Ferritin, RBP4, and sTfR were categorized as; ferritin < 15 μg/L to be iron storage deficiency [25], RBP4 < 0.7 μmol/L [26], functional vitamin A deficiency and sTfR > 8.3 mg/L [27] as functional iron deficiency. Here, RBP4 is linked to vitamin A deficiency.