Nonheme iron is absorbed as Fe2+, and not as Fe3+; ascorbic acid in the intestinal lumen will both maintain iron in the reduced state and also chelate it, thus increasing absorption. A dose of 25 mg of vitamin C taken together with a semisynthetic meal increases the absorption of iron 65%, whereas a 1-g dose gives a nine-fold increase (Hallberg, 1982).
This is an effect of ascorbic acid present together with the test meal; neither intravenous administration of vitamin C nor supplements several hours before the test meal has any significant effect on iron absorption, although the ascorbate secreted in gastric juice should be effective (Mowat and McColl, 2001). A variety of other reducing agents, including alcohol and fructose, also enhance the absorption of inorganic iron.
Ascorbate is also active in the reduction of Fe3+ in the plasma transport protein, transferrin, to Fe2+ for storage in ferritin in the liver or for heme synthesis. It is not clear to what extent this represents specific actions of ascorbate, because other reducing reagents, including glutathione, also enhance heme synthesis, and the NADH-dependent flavoprotein ferriductase is the major factor controlling the transfer of iron between transferrin and ferritin.
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