Two investigators (Drs. Wu YH and Ye Y) independently
extracted data from each study and reached a consensus. The
following information was extracted from each study: author, year
of publication, study design (case-control or cohort), country of
origin, number of participants, characteristics of participants (e.g.,
gender and age), variables adjusted for in the analysis, RR (or OR)
estimates with 95% CIs for the highest versus lowest categories of
vitamin A intake, retinol intake or blood retinol level. ORs (RRs) that
reflected the greatest degree of control for potential confounders
were adopted in this meta-analysis