Mediterranean diet and COVID-19 symptoms.

Background
Mediterranean Diet has been reported to possess immunomodulatory and anti-inflammatory properties. These properties are closely associated with the immunopathogenesis of COVID-19.
Objective
The present systematic review aimed to determine the association between Mediterranean Diet and COVID-19, COVID-19 symptoms, and COVID-19 severity.
Methods
The protocol for this systematic review was registered in International Prospective Register of Systematic Reviews (PROSPERO) with identification number CRD42023451794. The literature search was conducted through Pubmed, Proquest, and Google Scholar on August 2023. The inclusion criteria were studies with a population of human subjects, reported the association between Mediterranean diet adherence with risk of COVID-19 infection, COVID-19 symptoms, or COVID-19 severity, and full text must be available in English. The exclusion criteria were reviews, editorials, letters, replies, systematic reviews, meta-analyses, studies on animals, and duplicates. Risk of bias in included studies was assessed using Newcastle Ottawa Scale (NOS). Data was synthesized narratively. Each study was compared and a structured summary was developed.
Results
After selection process, 6 articles were included, with a sample size of 55,489 patients. All studies were observational studies and assessed Mediterranean diet adherence using food frequency questionnaires (FFQ), with scoring system varied between each study. Four studies found a significant correlation between increased adherence to Mediterranean Diet and reduced COVID-19 risk, while one study indicated non-significant association. One study reported a significant association between higher adherence to Mediterranean Diet and COVID-19 symptoms, but three studies reported non-significant association. One study found that individuals with higher adherence to Mediterranean Diet had reduced likelihood of developing severe COVID-19, however, two studies yielded inconclusive findings.
Limitations
All studies used self-administrated food frequency questionnaires (FFQs), which were prone to biased responses, such as recall and estimation bias.
Discussion
Lower trends of odds ratios (ORs) were consistently observed in higher Mediterranean diet adherence. In every outcome of the included studies, ORs ranged between 0.06–0.992, however, differing levels of significance were reported in each outcome.
Conclusion
Overall analyses suggest that high adherence to Mediterranean Diet is a protective factor against COVID-19, with unclear benefits against COVID-19 symptoms and severity.

研究背景：已有研究表明，地中海饮食（Mediterranean Diet）具有免疫调节与抗炎特性，而这些特性与新冠病毒感染（COVID-19）的免疫发病机制密切相关。
研究目的：本项系统综述旨在明确地中海饮食与新冠病毒感染、新冠感染相关症状以及新冠感染严重程度之间的关联。
研究方法：本项系统综述的研究方案已在国际前瞻性系统综述注册平台（International Prospective Register of Systematic Reviews, PROSPERO）注册，注册编号为CRD42023451794。本研究于2023年8月通过PubMed、ProQuest以及Google Scholar数据库开展文献检索。纳入标准如下：研究对象为人类受试者；报道了地中海饮食依从性与新冠感染风险、新冠感染症状或新冠感染严重程度之间的关联；且研究全文需为英文版本。排除标准包括综述类文章、社论、信函、回复、系统综述、荟萃分析、动物实验研究以及重复发表的文献。采用纽卡斯尔-渥太华量表（Newcastle Ottawa Scale, NOS）对纳入研究的偏倚风险进行评估。本研究采用叙述性方法对数据进行综合分析，对各项研究进行比对并形成结构化总结。
研究结果：经过文献筛选流程后，最终纳入6篇文献，总样本量为55489名受试者。所有纳入研究均为观察性研究，均通过食物频率问卷（food frequency questionnaires, FFQ）评估受试者的地中海饮食依从性，但各研究采用的评分系统存在差异。其中4项研究发现，更高的地中海饮食依从性与新冠感染风险降低之间存在显著相关性，另有1项研究未观察到显著关联。1项研究显示，更高的地中海饮食依从性与新冠感染症状改善存在显著关联，但其余3项研究未发现此类关联。1项研究发现，地中海饮食依从性更高的人群发生重型新冠感染的风险降低，然而另有2项研究得出了不确定性结论。
研究局限：所有纳入研究均采用自评式食物频率问卷（FFQs），此类问卷易受回忆偏倚、估算偏倚等应答偏倚的影响。
讨论：在地中海饮食依从性更高的人群中，始终观察到比值比（odds ratios, ORs）呈降低趋势。纳入研究的各项结局指标中，OR值区间为0.06~0.992，但各结局指标的显著性水平存在差异。
结论：综合分析结果表明，高依从性地中海饮食是新冠病毒感染的保护性因素，但该饮食模式对新冠感染症状及感染严重程度的保护效应尚不明确。