Iron Supplements: Forms, Absorption, and What to Know
Iron is one of the most common nutrient deficiencies worldwide, affecting an estimated 1.6 billion people according to the World Health Organization. Iron supplements can correct a deficiency, but they come in several forms with major differences in absorption and side effects. Here is what you should know before buying one.
What Does Iron Do in Your Body?
Iron is an essential mineral that your body uses primarily to make hemoglobin, the protein in red blood cells that carries oxygen from your lungs to every tissue in your body. It also plays a role in myoglobin (oxygen storage in muscles), energy production, and immune function.
Your body cannot produce iron on its own. You get it from food in two forms: heme iron (from animal sources like red meat, poultry, and fish) and non-heme iron (from plant sources like spinach, lentils, and fortified cereals). Heme iron is absorbed 2-3 times more efficiently than non-heme iron.
Signs of Iron Deficiency
Iron deficiency develops in stages. Early depletion may show no symptoms at all. As stores drop further, you may notice:
- • Fatigue and weakness: The most common symptom. Less oxygen reaching your tissues means less energy production.
- • Pale skin and brittle nails: Reduced hemoglobin affects skin color. Nails may become spoon-shaped (koilonychia) in more advanced deficiency.
- • Shortness of breath: Your body compensates for low oxygen-carrying capacity by increasing breathing rate, especially during physical activity.
- • Cold hands and feet: Poor oxygen delivery affects circulation to the extremities.
- • Restless legs and unusual cravings: Some people with iron deficiency develop restless leg syndrome or crave non-food items like ice or dirt (a condition called pica).
These symptoms overlap with many other conditions. A blood test measuring serum ferritin, serum iron, TIBC (total iron-binding capacity), and transferrin saturation is the only reliable way to confirm iron deficiency.
Forms of Iron Supplements
Iron supplements vary widely in absorption rate, elemental iron content, and how well they are tolerated. The form you choose matters more than the total milligrams on the label.
Ferrous Sulfate
The most commonly prescribed and least expensive form. Contains about 20% elemental iron (a 325mg tablet provides roughly 65mg of elemental iron). Well-studied but known for causing constipation, nausea, and stomach cramps in many users.
Ferrous Bisglycinate (Iron Bisglycinate Chelate)
Iron bonded to the amino acid glycine. Research shows it is absorbed as well as or better than ferrous sulfate at lower doses, with significantly fewer GI side effects. Often preferred for people who cannot tolerate ferrous sulfate. More expensive per dose.
Ferrous Gluconate
Contains about 12% elemental iron. Generally better tolerated than ferrous sulfate due to the lower iron content per dose, though this also means you may need more tablets to reach the same elemental iron intake.
Ferrous Fumarate
Contains about 33% elemental iron, the highest percentage among common forms. Effective but can cause similar GI issues as ferrous sulfate due to the high elemental iron content.
Polysaccharide Iron Complex
Iron surrounded by a polysaccharide shell that dissolves in the small intestine. Marketed as gentler on the stomach. Some studies support fewer side effects, though absorption data is more limited compared to ferrous forms.
What Affects Iron Absorption
How much iron your body actually absorbs depends on several factors beyond the supplement form:
Absorption enhancers and blockers
Increases absorption
- Vitamin C: Taking iron with 100-200mg of vitamin C can increase absorption by 2-3 times. A glass of orange juice works.
- Empty stomach: Iron is best absorbed without food, though this increases side effects.
- Meat factor: A compound in animal protein that enhances non-heme iron absorption.
Decreases absorption
- Calcium: Competes with iron for absorption. Separate iron and calcium supplements by 2+ hours.
- Tannins: Found in tea and coffee. Wait at least 1 hour after taking iron before drinking either.
- Phytates: Found in whole grains, beans, and nuts. Can reduce iron absorption by 50-65%.
Who Needs Iron Supplements
Iron supplements are not for everyone. Unlike water-soluble vitamins, excess iron accumulates in the body and can cause harm. You should only supplement if blood work confirms a deficiency or your doctor recommends it. Groups at higher risk include:
- • Premenopausal women: Monthly blood loss through menstruation makes iron deficiency much more common in women of reproductive age. The RDA for this group is 18mg per day, more than double the 8mg recommended for men.
- • Pregnant women: Blood volume increases by about 50% during pregnancy, driving iron needs up to 27mg per day. Many prenatal vitamins include iron for this reason.
- • Vegetarians and vegans: Plant-based iron (non-heme) is less efficiently absorbed. The NIH suggests vegetarians may need 1.8 times more iron than meat eaters.
- • Endurance athletes: Intense training can increase iron losses through sweat, GI bleeding, and red blood cell breakdown (foot-strike hemolysis in runners).
- • People with chronic conditions: Celiac disease, inflammatory bowel disease, chronic kidney disease, and heart failure can all affect iron status through reduced absorption or increased loss.
Important: People with hemochromatosis (hereditary iron overload) should not take iron supplements. This genetic condition affects about 1 in 200 people of Northern European descent and causes the body to absorb too much iron from food alone.
What to Look For in an Iron Supplement
Elemental Iron Content
The label should list elemental iron, not just the total weight of the iron compound. A 325mg ferrous sulfate tablet only contains about 65mg of actual iron. This is the number that matters for dosing.
Tolerability
If you have a sensitive stomach, look for ferrous bisglycinate or polysaccharide iron complex. GI side effects are the number one reason people stop taking iron supplements.
Third-Party Testing
Look for products verified by USP, NSF, or ConsumerLab. Iron supplements have been flagged in past testing for containing less iron than claimed or having heavy metal contamination.
Added Vitamin C
Some iron supplements include vitamin C to boost absorption. This is a convenience but not required. You can achieve the same effect by taking iron with a vitamin C source.
Looking for a quality supplement?
See our expert-ranked supplement picks for 2026.
View Best Health Supplements →