Table 2.

Subgroup analyses of diabetes mellitus and Parkinson’s disease

		Diabetes mellitus and Parkinson’s disease
		n	Relative risk (95% CI)	I2 (%)	P h 1	P h 2	References
All studies		14	1.27 (1.20–1.35)	82.3	< 0.0001		8–21
Sex
Men		8	1.29 (1.24–1.34)	31.1	0.18	0.49/ 0.933	9,10,12,14–16,19,21
Women		7	1.31 (1.17–1.47)	90.3	< 0.0001		9,12,14–16,19,21
Men and women		6	1.23 (1.12–1.35)	52.5	0.06		8,11,13,17,18,20
Follow-up
<5 years		1	1.32 (1.18–1.48)			0.57	13
5-<10 years		7	1.27 (1.20–1.35)	89.1	< 0.0001		11,14–17,20,21
10-<15 years		2	0.91 (0.71–1.15)	0	0.82		12,18
15-<20 years		2	1.36 (0.78–2.37)	76.7	0.04		8,9
≥20 years		2	1.57 (1.13–2.19)	58.3	0.12		10,19
≥15 years		4	1.47 (1.12–1.92)	60.9	0.05	0.79	8,9,10,19
Excluding early follow-up
Yes		3	1.33 (1.18–1.50)	23.0	0.27	0.44	9,11,15
No		11	1.24 (1.14–1.35)	85.6	< 0.0001		8,10,12–14,16–21
Geographic location
Europe		5	1.29 (1.19–1.38)	75.4	0.003	0.98	9,13,15,20,21
America		5	1.20 (1.05–1.36)	53.9	0.07		8,10–12,17
Asia		4	1.31 (1.11–1.55)	84.3	< 0.0001		14,16,18,19
Number of cases
Cases < 500		2	1.52 (1.08–2.14)	75.5	0.04	0.60	13,19
Cases 500-<1000		5	1.15 (0.90–1.46)	60.8	0.04		8–10,12,18
Cases ≥ 1000		7	1.27 (1.20–1.35)	89.1	< 0.0001		11,14–17,20,21
Parkinson's disease assessment
No validation or independent assessment		8	1.29 (1.21–1.37)	85.3	< 0.0001	0.71	13–20
Validated or independent assessment		6	1.25 (1.09–1.43)	58.8	0.03		8–12,21
Diabetes type
Any diabetes		10	1.26 (1.14–1.38)	83.7	< 0.0001	0.72	9,10,15,21
Type 2 diabetes		4	1.29 (1.17–1.41)	81.5	0.001		8,11–14,16–20
Assessment of diabetes
Self-reported		4	1.34 (1.05–1.72)	69.8	0.02	0.58	10–12,19
Self-reported and validated		1	1.04 (0.74–1.46)				8
Self-report and medical records or fasting glucose		9	1.26 (1.19–1.34)	86.9	< 0.0001		9,13–18,20,21
Timing of diabetes diagnosis
Prevalent (baseline)		10	1.31 (1.23–1.40)	81.8	< 0.0001	0.21	9,11–13,15,16,18–21
Prevalent and incident		3	1.17 (1.11–1.24)	0	0.51		8,10,17
Incident (newly diagnosed)		1	1.19 (1.08–1.32)				14
Diabetes duration
<5 years		2	1.38 (0.95-2.00)	83.0	0.02	0.89	10,26
≥5 years		2	1.32 (0.99–1.77)	66.0	0.09		10,26
Diabetes complications
Yes		3	1.54 (1.32–1.80)	89.6	< 0.0001	0.18	10,15,16
No		3	1.26 (1.16–1.38)	86.3	0.0007		10,15,16
Study quality, modified NOS scale
0–3 stars		0				0.76
>3–6 stars		10	1.26 (1.17–1.36)	72.0	< 0.0001		9,14,16,21
>6–8 stars		4	1.30 (1.14–1.48)	92.2	< 0.0001		8,10–13,15,17–20
Adjustment for confounding factors
Age	Yes	14	1.27 (1.20–1.35)	82.3	< 0.0001	NC	8–21
	No	0
Education	Yes	4	1.20 (0.89–1.63)	75.6	0.006	0.76	9,11,12,18
	No	10	1.28 (1.20–1.35)	85.3	< 0.0001		8,10,13–17,19–21
Alcohol	Yes	6	1.28 (1.13–1.46)	68.0	0.008	0.64	9,10,12,13,16,18
	No	8	1.25 (1.17–1.34)	80.2	< 0.0001		8,11,14,15,17,19–21
Smoking	Yes	9	1.27 (1.15–1.40)	82.7	< 0.0001	0.99	8–13,16,18,21
	No	5	1.27 (1.16–1.39)	83.0	< 0.0001		14,15,17,19,20
BMI or obesity	Yes	7	1.31 (1.19–1.45)	84.4	< 0.0001	0.42	9–11,13,16,18,21
	No	7	1.23 (1.13–1.35)	79.8	< 0.0001		8,12,14,15,17,19,20
Physical activity	Yes	6	1.29 (1.12–1.50)	65.8	0.01	0.40	9–12,16,18
	No	8	1.25 (1.17–1.33)	79.6	< 0.0001		8,13–15,17,19–21
Hypertension	Yes	2	1.41 (1.37–1.45)	0	0.72	0.20	10,16
	No	12	1.25 (1.17–1.33)	76.4	< 0.0001		8,9,11–15,17–21
Serum cholesterol	Yes	2	1.51 (1.12–2.03)	33.3	0.22	0.33	10,11
	No	12	1.26 (1.19–1.34)	84.5	< 0.0001		8,9,12–21
Coffee or caffeine	Yes	3	1.31 (0.92–1.86)	75.3	0.02	0.70	9,11,12
	No	11	1.27 (1.19–1.34)	84.7	< 0.0001		8,10,13–21

n denotes the number of risk estimates (one publication reported a combined risk estimate for two studies), BMI, body mass index, NC, not calculable because no studies were present in one of the subgroups

¹P for heterogeneity within each subgroup

²P for heterogeneity between subgroups with meta-regression analysis

³P for heterogeneity between men and women (excluding studies with both sexes) with meta-regression analysis