The proportion of dietary iron absorbed is determined by the iron requirement of the individual. Absorption is regulated by the size of the body iron store in healthy humans (percentage absorption is inversely proportional to serum ferritin concentration) (Cook et al., 1974). There is a several-fold difference in absorption from a meal between an individual who is iron deficient and some-one with sizeable iron stores. The calculation of dietary requirements must be based on the maintenance of a well-defined iron status. This has been accomplished by setting the need for the maintenance of a minimal iron store (serum ferritin concentration cutoff of 15 μg/L) as the surrogate indicator of functional adequacy.
The other major factor to take into account when computing dietary iron requirements is iron bioavailability based on the composition of the diet. Iron is present in food as either part of heme, as found in meat, poultry, and fish, or as nonheme iron, present in various forms in all foods. As previously discussed, the absorption mechanisms are different. Heme iron is always well absorbed and is only slightly influenced by dietary factors. The absorption of nonheme iron is strongly influenced by its solubility and interaction with other meal components in the lumen of the upper small intestine.
Decreased stomach acidity, due to overconsumption of antacids, ingestion of alkaline clay, or pathologic conditions such as achlorhydria or partial gastrectomy, may lead to impaired iron absorption (Conrad, 1968; Kelly et al., 1967).
Ascorbic Acid. Ascorbic acid strongly enhances the absorption of nonheme iron. In the presence of ascorbic acid, dietary ferric iron is reduced to ferrous iron which forms a soluble iron-ascorbic acid complex in the stomach. Allen and Ahluwalia (1997) reviewed various studies in which ascorbic acid was added to meals consisting of maize, wheat, and rice. They concluded that iron absorption from meals is increased approximately two-fold when 25 mg of ascorbic acid is added and as much as three- to six-fold when 50 mg is added.