Iron is abundant in the world and needed by every living organism. For this reason early in evolution multi-cellular organisms evolved antimicrobial peptides(1). Without them we just wouldn't be able to survive the onslaught of microbial growth (1).
In addition, free iron in plasma has redox capabilities that can also be toxic. The toxicity is due to the Haber-Weiss-Fenton sequence, which forms hydroxyl radicals due to superoxide after reduction of dioxygen (1). The hydroxyl radicals can then act detrimentally on proteins, nucleic acids, carbohydrates and also cause lipid peroxidation (1).
In animals, heme is the moiety that holds a central iron atom within a porphyrin ring structure (1). Most of the body’s iron is contained as heme acting in various functions, but mainly necessary as an oxygen carrier within hemoglobin of blood cells and myoglobin in muscles (1). Heme synthesis requires iron to be in a reduced ferrous state, which is why ferriductases are important for iron metabolism (1).
Of the total amount of iron received from the diet, only 10 percent is generally absorbed. Of clinical importance, dietary iron is best if derived from animal products, which contains heme iron (1). Nonheme iron, found in both animal and plant-based foods, is not absorbed as well, although absorption of nonheme iron is increased at times of iron deficiency (1).
1. McCord JM. Iron, free radicals, and oxidative injury. J Nutr 2004;134:3171S-2S. Available at: http://jn.nutrition.org/cgi/content/full/134/11/3171S.
2. Modern Nutrition in Health and Disease. Baltimore, MD: Lippincott Williams & Wilkins, 2009.