Systematic review with meta-analysis of the epidemiological evidence relating smoking to type 2 diabetes

Table 3 Meta-analysis random effect relative risks for current smoking

Grouping1		Current vs never smoking			Current vs non-smoking
		n2	RR (95%CI)	P	n	RR (95%CI)	P
Overall		99	1.33 (1.28-1.38)	P < 0.001, P < 0.05	156	1.28 (1.24-1.32)	P < 0.001, P < 0.05
Sex	Female	25	1.30 (1.23-1.37)		31	1.26 (1.21-1.31)
	Male	36	1.40 (1.32-1.49)		47	1.30 (1.24-1.36)
	Combined	38	1.28 (1.18-1.39)	NS3	78	1.26 (1.18-1.34)	NS
Continent	Asia	44	1.36 (1.30-1.43)		57	1.36 (1.29-1.43)
	Europe	32	1.34 (1.27-1.42)		60	1.25 (1.20-1.30)
	North and South America	19	1.27 (1.18-1.37)		34	1.18 (1.12-1.25)
	Oceania	4	1.05 (0.68-1.62)	NS	5	1.54 (1.28-1.85)	P < 0.001
Publication year	Up to 2005	13	1.41 (1.27-1.56)		23	1.24 (1.16-1.33)
	2005-2014	47	1.36 (1.30-1.43)		66	1.31 (1.27-1.35)
	2015 or later	39	1.27 (1.20-1.35)	NS	67	1.23 (1.17-1.30)	NS
Basis of diagnosis	Self-report only	12	1.32 (1.25-1.40)		17	1.34 (1.25-1.44)
	Medical records only	49	1.32 (1.25-1.38)		86	1.29 (1.23-1.34)
	Both	38	1.36 (1.27-1.46)	NS	53	1.24 (1.17-1.32)	NS
Population	General	93	1.32 (1.28-1.37)		147	1.28 (1.24-1.32)
	Pre-diabetics only	2	3.29 (1.51-7.21)		3	1.23 (0.79-1.90)
	Pre-diabetics excluded	4	1.61 (1.30-1.99)	P < 0.05	6	1.38 (1.15-1.67)	NS
Number of adjustment factors	0	17	1.15 (1.00-1.33)		33	1.19 (1.08-1.31)
	1 to 5	18	1.36 (1.25-1.47)		30	1.38 (1.27-1.51)
	6 to 10	43	1.40 (1.32-1.48)		64	1.29 (1.25-1.33)
	11 or more	21	1.28 (1.20-1.37)	P < 0.05	29	1.22 (1.15-1.30)	P < 0.1
Cohort size	< 5000	35	1.36 (1.19-1.56)		58	1.31 (1.20-1.42)
	5000 to 20000	20	1.38 (1.25-1.53)		43	1.24 (1.17-1.32)
	> 20000	44	1.32 (1.26-1.37)	NS	55	1.29 (1.24-1.35)	NS
Number of type 2 diabetes cases	< 500	44	1.37 (1.23-1.52)		78	1.27 (1.19-1.35)
	500-999	18	1.50 (1.34-1.67)		24	1.40 (1.27-1.55)
	1000-2000	10	1.26 (1.15-1.38)		17	1.20 (1.11-1.30)
	2001+	27	1.29 (1.22-1.35)	P < 0.1	37	1.26 (1.20-1.33)	NS
Highest age at baseline	< 60	13	1.36 (1.23-1.51)		22	1.24 (1.16-1.32)
	60-74	27	1.44 (1.32-1.56)		38	1.36 (1.27-1.45)
	75+	59	1.29 (1.24-1.35)	P < 0.1	96	1.26 (1.21-1.31)	NS
Length of follow-up (yr)	< 5	14	1.27 (1.19-1.35)		25	1.24 (1.15-1.34)
	5-10	55	1.38 (1.30-1.47)		81	1.34 (1.28-1.40)
	> 10	30	1.31 (1.22-1.39)	NS	50	1.22 (1.17-1.28)	P < 0.05
Definition of smoking	Cigarette	47	1.32 (1.27-1.38)		63	1.25 (1.21-1.29)
	Smoking	50	1.36 (1.26-1.46)		89	1.30 (1.23-1.37)
	Tobacco	2	1.10 (0.94-1.29)	P < 0.1	4	1.16 (1.06-1.27)	P < 0.1
Adjusted for age	No	20	1.17 (1.04-1.32)		41	1.22 (1.12-1.33)
	Yes	79	1.35 (1.31-1.41)	P < 0.05	115	1.29 (1.25-1.33)	NS
Adjusted for sex	No	72	1.35 (1.29-1.41)		107	1.27 (1.23-1.32)
	Yes	27	1.29 (1.20-1.39)	NS	49	1.29 (1.20-1.38)	NS
Adjusted for BMI	No	29	1.24 (1.11-1.38)		55	1.22 (1.13-1.32)
	Yes	70	1.35 (1.30-1.41)	NS	101	1.30 (1.26-1.34)	NS
Adjusted for physical activity	No	41	1.27 (1.20-1.35)		87	1.27 (1.21-1.33)
	Yes	58	1.36 (1.30-1.43)	P < 0.1	69	1.28 (1.23-1.33)	NS
Adjusted for alcohol consumption	No	42	1.26 (1.19-1.34)		87	1.26 (1.20-1.32)
	Yes	57	1.37 (1.31-1.43)	P < 0.05	69	1.29 (1.25-1.33)	NS
Adjusted for family history of diabetes	No	61	1.28 (1.22-1.35)		99	1.23 (1.17-1.29)
	Yes	38	1.41 (1.33-1.49)	P < 0.05	57	1.34 (1.29-1.40)	P < 0.01
Adjusted for education	No	63	1.37 (1.31-1.44)		115	1.29 (1.24-1.35)
	Yes	36	1.28 (1.21-1.34)	P < 0.05	41	1.23 (1.18-1.28)	P < 0.1
Adjusted for diet	No	74	1.35 (1.29-1.41)		126	1.29 (1.24-1.34)
	Yes	25	1.30 (1.22-1.38)	NS	30	1.23 (1.18-1.28)	P < 0.1
Adjusted for blood pressure	No	53	1.31 (1.24-1.40)		88	1.27 (1.21-1.34)
	Yes	46	1.35 (1.29-1.41)	NS	68	1.28 (1.24-1.33)	NS
Adjusted for cholesterol	No	72	1.30 (1.25-1.35)		115	1.26 (1.22-1.31)
	Yes	27	1.40 (1.32-1.48)	P < 0.05	41	1.32 (1.25-1.39)	NS
Adjusted for glucose	No	79	1.30 (1.25-1.35)		116	1.26 (1.22-1.31)
	Yes	20	1.44 (1.35-1.54)	P < 0.01	40	1.34 (1.27-1.41)	NS
Adjusted for triglycerides	No	80	1.30 (1.25-1.36)		124	1.27 (1.22-1.31)
	Yes	19	1.45 (1.33-1.58)	P < 0.05	32	1.34 (1.24-1.44)	NS
Adjusted for waist circumference	No	82	1.34 (1.29-1.40)		136	1.28 (1.24-1.32)
	Yes	17	1.29 (1.19-1.41)	NS	20	1.25 (1.16-1.35)	NS
Adjusted for any other factors	No	37	1.30 (1.19-1.42)		62	1.28 (1.18-1.38)
	Yes	62	1.34 (1.29-1.40)	NS	94	1.27 (1.23-1.30)	NS

¹For sex, publication year, basis of diagnosis, number of adjustment factors, definition of smoking and age adjusted the grouping relates to characteristics of the relative risk. For other factors it relates to characteristics of the study.

²Number of estimates combined.

³NS means not significant, P ≥ 0.1. For the overall analysis, the first P value relates to heterogeneity between estimates and the second to publication bias. For the other analyses it relates to a test of heterogeneity between the random-effects estimates at each level. Information on publication bias by level of each factor studied is given in Supplementary file 6. CI: Confidence interval; RR: Relative risk; NS: Not significant; BMI: Body mass index.