Binding of iron by fiber of wheat and maize

Abstract

Iron II is firmly bound by neutral detergent fiber (NDF) prepared from wheat or maize and NDF accounts for nearly all of the iron binding capability of these cereals. The amount of iron bound depends upon iron concentration, pH, quantity of fiber, and the presence or absence and quantities of inhibitors of binding. Binding is minimal, although appreciable, below pH 4.0, but rises rapidly above pH 5.0 to a maximum near pH 7.0, the limit of stability of iron in the system used. The NDF of wheat binds about 0.38 mg of iron per gram of NDF; that of maize somewhat more than 0.3 mg/g at pH 6.45. Binding of iron by acid detergent fiber (cellulose and lignin) is largely accounted for by its cellulose, and it also is pH dependent but less so than NDF. Iron binding by fiber is strongly inhibited by ascorbic, citric, and phytic acids and by EDTA in low concentrations. Various amino acids produce inhibition, especially cysteine, which inhibits strongly, but others are inactive. Phosphate and calcium are strong inhibitors; taurocholic acid is moderately inhibitory. It appears that a high proportion of ingested nonheme iron combines with fiber of wheat or maize and becomes unavailable for absorption when intakes of these cereals are high unless it is released by surges of gastric acid or inhibitors of binding. The promotion of iron absorption by adjuvants such as ascorbic acid, fruit juices, and EDTA may depend in part upon their ability to release iron from its combination with dietary fiber.