With great interest, we read the article by Konings et al,1 in which the authors conducted a combined case–control and cohort study and found that dysphagia, gastroparesis, irritable bowel syndrome (IBS) without diarrhoea and constipation specifically predict subsequent newly onset idiopathic Parkinson’s disease (PD) through comparing patients with PD with matched negative controls and patients with Alzheimer’s disease (AD) and cerebrovascular diseases (CVDs). However, several methodological concerns should be carefully addressed before concluding that those gastrointestinal (GI) syndromes can predict the development of PD.

First, reverse causation from the diagnostic delay of neurodegenerative diseases is a big concern,2 3 and not considering it in the analysis might lead to spurious associations (eg, the short-term increased PD risk after Clostridium difficile infection4). Given this diagnostic delay, the temporal relationship between GI syndromes and neurodegenerative disease in Konings et al’s study1 is hard to be determined due to …