Table 2.

Quality assessment of the included studies

Study	Q1	Q2	Q3	Q4	Q5	Q6	Q7	Q8	Q9	Q10	Q11	Q12	Q13	Q14	Rating
Doupis 2011 [50]	Yes	Yes	NR	Yes	No	No	No	Yes	Yes	No	Yes	CD	NA	Yes	Poor
Du 2016 [20]	Yes	Yes	NR	Yes	No	No	No	Yes	Yes	No	Yes	CD	NA	Yes	Poor
Erman 2016 [21]	Yes	Yes	NR	No	No	No	No	NA	Yes	No	Yes	CD	NA	No	Poor
Gomez-Ambrosi 2010 [9]	Yes	No	NR	Yes	No	No	No	NA	Yes	No	Yes	CD	NA	No	Poor
Guo 2014 [22]	No	Yes	NR	Yes	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor
Hanefeld 2016 [23]	Yes	Yes	NR	CD	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor
Jain 2013 [24]	Yes	Yes	NR	Yes	Yes	No	No	NA	Yes	No	Yes	CD	NA	Yes	Fair
Jesmin 2013 [25]	Yes	Yes	NR	No	No	Yes	Yes	NA	Yes	No	No	CD	Yes	No	Fair
Kakizawa 2004 [26]	Yes	Yes	NR	Yes	No	Yes	Yes	NA	Yes	No	Yes	CD	Yes	Yes	Good
Kubisz 2010 [27]	Yes	Yes	Yes	Yes	No	No	No	NA	Yes	No	Yes	CD	NA	Yes	Fair
Lim 2004 [28]	Yes	Yes	NR	Yes	No	No	No	No	Yes	No	Yes	CD	NA	No	Poor
Litvinova 2014 [29]	Yes	Yes	NR	Yes	No	Yes	No	Yes	Yes	No	Yes	CD	Yes	Yes	Fair
Loebig 2010 [30]	Yes	Yes	NR	CD	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor
MacEneaney 2010 [31]	Yes	Yes	NR	Yes	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor
Mahdy 2011 [32]	Yes	Yes	NR	Yes	No	NA	NA	NA	Yes	Yes	Yes	CD	Yes	Yes	Good
Marek 2010 [33]	Yes	Yes	NR	Yes	No	No	No	NA	Yes	No	Yes	CD	NA	Yes	Fair
Mirhafez 2015 [34]	Yes	Yes	NR	Yes	No	No	No	Yes	Yes	No	Yes	CD	NA	Yes	Fair
Mirhafez 2016 [35]	Yes	Yes	NR	No	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor
Mysliwiec 2008 [36]	Yes	No	NR	No	No	No	No	No	Yes	No	Yes	CD	NA	No	Poor
Nandy 2010 [37]	Yes	Yes	NR	Yes	No	No	No	Yes	Yes	No	Yes	CD	NA	Yes	Fair
Ozturk 2009 [38]	Yes	Yes	NR	CD	No	No	No	NA	Yes	No	Yes	CD	NA	No	Poor
Ruszkowska-Ciastek 2014 [39]	Yes	Yes	NR	CD	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor
Schlingemann 2013 [40]	Yes	Yes	NR	CD	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor
Seckin 2006 [41]	Yes	Yes	NR	Yes	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor
Siervo 2010 [42]	Yes	Yes	Yes	Yes	No	No	No	NA	Yes	No	Yes	CD	NA	Yes	Fair
Siervo 2012 [43]	Yes	Yes	Yes	Yes	No	No	No	NA	Yes	No	Yes	CD	NA	Yes	Fair
Silha 2005 [44]	Yes	Yes	NR	No	No	No	No	Yes	Yes	No	Yes	CD	NA	Yes	Fair
Suguro 2008 [45]	Yes	Yes	NR	Yes	No	Yes	No	NA	Yes	No	Yes	CD	Yes	No	Poor
Valabhji 2001 [46]	Yes	Yes	NR	No	No	No	No	NA	Yes	No	Yes	CD	NA	No	Poor
Wada 2010 [47]	Yes	Yes	NR	Yes	No	No	No	NA	Yes	No	Yes	CD	NA	Yes	Fair
Wu 2017 [48]	Yes	Yes	NR	Yes	No	Yes	No	No	Yes	No	Yes	CD	NA	Yes	Fair
Zorena 2010 [49]	Yes	No	NR	No	No	No	No	Yes	Yes	No	Yes	CD	NA	No	Poor

CD cannot determine, NA not applicable, NR not reported

Q1. Was the research question or objective in this paper clearly stated?, Q2. Was the study population clearly specified and defined?, Q3. Was the participation rate of eligible persons at least 50%?, Q4. Were all the subjects selected or recruited from the same or similar populations (including the same time period)? Were inclusion and exclusion criteria for being in the study prespecified and applied uniformly to all participants?, Q5. Was a sample size justification, power description, or variance and effect estimates provided?, Q6. For the analyses in this paper, were the exposure(s) of interest measured prior to the outcome(s) being measured?, Q7. Was the timeframe sufficient so that one could reasonably expect to see an association between exposure and outcome if it existed?, Q8. For exposures that can vary in amount or level, did the study examine different levels of the exposure as related to the outcome (e.g., categories of exposure, or exposure measured as continuous variable)?, Q9. Were the exposure measures (independent variables) clearly defined, valid, reliable, and implemented consistently across all study participants?, Q10. Was the exposure(s) assessed more than once over time?, Q11. Were the outcome measures (dependent variables) clearly defined, valid, reliable, and implemented consistently across all study participants?, Q12. Were the outcome assessors blinded to the exposure status of participants?, Q13. Was loss to follow-up after baseline 20% or less?, Q14. Were key potential confounding variables measured and adjusted statistically for their impact on the relationship between exposure(s) and outcome(s)?