There are many factors that could affect the requirement of vitamin A including infections, parasites, protein-energy malnutrition, bioconversion efficiency of provitamin A, food matrix and so on. However, the toxicity of vitamin A should also be taken into account when establishing reference values for infants, young and preschool children. Serum and breast-milk retinol concentrations, retinol binding protein concentrations, the relative-dose-response test , the modified-relative-dose-response and the deuterated retinol isotope dilution test are the common tools to assess vitamin A status.