Effects of pH and chelating agents on iron binding by dietary fiber: implications for iron availability

Iron binding by isolated dietary fiber in the presence of iron ligands was measured. Iron-ligand solutions were mixed with isolated fiber and incubated. Bound and polymerized iron were estimated by centrifugation and ultrafiltration, respectively. Results showed that iron binding varied greatly with ligand used and with pH. Based on these studies, it was predicted that ligands would influence iron availability in the order ascorbate less than citrate = nitrolotriacetic acid less than EDTA. When added to a complex meal containing beef, bread, green beans, and milk, iron availability (measured by an in vitro method) was little affected by fiber, citrate, and ascorbate, but enhanced by nitrolotriacetic acid and EDTA. When added to a semisynthetic meal, fiber, ascorbate, citrate, nitrolotriacetic acid, and EDTA increased iron availability. It appears that the effect of dietary fiber on iron availability is greatly dependent on the ligands present in the meal.