plant-based diets, it is plausible that a difference in immune status might be observed between plant-based and omnivorous dietary patterns,” the researchers said in the study.

The study further added that plant-based dietary patterns are rich in antioxidants, phytosterols, and polyphenols, which positively affect several cell types implicated in immune function and exhibit direct antiviral properties.

To explore the potential impact of dietary patterns on Covid, the researchers studied 702 adult volunteers recruited between March and July 2022.

Participants were surveyed on their regular eating habits, food group frequency, lifestyle, medical history, and Covid vaccination status. They were categorised as either omnivorous (424) or predominantly plant-based (278). The plant-based group was further divided into flexitarians/semi-vegetarians (87) and vegetarians/vegans (191).

Out of the total participants, 47 percent reported having had Covid. Among them, 32 percent experienced mild symptoms and 15 percent experienced moderate to severe symptoms.

Compared to the plant-based dietary groups, omnivores reported a significantly higher incidence of Covid (52 percent vs 40 percent) and were more likely to experience moderate to severe infection (18 percent vs just over 11 percent).

It is to be noted that the omnivore group reported a higher incidence of medical conditions, lower physical activity, and a higher prevalence of obesity — factors associated with increased Covid infection risk and more severe symptoms.

Despite the absence of variations in the duration of symptoms, there was no discernible difference in symptom severity between omnivores and those adhering to plant-based diets, even after adjusting for potential influential factors such as weight, pre-existing medical conditions, and physical activity levels.