Table 2.

Newcastle-Ottawa Scale (NOS) quality assessment of each cohort study.

Study	Selection				Comparability	Outcome			NOS score
	Representativeness of the exposed cohort	Selection of the nonexposed cohort	Ascertainment of exposure	Demonstration that outcome of interest was not present at start of study	Comparability of cohorts on the basis of the design or analysis	Assessment of outcome	Was follow-up long enough for outcomes to occur	Adequacy of follow-up of cohorts
Tsumura et al. (1999) [29]	∗	∗	NA	∗	∗∗	∗	∗	∗	8
Noriyuki et al. (2003) [30]	∗	∗	NA	∗	∗∗	∗	∗	∗	8
Lee et al. (2003) [31]	∗	∗	NA	∗	∗∗	∗	NA	∗	7
Waki et al. (2005) [32]	∗	∗	NA	∗	∗∗	∗	∗	∗	8
Shi et al. (2013) [12]	∗	∗	NA	∗	∗∗	∗	NA	∗	7
Lee et al. (2017) [13]	∗	∗	NA	∗	∗∗	∗	∗	∗	8
Zhang et al. (2017) [14]	∗	∗	NA	∗	∗∗	∗	NA	∗	7
Yatsuya et al. (2018) [15]	∗	∗	NA	∗	NA	∗	∗	∗	6

^∗: 1 point, ^∗∗: 2 points, NA: no point.