Plant-based immunity? Study suggesting decrease in COVID-19 infection risk draws scrutiny

10 Jan 2024 --- A study linking plant-based diets to reduced COVID-19 risk is sparking a nutritional debate among experts. Scientists in Brazil reveal that a predominantly plant-based or vegetarian diet could be linked to a 39% lower chance of contracting COVID-19, suggesting that dietary choices might play a vital role in the susceptibility and response to the virus.

However, some experts are contesting the study’s conclusions.

According to the study, diets primarily consisting of plant-based foods might offer a richer array of nutrients that bolster the immune system’s capabilities, which could aid in the battle against viral infections.

“Plant-based dietary patterns are rich in antioxidants, phytosterols and polyphenols, which positively affect several cell types implicated in the immune function and exhibit direct antiviral properties,” the authors state.

The researchers also highlight that the study is observational and, therefore, unable to determine cause-and-effect relationships. They further emphasize that the research depends on individual recollection and subjective evaluation, both susceptible to inaccuracies.

Dr. Duane Mellor, a registered dietitian and senior lecturer at Aston University’s medical school in the UK, states that subjectivity is not the only issue with the study’s findings.

“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 could have meant that any association could be a statistical error,” he explains.

Furthermore, Dr. Margaret Rayman, a professor of nutritional medicine at the University of Surrey, states that the assertion of the adequacy of plant-based diets might present more of a dilemma.

“The authors say that ‘we recommend the practice of following plant-based diets or vegetarian dietary patterns,’” she states. “I think that this study can’t provide substantial evidence for this recommendation with regards to COVID-19 infection.”

Plant-based immunity?
The study, published in the BMJ Nutrition Prevention & Health, was conducted between March and July 2022 and involved 702 adult participants. The participants were categorized into two primary dietary groups — 424 as omnivorous and 278 as predominantly plant-based.

The latter category also included 87 flexitarians or semi-vegetarians who consumed meat no more than three times a week, and 191 participants who practiced vegetarianism or veganism. Of the total participants, 330 (47%) reported having contracted COVID-19. The incidence was significantly higher in the omnivorous group (52%) compared to the plant-based group (40%).

Moreover, the omnivores were more likely to experience moderate to severe infection. However, the study found no significant difference in the duration of symptoms between the two dietary groups.

The results also showed that participants who followed a plant-based or vegetarian diet tended to have higher — in most cases postgraduate — educational levels. The plant-based participants also had fewer medical conditions and lower rates of overweight and obesity, known risk factors for COVID-19.

“In light of these findings and the findings of other studies, and because of the importance of identifying factors that can influence the incidence of COVID-19, we recommend the practice of following plant-based diets or vegetarian dietary patterns,” the study reads.

Differing opinions
Rayman states that plant-based diets can have significant nutrient limitations, especially regarding individuals with greater nutritional requirements than the average person, like pregnant women.Though plant-based diets may reduce COVID-19 infections, experts say more robust research must be done.

Several experts raised this same convention following the publication of the EAT-Lancet Commission’s Diet for Planetary Health. Experts pointed out that the diet held significant micronutrient shortfalls, specifically for women of reproductive age — typically between 15 and 49.

“A recent study assessed 141 observational and intervention studies published since 2000 that looked at nutrient intake and status in adult populations consuming vegetarian, vegan or meat-eating diets,” Rayman reveals. “Intake of vitamin B12, iodine, iron, zinc, calcium and vitamin D and bone turnover markers were generally lower in plant-based versus meat-eater dietary patterns.”

“These micronutrients are mostly found in animal foods or, in the case of iron, zinc and calcium, have a lower bioavailability in plant food.”

Additionally, experts emphasize that plant-based diets often contain large amounts of phytate — an anti-nutrient found in many plants, which can reduce the bioavailability of micronutrients and minerals.

Also, a team of US-based researchers recently reported that although there are many health benefits of reducing meat intake, caution should be taken when removing meat from diets. The study underscored that, due to its nutrient density, meat provides essential proteins and nutrients that can be challenging to replicate in meat-free diets, playing a crucial role in overall health and development.

Making their case
The study’s researchers highlight that — after adjusting for factors like weight, pre-existing medical conditions and physical activity levels — the severity of symptoms showed no overall difference. Rather, those following the predominantly plant-based, flexitarian, vegetarian or vegan diet had a 39% lower risk of infection compared to omnivores.

The study also highlights the potential protective effects of a diet rich in vegetables, legumes and nuts and low in dairy and meat, against COVID-19 and opens avenues for further research. The researchers’ findings underscore the importance of dietary choices in managing health risks, particularly in the context of the ongoing pandemic.

“This research adds to the existing evidence, suggesting that diet may have a role in susceptibility to COVID-19 infection,” says Shane McAuliffe, a senior visiting academic associate at the NNEdPro Global Institute for Food, Nutrition and Health — a co-owner of the publishing journal.

“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 the risk of COVID-19 infection,” he concludes.

By William Bradford Nichols

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