Assessment of study quality¹ included in the meta-analysis.

<p>¹ Representativeness of the exposed cohort; 2 selection of the non-exposed cohort; 3 ascertainment of exposure; 4 demonstration that outcome interest was not present at start of study; 5 comparability of cohorts on the basis of the design or analysis; 6 assessment of outcome; 7 was follow-up long enough for outcomes to occur; 8 adequacy of follow-up cohorts</p><p>^a Studies that controlled for age received one score, whereas studies that controlled for other important confounders received an additional score</p><p>^b Study with follow-up time >5 years was assigned one score</p><p>^c Study with follow-up rate >70% was assigned one score</p><p>Assessment of study quality<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0132424#t003fn001" target="_blank">¹</a> included in the meta-analysis.