Table 2. Quality assessment of eligible studies.

First author, year	Representativeness of exposed cohort	Selection of non-exposed cohort	Ascertainment of exposure	Demonstration that outcome was not present at start of study	Comparability based on the design or analysis	Assessment of outcome	Follow-up long enough for outcomes to occur	Adequacy of follow-up of cohorts	Total NOS score
Ide et al., 2007	*	*		*	*	*	*	*	7
Sun et al., 2007	*	*	*	*	*	*	*	*	8
Miyake et al. 2008	*	*	*	*	*		*	*	7
Zhang et al., 2010	*	*		*	*	*	*	*	7
Zhao et al., 2012	*	*	*	*			*	*	6
Zhu et al., 2013	*	*	*	*	*	*	*	*	8
Matsuo et al., 2014	*	*	*	*			*	*	6
Qin et al., 2015	*	*	*	*	*	*	*	*	8
Leppanen et al., 2018	*	*	*	*	*		*	*	7
Wang et al., 2019	*	*	*	*	*		*	*	7
Wang et al., 2019	*	*	*	*	*			*	6

A study can be awarded a maximum of one star for each numbered item within the selection and outcome categories. A maximum of two stars can be given for comparability. NOS, New Ottawa Scale.