Table 2.

Quality assessment of cohort studies using New Castle Ottawa Scale

Author	Selection				Comparability		Outcome
	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	The study controls for age, sex and marital status	Study controls for other factors	Assessment of outcome	Was follow-up long enough for outcomes to occur	Adequacy of follow-up of cohorts
Hujoel et al., 2003[21]	*	*	*	*	*	*		*	*
Michaud et al., 2008[23]	*	*	*	-	*	*	-	*	*
Arora et al., 2010[24]	*	*	-	-	*	*	*	*	*
Wen et al., 2014[25]	*	*	*	-	*	*	*	*	-
Michaud et al., 2016[27]	*	*	*	-	*	*	-	*	*
Nwizu et al., 2017[29]	*	*	-	*	*	-	*	*	*
Michaud et al., 2018[30]	*	*	*	-	*	*	-	*	*
Tai et al., 2019[31]	*	-	*	-	-	*	*	*	*

*significant and -nonsignificant