Well-Water Consumption and Risk of Parkinson’s Disease: A Meta-Analysis of 15 Observational Studies

Figures & data

Figure 1 Flow diagram of study selection.

Table 1 Characteristics of Studies Included in the Meta-Analysis

Study	Selection	Comparability	Exposure	Total Score
Liou 1997^Citation12	★★	★★	★	★★★★★
Behari 2001^Citation13	★★★	★★	★	★★★★★★
Michele 1996^Citation6	★★★	★★	★	★★★★★★
Zorzon 2002^Citation33	★★★	★★	★	★★★★★★
Cho 2008^Citation8	★★★	★	★	★★★★★
Hristina 2010^Citation10	★★★	★★	★	★★★★★★
Morano 1994^Citation36	★★★	★★	★	★★★★★★
Hancock 2008^Citation15	★★★	★★	★	★★★★★★
Firestone 2005^Citation7	★★★	★★	★★	★★★★★★★
Wang 1993^Citation37	★★★	★★	★	★★★★★★
Sanyal 2010^Citation11	★★★	★★	★	★★★★★★
Semchuk 1995^Citation34	★★★★	★★	★★★	★★★★★★★★★
Nuti 2004^Citation14	★★★★	★★	★★	★★★★★★★★
Gatto 2009^Citation9	★★★★	★★	★★★	★★★★★★★★★
Wang 1994^Citation35	★★★★	★★	★★★	★★★★★★★★★

Notes: One “★” will be given if one standard is reached. The more stars, the better the quality of the research.

Table 2 Quality Analysis Diagrams of the Included Studies

Study	Country	Design	Sample Size	Blind Method	Adjustments	Exposure Definition	Study Quality
Liou 1997^Citation12	China	HCC	120/240	N	1,2	Ever	5
Behari 2001^Citation13	India	HCC	377/377	N	2	>1year	6
Michele 1996^Citation6	Italy	HCC	116/116	N	1,2,3	>1year	6
Zorzon 2002^Citation33	Italy	HCC	136/272	N	1,2	Before 20 years old	5
Cho 2008^Citation8	Korea	HCC	235/77	N	5	Ever	5
Hristina 2010^Citation10	Serbia	HCC	110/220	N	1,2,5	>6mouths	6
Morano 1994^Citation36	Spain	HCC	74/148	N	1	>1year	6
Hancock 2008^Citation15	USA	PCC	319/296	N	4	Ever	6
Firestone 2005^Citation7	USA	HCC	250/388	N	1,2	Ever	7
Wang 1993^Citation37	China	HCC	93/186	N	1,2	Ever	6
Sanyal 2010^Citation11	India	HCC	175/350	N	1,2,4	>5 years	6
Semchuk 1995^Citation34	Canada	PCC	97/194	Y	1,2	Before 15 years old	9
Nuti 2004^Citation14	Italy	PCC	190/190	N	1,2,6	Ever	8
Gatto 2009^Citation9	USA	PCC	368/341	Y	2,5	Ever	9
Wang 1994^Citation35	Canada	PCC	40/97	Y	4	Ever	9

Note: Matching or adjustments were: (1) sex, (2) age, (3) institution, (4) gene, (5) residential area, (6) socio-cultural factors.

Abbreviations: PCC, population-based case–control study; HCC, hospital-based case–control study.

Table 3 Results of Subgroup Analyses Stratified by Studied Design, Geographic Area and Study Quality

Group	Number of Studies	Summary Effect		Heterogeneity
		OR(95% CI)	p value	I-Squared	p
All studies	12	1.245(1.011–1.533)	0.039	61.2%	0.003
Study design
HCC	8	1.471(1.124–1.923)	0.005	61.1%	0.012
PCC	4	0.962(0.734–1.261)	0.781	41.1%	0.165
Geographic area
America	4	0.969(0.757–1.240)	0.803	41.2%	0.164
Asia	4	1.286(1.045–1.582)	0.017	0.0%	0.460
Europe	4	1.798(1.014–3.187)	0.045	73.2%	0.011
Study quality
High	4	0.919(0.717–1.179)	0.507	32.3%	0.219
Low	8	1.495(1.177–1.898)	0.001	50.9%	0.047

Figure 2 Sensitivity analysis.

Abbreviation: CI, confidence interval.

Figure 3 Forest plots for ever-well-water drinkers versus non-drinkers (95% CI).

Abbreviations: CI, confidence interval; OR, odd ratio.

Table 4 Results of Individual Studies

Study	Country	Tevent	Tnoevent	Cevent	Cnoevent	OR (95% CI)
Liou 1997^Citation12	China	90	30	177	63	1.07(0.65,1.77)
Behari 2001^Citation13	India	156	221	140	237	1.19(0.89,1.60)
Michele 1996^Citation6	Italy	51	65	68	264	1.89(1.10,3.26)
Cho 2008^Citation8	Korea	34	102	35	237	1.83(1.08,3.08)
Morano 1994^Citation36	Spain	156	79	40	37	3.28(0.93,11.51)
Hancock 2008^Citation15	USA	76	34	102	118	1.12(0.81,1.54)
Firestone 2005^Citation7	USA	71	3	130	18	0.94(0.69,1.30)
Wang 1993^Citation37	China	197	122	175	121	1.39(0.84,2.29)
Nuti 2004^Citation14	Italy	133	117	212	176	0.93(0.59,1.44)
Gatto 2009^Citation9	USA	45	48	75	111	1.09(0.79,1.50)
Wang 1994^Citation35	Canada	32	10	60	24	0.45(0.21,0.96)

Table 5 Results of Subgroup Analyses Stratified by Studied Design, Geographic Area and Study Quality

Group	Number of Studies	Summary Effect		Heterogeneity
		OR(95% CI)	p value	I-Squared	p
All studies	11	1.158(0.968–1.386)	0.109	44.52%	0.054
Study design
HCC	7	1.314(1.044–1.654)	0.020	40.9%	0.118
PCC	4	0.962(0.734–1.261)	0.781	41.1%	0.165
Geographic area
America	4	0.969(0.757–1.240)	0.803	41.2%	0.164
Asia	4	1.286(1.045–1.582)	0.017	0.0%	0.460
Europe	3	1.542(0.793–3.001)	0.202	67.6%	0.046
Study quality
High	4	0.919(0.717–1.179)	0.507	32.3%	0.219
Low	7	1.331(1.100–1.611)	0.003	18.6%	0.288

Figure 4 Funnel plot analysis to detect publication bias.

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