Mediterranean diet protective against COVID-19?

A higher adherence to the Mediterranean diet (MedDiet) appears to reduce the odds of developing COVID-19, a systematic review suggests.

The review included six observational studies with a sample size of 55,489 participants. All studies assessed MedDiet adherence using food frequency questionnaires (FFQs). [PLoS One 2024;19:e0301564]

Three of the five studies evaluating the correlation between MedDiet adherence and COVID-19 showed a significantly lower COVID-19 risk, with odds ratios (ORs) ranging between 0.75 and 0.95.

“[I]nterestingly, all studies reported ORs <1, although there were different reports on the significance of this association,” said the researchers. “[A]lthough the magnitude of the effect estimate was probably minor and was thus only observable in a larger population … [the findings suggest] that higher MedDiet adherence was a protective factor against COVID-19.”

Three studies looked at the association of individual MedDiet components with COVID-19 risk. According to the researchers, these showed that higher olive oil and fruit and nut consumption, moderate alcohol intake, and lower red meat and cereal intake reduced COVID-19 risk. Also, higher consumption of vegetables, fruits, legumes, nuts, whole grains, and fish reduced the odds of severe COVID-19.

The researchers attributed the reduced COVID-19 risk to fruit components such as flavonoids and antiviral and immunostimulatory compounds. Vitamin C, for instance, cuts pro-inflammatory cytokines and improves neutrophil chemotaxis. These, in turn, curtail viral pathogenesis and enhance recovery while preventing respiratory viral infections. [Nutrients 2021;13:2980; Infect Chemother 2020;52:461-477; Nutr Rev 2021;79:382-393]

Olive oil and fish are excellent sources of MUFAs/PUFAs*, which are known for their immunomodulatory properties. [Biology (Basel) 2023;12:279] Virgin olive oil has phenolic compounds that exert potent anti-inflammatory actions, while legumes contain bioactive compounds that have anti-inflammatory activity. [Curr Pharm Des 2011;17:754-768; Br J Nutr 2022;128:2158-2169] Red wine contains antioxidants that neutralize free oxygen radicals released by neutrophils, thereby minimizing cellular damage. [Molecules 2021;26:5537]

“The overall beneficial anti-inflammatory properties of these food groups explain their effects in diminishing the risks of COVID-19 and severe COVID-19,” said the researchers.

COVID-19 symptoms, severity

In the studies evaluating the link between MedDiet adherence and COVID-19 symptoms, one study showed that higher MedDiet adherence significantly reduced the odds for all reported COVID-19 symptoms, with ORs ranging between 0.06 and 0.34 for each symptom.

When looking at the association between MedDiet adherence and COVID-19 severity, one study found that participants with top tertile MedDiet score were less likely to have severe COVID-19 than those with bottom tertile score (OR, 0.23; p<0.001).

“[D]espite the difference in significance, the effect estimates for both COVID-19 symptoms and severity outcomes across all studies showed a consistent trend of OR <1. It was necessary to consider that the consistent effect of MedDiet adherence in all studies may be more critical than the statistically insignificant p-value,” the researchers noted.

“In fact, a nonsignificant result does not mean that there is no effect, albeit it might be too presumptive to assume a protective effect of higher MedDiet adherence against COVID-19 symptoms and severity without conclusive evidence,” they continued.

Implications

“As more countries have loosened [their COVID-19] regulations, a nutritional strategy may be more feasible and beneficial in the long term. The results of the present study may shed some light on the additional benefits of MedDiet against COVID-19,” said the researchers.

They added that certain food groups in the MedDiet may also be more important in reducing the odds of COVID-19. However, further studies are warranted before definitive conclusions can be drawn.

The main limitation was that diet adherence was self-reported across all studies, the researchers noted. “Measurement errors in FFQs are known to affect results and may have biased the observed effect estimates. FFQs are prone to biased responses … However, FFQs are often used in large cohort studies because they are realistic and logistically feasible.”