The researchers noted that the survey is observational, and does not establish a causative relation between diet and COVID-19 severity, and caution is needed in the interpretation of the findings.
The survey results, published in the journal BMJ Nutrition Prevention & Health on Tuesday, indicate that plant and fish based diets were associated with 73 per cent and 59 per cent lower odds, respectively, of severe disease.
Several studies have suggested that diet might have an important role in symptom severity and illness duration of COVID-19 infection.
However, there’s little evidence to confirm or refute this theory.
The researchers, including those from Johns Hopkins Bloomberg School of Public Health, US, drew on the survey responses of 2,884 frontline doctors and nurses with extensive exposure to SARS-CoV-2, the virus behind COVID-19, working in France, Germany, Italy, Spain, the UK and the US.
The online survey, conducted between July and September 2020, was designed to elicit detailed information about respondents’ dietary patterns over the previous year.
It was based on a 47-item food frequency questionnaire, and the severity of any COVID-19 infections the respondents had.
The survey also gathered information on personal background, medical history, medication use, and lifestyle.
The various diets were combined into plant-based — higher in vegetables, legumes, and nuts, and lower in poultry and red and processed meats -– pescatarian/plant-based with added fish or seafood, and low carb-high protein diets.
As many as 568 respondents said they had symptoms consistent with COVID-19 infection or no symptoms but a positive swab test for the infection.
Around 2,316 said they had no symptoms or had not tested positive.
Among the 568 cases, 138 clinicians said they had moderate to severe COVID-19 infection, while as the remaining 430 said they had had very mild to mild COVID-19 infection.
The respondents who said they ate plant-based diets’ or plant-based/fish diets had, respectively, 73 per cent and 59 per cent lower odds of moderate to severe COVID-19 infection.
Compared to people who said they ate a plant-based diet, those who said they ate a low carb-high protein diet had nearly four times the odds of moderate to severe COVID-19 infection, according to the researchers.
These associations held true when weight (BMI) and co-existing medical conditions were also factored in, they said.
However, the researchers did not observe any association between any type of diet and the risk of contracting COVID-19 infection or length of the subsequent illness.
The survey relied on individual recall rather than on objective assessments, and the definition of certain dietary patterns may vary by country, the researchers noted.
Men outnumbered women in the study, so the findings may not be applicable to women, they added.
However, plant-based diets are rich in nutrients, especially phytochemicals, vitamins and minerals, all of which are important for a healthy immune system, the researchers said.
Also, fish is an important source of vitamin D and omega-3 fatty acids, both of which have anti-inflammatory properties, they said.
“Our results suggest that a healthy diet rich in nutrient dense foods may be considered for protection against severe COVID-19,” the researchers noted.
“The trends in this study are limited by study size and design (self-reporting on diet and symptoms) so caution is needed in the interpretation of the findings,” said Shane McAuliffe, Deputy Chair of the NNEdPro Nutrition and COVID-19 Taskforce, a UK based think-tank that works on nutrition education, research and innovation.
“However, a high quality diet is important for mounting an adequate immune response, which in turn can influence susceptibility to infection and its severity,” McAuliffe, who was not involved in the study, said.
Other researchers in the study were from Brigham & Women’s Hospital, Cedars-Sinai Medical Center, Stamford Hospital, and Columbia University Vagelos College of Physicians and Surgeons, in the US.
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