The 'Dual Hit' hypothesis, stating that Parkinson's disease (PD) begins via olfactory pathways and the gut, and the gastrointestinal symptoms PD individuals face, have largely driven the interest of the gut's involvement in PD. Studies have since observed gut microbiota differences between PD groups and controls, with these alterations potentially relating to PD pathophysiology. However, differences in the studies' methodologies precludes unanimity on the relationships of gut microbiota to PD. Thirteen observational case-control studies investigating gut microbiota in PD and controls were reviewed to assess how microbiota abundance and diversity relates to PD. Nine studies showed butyrate producing gut microbiota had lower abundances in PD compared to controls. Three studies reported α-diversity was higher, with one reporting it was lower, in PD compared to controls. Given most studies show abundance, not diversity, differences of butyrate producing bacteria between groups, we propose abundance differences are more associated with PD than microbiota diversity. As current research is observational, investigating how specific bacteria and their metabolites may alter throughout PD progression is warranted.

中文翻译：

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健康的老年人与帕金森氏病患者之间的肠道菌群差异：系统评价。

“双重打击”假说指出帕金森氏病（PD）是通过嗅觉途径和肠道开始的，PD个体所面对的胃肠道症状在很大程度上推动了肠道对PD的兴趣。此后研究已经观察到PD组和对照组之间的肠道菌群差异，这些变化可能与PD病理生理有关。但是，研究方法的差异排除了肠道菌群与PD关系的一致。十三项观察性病例对照研究调查了PD中的肠道菌群和对照，以评估微生物菌群的丰富度和多样性如何与PD相关。九项研究表明，与对照组相比，产生丁酸盐的肠道菌群的PD含量较低。三项研究表明，α多样性更高，与对照组相比，有一项报告的PD较低。鉴于大多数研究表明组之间丁酸产生细菌的差异是丰富而不是多样性，因此我们认为丰度差异与PD的相关性高于微生物群的多样性。由于目前的研究是观察性的，因此有必要调查特定细菌及其代谢产物如何在PD整个发展过程中发生变化。