Table 1.

Summary of characteristics for eight studies included in the meta-analysis

Author (year)	Study design	Country	Study period	Outcome	Outcome ascertainment	Sex	Sample size (n); characteristics				Blood sample type	Vitamin D testing method
							Case or control	No.	Male (%)	Mean age (SD)/median age (range)
Budhathoki et al (2018)30	Nested case-cohort	Japan	1990–2009	Colorectal cancer	NA	All*	Case	637	52	56.2 (7.5)	Plasma	Chemiluminescent enzyme immunoassay
							Control	4044	34	53.7 (7.9)
Choi et al (2015)37	Case-control	Korea	2011–2012	Colorectal adenoma	Colonoscopy	All	Case	112	51	60.3 (5.3)	Serum	Chemiluminescent enzyme immunoassay
							Control	112	51	59.5 (5.4)
						Men	Case	57	100	NA	Serum
							Control	57	100	NA
						Women	Case	55	0	NA	Serum
							Control	55	0	NA
Hong et al (2012)26	Case-control	Korea	2009–2010	Colorectal adenoma	Colonoscopy	All	Case	143	68	58.7 (6.0)	Serum	Direct competitive electrochemiluminescence immunoassay
							Control	143	68	58.7 (6.0)
Otani et al (2007)24	Case-control	Japan	1990–2003	Colorectal cancer	Pathologically confirmed	All	Case	323	52	NA	Plasma	Competitive protein-binding assay
							Control	621	52	NA
						Men	Case	163	100	56.9
							Control	324	100	56.9
						Women	Case	160	0	56.5
							Control	297	0	56.4
Takahashi et al (2010)25	Case-control	Japan	1997–2004	Colorectal adenoma	Colonoscopy	Men	Case	656	100	52.0 (1.4)	Plasma or Serum	Radioimmunoassay method
							Control	648	100	51.8 (1.5)
Yamaji et al (2012)27	Case-control	Japan	2004–2005	Colorectal adenoma	Colonoscopy	All	Case	737	67	NA	Plasma	Radioimmunoassay method
							Control	703	65	NA
Ying et al (2015)36	Case-control	China	2010–2014	Colorectal cancer	Colonoscopy	All	Case	212	NA	63(56-73)	Plasma	Direct competitive ELISA
							Control	212	NA	63(57-73)
Yurekli et al (2015)29	Case-control	Turkey	2012	Colorectal cancer	Colonoscopy	All	Cases	96	67	57.8 (10.0)	Serum	NA
							control	195	54	51.2 (12.9)

All*: all participants, including men and women

NA, not available; SD, standard deviation.