After searching PUBMED and EMBASE, 2331 articles were
identified. After exclusion of 135 duplicated articles, 2196 articles
were assessed. Review of the titles and abstracts resulted in the
exclusion of 2072 articles. For the remaining 124 full-text articles,
reasons for exclusion were articles not on the right topic or targeted
population (56 papers), insufficient data (10 papers), multiple
publications (2 papers), language in neither English nor Chinese (8
papers), reviews or systemic reviews (17 papers). Finally, 31 studies
with approximate 300,000 participants were included in this study,
as detailed in Fig.1. Among the included studies 15 articles assessed
vitamin A intake, 19 articles investigated retinol intake and 5 articles
was on blood retinol level.