Iron deficiency is one of the most common types of nutrient deficiencies around the world. It causes anemia and can lead to weakness and fatigue, so if you're diagnosed with an iron deficiency, your doctor will recommend you take an iron supplement. Don't take one without speaking with your doctor, as high amounts can be toxic. Some iron supplements are easier to absorb or more readily tolerated without causing side effects than others -- your doctor can make an informed choice on which type might be best for you.
Type of Iron
The most common type of iron supplement is ferrous sulfate, but some supplements contain ferrous lactate, ferrous gluconate, ferrous glutamate, ferrous glycine, ferrous succinate, ferrous fumarate, ferric ammonium citrate, ferric sulphate and ferric citrate. These are all sometimes called iron salts. Another option available in the United States is carbonyl iron, which contains just a form of iron instead of iron combined with another substance to form an iron salt. Less commonly used forms of iron supplements include heme iron polypeptides, which are capsules containing hemoglobin from pig blood cells; iron amino-acid chelates, which are sometimes called chelated iron; and polysaccharide-iron complexes, which combine ferric iron with starches.
Differences in Absorption
Iron supplements in general are better absorbed when taken with vitamin C and less absorbed when taken with calcium, zinc, manganese, copper, soy and certain medications. You will only absorb half as much iron if you take your iron with instead of between meals, according to an August 2008 article in Pharmacist's Letter.
The ferrous forms are about three to four times better absorbed than the ferric forms, according to a review article published in Scientific World Journal in 2012, and even then only about 10 to 15 percent of the iron in supplements is absorbed.
Carbonyl iron is very slowly absorbed over the course of about one to two days and is less toxic than the other forms of iron, according to the Pharmacist's Letter article.
Heme-iron polypeptides is more bioavailable than iron salts, while polysaccharide-iron complexes have about the same bioavailability as ferrous iron salts.
Potential for Side Effects
The main side effects associated with iron supplements are an upset stomach, nausea, heartburn, diarrhea and constipation. These supplements can also darken your stool. Starting with just half of the recommended amount and working your way up to the full amount may help make an upset stomach less likely, and taking a stool softener can help you deal with any constipation you experience.
Taking extended-release forms of supplements or coated versions may help limit side effects, but these types of supplements aren't as well-absorbed. Polysaccharide-iron complexes, heme iron polypeptides, carbonyl iron and chelated iron may all have fewer side effects than the typically prescribed ferrous or ferric salts, according to the Office of Dietary Supplements.
Chelated Iron Considerations
Chelated forms of iron supplements, such as ferrous bisglycinate, consist of ferrous iron combined with an amino acid. The theory behind increased bioavailability with these supplements is that these compounds may be less likely to bind with inhibitory substances found in food, such as oxalates and phytates, so more iron is available for absorption. Chelated iron supplements may also be absorbed the same way as amino acids, rather than the way minerals are normally absorbed, which helps limit the risk of other minerals competing with the iron for absorption.