COVID-19: Do plant-based and fish diets play a role?

• A new small study finds a link between plant-based and pescatarian diets and milder cases of COVID-19.
• The study is based on the experiences of doctors and nurses with heavy exposure to SARS-CoV-2.
• People whose diet is plant- or fish-based had a significantly lower risk of developing severe COVID-19.

For some people, COVID-19 is a life threatening illness, while for others, it is little more than a short-term annoyance. To some extent, this is a function of an individual’s comorbidities. However, why SARS-CoV-2 manifests so differently in different people remains a mystery.

Some research has indicated that this may have to do with a person’s diet, and a new six-country study involving people working on the front lines of COVID-19 care provides striking new evidence.

Healthcare professionals eating a plant-based diet were 73% less likely to experience moderate to severe COVID-19, and those following a plant- or fish-based diet were 59% less likely to get seriously ill.

The study’s authors write, “Our results suggest that a healthy diet rich in nutrient-dense foods may be considered for protection against severe COVID-19.”

The study appears in the journal BMJ Nutrition, Prevention & Health.

Doctors and nurses on the front lines

The researchers surveyed 2,884 doctors and nurses whose work with patients exposed them to SARS-CoV-2. The participants were from the United States, the United Kingdom, Spain, Italy, France, and Germany. The survey was designed to reveal “the association between self-reported diets and COVID-19 infection, severity, and duration of symptoms.”

Of the entire group, 2,316 of the participants had not experienced any COVID-19 symptoms or tested positive for the virus. These individuals served as the study’s control group.

The remaining 568 individuals either had symptoms consistent with COVID-19 or a positive swab test. Of these, 138 people reported moderate to severe COVID-19. The rest had only mild or very mild cases of the illness.

In their analysis, the researchers factored in age, ethnicity, medical profession, weight, body mass index, comorbidities, and lifestyle behaviors, including smoking and level of physical activity.

Th researchers asked the participants to choose a diet from among 11 selections that most closely resembled their own for the last year. The researchers then created dietary patterns by combining participants’ choices into groupings, such as whole food, plant-based diets; vegetarian diets; or pescatarian diets.

Compared with those following plant-based diets, people who reported eating a low carb, high protein diet were four times more likely to develop moderate to severe COVID-19 symptoms.

The study found no link between diet and the likelihood of developing COVID-19 or the duration of the illness.

Dr. Scott Kaiser, of Saint John’s Health Center in Santa Monica, CA, considers this research valuable.

He told Medical News Today: “Studies like this are really great, because they’re hypothesis-generating. It’s important to do these population-level studies and look at associations and generate additional hypotheses to investigate further.”

Understanding the study’s conclusions

While the study establishes a correlation between these diets and the severity of COVID-19, it does not establish causality. MNT asked Cleveland Clinic dietitian Kristin Kirkpatrick to speculate on why there might be such a strong link between these diets and COVID-19 severity:

“This may be due to the fact that plant-based and pescatarian patterns are also associated with reductions of anti-inflammatory markers. [O]ther studies have indicated that worsening COVID-19 symptoms may have something to do with inflammatory factors.”

The study suggests the nutrients in a plant-based diet that might be helping people with COVID-19 are phytochemicals, such as polyphenols and carotenoids, and minerals and vitamins, all of which support healthy immune systems. Pescatarians get beneficial omega-3 fatty acids and vitamin D — all anti-inflammatory substances — from fish.

The findings may be more broadly applicable than simply to COVID-19. Kirkpatrick said: “My belief is that any diet that includes nutrient-dense options can truly help […] mitigate a number of chronic conditions. This is clearly seen in over a decade of data.”

Significant limitations to the study

Shane McAuliffe, deputy chair of the NNEdPro Nutrition and COVID-19 Taskforce, pointed out that the study has two main limitations: small size and reliance on self-reporting.

Dr. Kaiser hopes for research that tracks “the actual biology of what’s going on and try and measure, for example, the levels of certain nutrients, minerals, vitamins, etc., in patients, actually measure the blood levels.”

There are several other important limitations to this study. For instance, as the authors note, COVID-19 tests were not always available during the time of the study, and we now know people can have the disease but show no symptoms. Therefore, some participants may have been wrongly classified as “controls” in the study, when, in reality, they may have had COVID-19. This could affect the accuracy of the results.

In addition, the relatively small sample size meant that there were wide margins of error in the estimates of how much of an impact diet had on people’s risk of getting seriously ill. For instance, for people following a plant-based diet, the odds of becoming seriously ill were anywhere between 19 and 90% lower.

The authors also found that when they adjusted for access to personal protective equipment (PPE), the results for the plant-based diet seemed to hold, but the plant- or fish-based diet stopped being statistically significant. Therefore, it is possible for the relationship between diet and severity of illness to be explained by other factors, such as PPE, that the researchers did not adjust for.

Finally, the majority of the people in this study were white male Western doctors. Therefore, we do not yet know how generalizable these results are to the rest of the population.

As McAuliffe concluded:

“This study highlights the need for better designed prospective studies on the association between diet, nutritional status, and COVID-19 outcomes.”

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