Table 2. Summary risk estimates of the association between vitamin C and lung cancer risk.

	No.	No.		Heterogeneity test
Subgroups	(cases)	studies	Risk estimate (95% CI)	I2 (%)	P-value
All studies	8938	21	0.829(0.734–0.937)	57.8	0.001
Study design
Prospective	5485	14	0.829(0.729–0.942)	48.0	0.023
Case-control	3453	7	0.838(0.620–1.133)	73.2	0.001
Geographic locations
America	7104	17	0.849(0.735–0.982)	63.4	0.000
Europe	993	2	0.642(0.397–1.040)	46.8	0.170
Asia	841	2	0.824(0.660–1.029)	0.0	0.873
Sex
Males	3474	8	0.740(0.631–0.868)	31.9	0.173
Females	2037	8	0.999(0.751–1.329)	59.5	0.016
Histological type
Squamous cell carcinoma	1009	3	0.634(0.524–0.768)	0.0	0.852
Adenocarcinoma	482	3	0.713(0.549–0.926)	0.0	0.632
Sources of control (case-control studies)
Population-based	2184	7	0.808(0.590–1.107)	73.4	0.001
Hospital-based	1794	2	0.807(0.531–1.225)	71.4	0.062
History of smoking
Never-smokers	262	3	1.025(0.640–1.642)	0.0	0.474
Current smokers	1044	4	0.641(0.445–0.922)	52.2	0.099
Former smokers	702	4	0.901(0.712–1.139)	0.0	0.926