Table 2. Methodological quality of observational studies included in the meta-analysis1.

First author	Representativeness of the exposed cohort	Selection of the unexposed cohort	Ascertainment of exposure	Outcome of interest not present at the start of the study	Control for important factor or additional factor	Outcome assessment	Follow-up long enough for outcomes to occur	Adequacy of the follow-up of cohorts	Total quality scores
Meissner/2003 [12]	☆	—	☆	☆	—	☆	☆	☆	6
Hu/2012 [13]	☆	—	☆	☆	—	☆	☆	☆	6
Wang/2017 [14]	☆	—	☆	—	—	☆	☆	☆	5
Fukutomi/2018 [15]	☆	☆	☆	☆	—	☆	☆	☆	7
Herrera/2018 [16]	☆	—	☆	☆	—	☆	☆	☆	7
Thornton/2019 [17]	☆	—	☆	☆	—	☆	☆	☆	6

¹ A study could be awarded a maximum of one star for each item except for the item Control for important factor or additional factor.