The articles were included if they met all the following criteria:
(1) the outcome of interest was gastric cancer; (2) the study of
interest was dietary vitamin A, retinol or blood retinol level; (3) the
relative risk (RR), or odds ratio (OR) estimates and their 95% confidence
intervals (95% CI) were given or sufficient data were
available for evaluation; (4) the articles were written in English or
Chinese. For duplicate studies, the most recent or most detailed one
was included