Study: Relevance of Mediterranean diet as a nutritional strategy in diminishing COVID-19 risk: A systematic review. Image Credit: Brian A Jackson / Shutterstock

Systematic review found that high adherence to the Mediterranean diet is associated with a reduced risk of COVID-19, but its impact on symptoms and severity remains unclear.

A recent systematic review published in the journal PLoS ONE determined the relationship between the Mediterranean diet and coronavirus disease 2019 (COVID-19) risk, symptoms, and severity.

Background

COVID-19, declared a pandemic in March 2020, has significantly impacted individuals worldwide. Symptoms include fever, cough, shortness of breath, sore throat, and nasal congestion. Diarrhea, headaches, and nausea are typical symptoms. Clinicians categorize COVID-19 as mild, moderate, severe, or critical based on symptomatology, oxygen saturation, and pro-inflammatory biomarkers. COVID-19 severity has a considerable influence on mortality.

Inflammation has a vital role in the disease’s progression and severity. Cytokine storms in COVID-19 are robust inflammatory responses with increased production of inflammatory cytokines that can result in severe infections and catastrophic complications. The plant-based Mediterranean diet exerts anti-inflammatory and immunomodulatory effects. The diet can minimize the likelihood of viral infection and promote viral clearance, indicating that it may protect against COVID-19.

About the systematic review

In the present review, researchers investigated the protective effects of increased adherence to the Mediterranean diet against the development, symptoms, and severity of COVID-19.

The researchers searched the Google Scholar, Pubmed, and Proquest databases on August 16, 2023, for human studies published in English that determined the relationship between adherence to the Mediterranean diet and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection risk, symptoms, or severity, with full text available. There were no restrictions on the study design or the date of publication. They excluded editorials, reviews, systematic reviews, replies, letters, meta-analyses, animal studies, and duplicate records.

Two reviewers independently performed title-abstract followed by full-text screening for relevant records, and a third reviewer resolved disagreements. Data extracted included the first author’s name, publication year, study design, study location, sample population, individual characteristics, and association of the Mediterranean diet association with COVID-19 development risk, symptoms, and severity. Researchers synthesized data narratively and compared the studies to develop a structured summary. The study outcomes were measured using odds ratios (OR).

The Newcastle Ottawa Scale (NOS) evaluated the risk of bias for non-randomized studies and the revised version of the Cochrane risk of bias (RoB2) tool for randomized studies. The Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) approach determined evidence certainty. The Rayyan software deduplicated records. The systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.

Results and discussion

Initially, researchers identified 325 records, of which 209 remained after excluding duplicates, and 13 underwent eligibility assessments. They included six records for the final review. The selected records, published between 2020 and 2023, incorporated observational designs and included 55,489 individuals. All studies used food frequency questionnaires (FFQ) to assess adherence to the Mediterranean diet. Four studies reported significant correlations between increased Mediterranean dietary adherence and a lowered risk of COVID-19, whereas one documented non-significant associations. A study documented significant associations between increased Mediterranean dietary adherence and SARS-CoV-2 infection symptoms, whereas three found non-significant associations. One study showed that increased adherence to the Mediterranean diet was related to a decreased probability of severe SARS-CoV-2 infection, though two studies reported inconclusive results.

The team repeatedly found lower OR values for COVID-19 associated with increased adherence to the Mediterranean diet. The OR values for each study outcome ranged from 0.06 to 0.99, with varying degrees of statistical significance. The studies exhibited a low to moderate risk of reporting bias. The authors rated the evidence for COVID-19 risk results as moderate confidence, whereas that for COVID-19 symptomatology and severity was of medium certainty. Higher intake of olive oil, less red meat, less cereal, moderate alcohol intake, and more nuts and fruits lowered the risk of COVID-19, and increased consumption of fruits, vegetables, nuts, legumes, fish, and whole grains reduced COVID-19 severity.

Fruit contains flavonoids, vitamin C, and antioxidants that lessen the incidence of COVID-19. Vitamin C suppresses the nucleotide-binding domain, leucine-rich-containing family, and pyrin domain-containing-3 (NLRP3) inflammasomes, which reduces inflammatory cytokines while increasing neutrophil chemotaxis. Fish and olive oil are rich in immunomodulatory monounsaturated and polyunsaturated fat. Olive oil also contains phenolic compounds, whereas legumes have bioactive substances that inhibit the enzymes cyclooxygenase-1 (COX-1) and COX-2. Red wine, a popular alcoholic beverage in the Mediterranean diet, provides antioxidants that inhibit reactive oxygen species generated by neutrophils, reducing cellular damage.

The study suggests that higher adherence to the Mediterranean diet may protect against developing COVID-19, although the advantages against disease symptomatology and severity are unknown. As governments relax social distancing restrictions and personal protection equipment (PPE) use, a nutritional approach may become more viable and helpful in the long run. Specific Mediterranean diet components may be critical in lowering COVID-19 risk. However, all studies used food frequency surveys and thus were prone to skewed replies, warranting further research before drawing conclusive conclusions.