Toxicology Brief: The toxicity of iron, an essential element

Toxicology Brief: The toxicity of iron, an essential element

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Feb 01, 2006

Iron is the most abundant trace mineral in the body and is an essential element in most biological systems.1,2 It is likely that iron was essential for developing aerobic life on Earth.3 But iron is toxic to cells in excessive amounts. Acute iron poisoning is common and potentially lethal in dogs, cats, and many other animals. Iron is also a leading cause of unintentional poisoning deaths in children less than 6 years old.

Normal iron content and storage

About 70% of the iron in mammals is found in hemoglobin, and about 5% to 10% is found in myoglobin. When bound to normal hemoglobin and myoglobin, iron is in the ferrous (Fe2+ ) form.1,2,4 Up to 25% of iron in the body is in the ferric (Fe3+ ) form and is stored in hemosiderin, ferritin, and transferrin in the liver, spleen, and bone marrow.1,2,5 Ferric iron is used in iron-containing enzymes, such as peroxidase, catalase, and cytochrome-c.

Sources

One reason iron toxicosis is such an important problem is that the general public is often unaware of the potential toxicity of products that are considered natural and necessary for our health.6

Another reason is that many pharmaceutical preparations contain iron. Multivitamins containing iron are readily available. Many are brightly colored and sugarcoated, making them attractive to animals and small children. In addition, several iron supplements are available over the counter. Another frequent source of iron overdose in pets is prenatal vitamins. Many prescription prenatal vitamins contain more than 60 mg of elemental iron in each pill, so animals can develop severe iron toxicosis even if only a few tablets are ingested.

Numerous other products contain iron, including one-time-use heating pads. Iron can also be found in fertilizers and pesticides and in the soil.1,2,4


Table 1. Iron Salts and the Percentage of Elemental Iron*
Iron is also used in injectable products and is bound to proteins in supplements (chelated iron) to treat iron deficiencies in animals. There are several forms of injectable iron (iron dextran, iron dextrin, iron sorbitol, ferric ammonium citrate) and several chelated forms of iron. Chelated iron is almost as effective in treating iron deficiencies as other salts are but is about a fourth as toxic.1,2,4 Product labels do not always indicate if the iron is chelated. Most products that contain iron have it in a salt form. Table 1 lists several iron salts and the percentage of elemental iron in each.

Iron absorption

Iron absorption is a two-step process. First, iron ions are absorbed from the intestinal lumen into mucosal cells. Ferrous iron is better absorbed than ferric iron because ferric iron precipitates out of solution at around pH 7 or under normal physiologic conditions.7 However, both forms can be absorbed if they are ionized.1,2,5 Because iron must be ionized to be absorbed, metallic iron and iron oxide (rust) are not generally of concern when they are ingested.1,2 Most iron absorption occurs in the duodenum and upper jejunum, but in animals with iron toxicosis, the iron seems to be well-absorbed along all parts of the intestinal tract.1,2,5,6 A diet high in sugar and vitamin C increases iron absorption, while a high-phosphate diet reduces iron absorption.1,2,4,5 But in acute overdoses, the iron seems to be absorbed in a passive, concentration-dependent fashion, similar to how most other metals are absorbed.