On The Shelf
Mary Warner
Iron, a mineral that is naturally present in many foods, added to some food products, and available as a dietary supplement, is an essential component of hemoglobin and myoglobin, two proteins that supply oxygen for metabolism. Iron also supports muscle metabolism and is necessary for physical growth, neurological development, cellular functioning, and hormone synthesis.
Function
Iron plays an important role in both oxygen and electron transport. It is either functional, as hemoglobin in red blood cells, myoglobin in muscle cells, in tissue enzymes, or bound to transferrin; or stored in the liver, spleen, intestinal muscosa, and bone marrow as ferritin or hemosiderin.
Daily requirements and dietary sources
The RDA for iron varies by age, sex, and diet (see table for RDAs for nonvegetarians). The RDAs for vegetarians are 1.8 times higher than for people who eat meat because heme iron from meat is more bioavailable than nonheme iron from plant-based foods, and meat, poultry, and seafood increase the absorption of nonheme iron.
Dietary iron has two main forms: heme and nonheme. Plants and iron-fortified foods contain nonheme iron only, whereas meat, seafood, and poultry contain both heme and nonheme iron. Heme iron, which is formed when iron combines with protoporphyrin IX, contributes about 10% to 15% of total iron intake in western countries.
According to the National Institutes of Health, the bioavailability of iron is approximately 14% to 18% from mixed diets that include substantial amounts of meat, seafood, and vitamin C (which enhances the bioavailability of nonheme iron) and 5% to 12% from vegetarian diets.
In addition to ascorbic acid, meat, poultry, and seafood can enhance nonheme iron absorption, whereas phytate (present in grains and beans) and certain polyphenols in some nonanimal foods (such as cereals and legumes) can decrease it.
Iron supplements
Pregnant women, infants and young children, women with heavy menstrual bleeding, frequent blood donors, and people with cancer, GI disorders, or heart failure are all at increased risk for iron deficiency and are often advised to take iron supplements to ensure adequate iron bioavailability. Multivitamin and mineral supplements with iron, especially those designed for women, typically provide 18 mg of iron. Multivitamin and mineral supplements for men and older adults frequently contain less or no iron, while iron-only supplements often provide 65 mg of iron.
Iron is most commonly included in supplements as ferrous or ferric iron salts, including ferrous sulfate, ferrous gluconate, ferric citrate, and ferric sulfate. Because of its higher solubility, ferrous iron in dietary supplements is more bioavailable than ferric iron. Supplements containing iron polypeptides, carbonyl iron, iron amino-acid chelates, or polysaccharide-iron complexes may have fewer GI adverse effects than ferrous or ferric salts.
The different forms of iron in supplements contain varying amounts of elemental iron. For example, ferrous fumarate is 33% elemental iron by weight, whereas ferrous sulfate is 20% and ferrous gluconate is 12% elemental iron. The Supplemental Facts panel, however, provides the amount of elemental iron and should always be used to determine dosage.
What to tell your patients
Ensure patients understand that iron can be toxic and that high doses of supplemental iron (more than 40 mg/d) may cause GI adverse effects such as nausea and constipation. Symptoms of acute iron poisoning (ingestion of more than 50 mg/kg of elemental iron) include abdominal pain, vomiting, diarrhea, electrolyte imbalances, and shock.
Because some medications, including levodopa, levothyroxine, and proton pump inhibitors, can have an adverse effect on iron levels by affecting bioavailability, pharmacists should discuss iron status with patients prescribed these medications.
Calcium can also interfere with iron absorption, so patients may wish to take calcium and iron supplements separately at different times of the day. Food may decrease the amount of iron absorbed by as much as 50%, so iron should be taken on an empty stomach and only with food if GI adverse effects occur.