Table 1.

The association between Parkinson's disease and intestinal disorders.

Intestinal disorders	Connection with Parkinson's disease (PD)	Publication trends on web of science (accessed on 7 May 2022)	References
Colorectal cancer (CRC)	1. PD patients had a reduced risk of CRC	“Parkinson's disease” and “Colorectal cancer” (176)	Xie et al., 2017; Fang et al., 2021
	2. CRC occurrence was significantly lower in patients with PD
Constipation	1. Constipation patients are at a 2.27-fold increased risk of developing PD compared to the control group, and this phenomenon emerges up to 20 years before diagnosis	Parkinson's disease and “Constipation” (1,016)	Kaye et al., 2006; Adams-Carr et al., 2016; Svensson et al., 2016; Gan et al., 2018; Zhou et al., 2019; Camacho et al., 2021; Kang et al., 2022; Santos García et al., 2022; Zheng et al., 2022
	2. Compared with people without constipation, regional neural activity and functional connectivity in the brain show much difference in PD patients with constipation
	3. Constipation is associated with a sustained increased risk of a PD diagnosis and progression of neurodegenerative pathology, and there was a higher incidence for men than women
	4. Constipation is associated with cognitive decline in PD patients
	5. Constipation is associated with the increased severity of motor symptoms and decreased dopamine levels in PD patients in a dose-dependent manner. Simultaneously, the different constipation-loading times could lead to different clinical characteristics, especially in motor symptoms
Irritable bowel syndrome (IBS)	1. Patients with IBS are at an increased risk of developing PD in Taiwan in both genders in an age-dependent manner	“Parkinson's disease” and “Irritable bowel syndrome” (134)	Lai et al., 2014; Mertsalmi et al., 2017; Liu B. et al., 2021; Zhang J. et al., 2021; Lu et al., 2022; Yoon et al., 2022; Zhang X. et al., 2021
	2. IBS increased PD risk only in individuals ≥ 65 years
	3. PD patients with IBS-like symptoms had more non-motor symptoms
Inflammatory bowel disease (IBD)	1. The overall risk of PD in IBD, both Crohn's disease and ulcerative colitis is significantly higher than in controls	Parkinson's disease and “Inflammatory bowel disease” (302)	Lin et al., 2016; Killinger et al., 2018; Peter et al., 2018; Park et al., 2019; Villumsen et al., 2019; Zhu et al., 2019, 2022; Fu et al., 2020; Noh et al., 2020; Herrick and Tansey, 2021; Kim et al., 2022
	2. IBD is associated with increased PD risk regardless of sex, especially in patients over 65 years of age. Furthermore, the therapies for IBD using corticosteroids, anti-TNF and early anti-inflammatory methods may decrease the risk of PD
	3. The risk of neurodegenerative diseases is higher in IBD patients than in the non-IBD population
	4. Abnormal changes in the intestinal environment trigger the onset of PD via the brain-gut axis
	5. Gut inflammation and higher LRRK2 levels in Crohn's disease (IBD) may be a biomarker of increased risk for sporadic PD
Small intestinal bacterial overgrowth syndrome (SIBO)	1. The risk of SIBO is higher in PD patients than in non-PD patients, and SIBO could influence the progression of PD using negative and positive manners	“Parkinson's disease” and “Small intestinal bacterial overgrowth syndrome” (128)	Marrinan et al., 2014; Tan et al., 2014; Niu et al., 2016; Dǎnǎu et al., 2021
	2. SIBO may lead to fluctuation in the absorption of medications utilized to therapy PD, which could further influence the treatment of PD
	3. SIBO is associated with increased motor fluctuations present in PD patients compared with individuals without SIBO