ABSTRACT
Dietary iron occurs in two fundamental forms in the human diet: heme and nonheme iron.[3] Heme iron refers to all forms from plant and animal sources in which the iron molecule is tightly bound within the porphyrin ring structure, as is found in both myoglobin and hemoglobin. Nonheme iron refers to all other forms. Contaminant iron that is derived from dust and soil iron are relatively unavailable to the absorptive cells, but may constitute a significant amount of iron intake in developing countries. There is substantial information that demonstrates that nearly all nonheme dietary iron mixes together in a lumenal ‘‘pool’’ of iron in the upper gastrointestinal (GI) track as a result of acidification in the stomach and subsequent exposure to pancreatic and GI enzymes. Inorganic iron is solubilized and ionized by gastric acid juice, reduced to the ferrous form, and kept soluble in the upper GI tract by chelation to compounds such as citrate and ascorbic acid. The type and amount of other materials such as ascorbic acid that can chelate iron to keep it in solution also determine the amount of nonheme iron in a soluble lumenal pool. The number of ‘‘inhibitors’’ of nonheme iron absorption is substantial, with phytate, polyphenols, and tannins leading the list. These inhibitors typically bind either ferric or ferrous iron in a tight complex in the lumen of the gut and make it unavailable for the absorptive proteins. Thus, it should be clear that a diet containing a large amount of unrefined grains, nondigestible fibers, etc. will have poor iron bioavailability. In contrast, a diet that is highly refined has little roughage, and substantial portions of meat will have a greater iron bioavailability regardless of other factors. The American diet typically obtains about 50% of its iron intake from grain products, in which the iron concentration is between 0.1 and 0.4mg per serving. Some fortified cereals, however, may contain as much as 24mg of iron in a single serving. Heme iron is more highly bioavailable than nonheme iron, and its bioavailability is less affected by other components of the diet. Only about 10% of total dietary iron intake is represented by heme iron in many Western countries.
