Table 2. Newcastle Ottawa Quality Matrix for quality appraisal of systematic review.

NOS Items	Ilie et al. [13]	Alcala et al. [14]	Carlsen et al. [15]
Representativeness of the exposed cohort	0	0	1
Selection of the non-exposed cohort	0	0	1
Ascertainment of exposure	1	1	1
Demonstration that outcome of interest was not present at the start of the study	1	0	1
Comparability of cohorts on the basis of the design and analysis	1	0	1
Assessment of outcome	1	1	1
Was follow-up long enough for outcomes to occur	1	0	0
Adequacy of follow-up of cohorts	0	0	0
Total	5	2	6