All subjects met the inclusion criteria were randomly divided into four groups using random number table : sole VA supplementation group ( VA group , a single oral dose of VA at about 200 000 units ) , sole iron supplementation group ( FE group , daily oral supplementation with the element iron 1-2 mgkg ( -1 ) d ( -1 ) for five days a week , lasting for 6 months ) , the combined supplementation of VA and iron group ( VF group ) and control group ( CO group , no VA and iron supplementation ) .