Advocates of plant-based diets suggest they can reduce the risk of chronic illnesses such as heart disease and help the environment.

Now, new research suggests they may provide another health benefit: lowering severity from the coronavirus disease 2019 (Covid-19).

A plant-based diet was associated with 73-percent lower odds of moderate to severe Covid-19 infection in the study. A pescatarian diet, which includes fish but limits or eliminates meat, was associated with 59-percent lower odds.

Compared to those who ate a plant-based diet, those with a low-carb, high-protein diet had nearly four times the odds of moderate to severe Covid-19 infection, according to the study.

“Our results suggest that a healthy diet rich in nutrient-dense foods may be considered for protection against severe Covid-19,” said the researchers led by Dr. Sara Seidelmann of Stamford Health in Connecticut.

Plant-based diets were described as high in vegetables, legumes and nuts and low in poultry and red and processed meats.

The researchers noted that plant-based diets are rich in nutrients, especially phytochemicals — polyphenols, carotenoids — vitamins and minerals, which are important for a healthy immune system.

Fish is an important source of vitamin D and omega-3 fatty acids, both of which have anti-inflammatory properties.

Researchers drew on the survey responses of nearly 2,900 frontline doctors and nurses with extensive exposure to Covid-19 infection. The doctors were working in the United States, France, Germany, Italy, Spain and Britain.

The survey asked about their dietary patterns over the past year, based on a food frequency questionnaire. It also asked about the severity of any Covid-19 infections they had.

Researchers learned that 568 respondents had symptoms consistent with Covid-19 infection or no symptoms but a positive swab test for the infection.

Among the 568 cases, 138 clinicians said they had moderate to severe Covid-19 infection. The remaining 430 said they had very mild to mild Covid-19 infection.

The study was published in the online journal BMJ Nutrition Prevention & Health.

The team factored in age, ethnicity, medical specialty and lifestyle, such as smoking and physical activity. They also factored in weight and coexisting medical conditions. No association was found between diet and risk of contracting Covid-19.

The study can establish correlation, not cause. Also, it relied on individual recall rather than objective assessments.

Because of those limitations, Shane McAuliffe, a registered dietitian and deputy chair of Britain’s Nutrition and Covid-19 Taskforce, urged caution in interpreting the findings.

“A high-quality diet is important for mounting an adequate immune response, which in turn can influence susceptibility to infection and its severity,” McAuliffe said in a journal news release.

However, he said, “this study highlights the need for better designed prospective studies on the association between diet, nutritional status and Covid-19 outcomes.”