Iron is an important mineral that helps maintain healthy blood. A lack of iron is called iron-deficiency anemia, which affects about 4-5 million Americans yearly.  It is the most common nutritional deficiency worldwide, causing extreme fatigue and lightheadedness. It affects all ages, with children, women who are pregnant or menstruating, and people receiving kidney dialysis among those at highest risk for this condition.

Iron is a major component of hemoglobin, a type of protein in red blood cells that carries oxygen from your lungs to all parts of the body. Without enough iron, there aren’t enough red blood cells to transport oxygen, which leads to fatigue. Iron is also part of myoglobin, a protein that carries and stores oxygen specifically in muscle tissues. Iron is important for healthy brain development and growth in children, and for the normal production and function of various cells and hormones.

Iron from food comes in two forms: heme and non-heme. Heme is found only in animal flesh like meat, poultry, and seafood. Non-heme iron is found in plant foods like whole grains, nuts, seeds, legumes, and leafy greens. Non-heme iron is also found in animal flesh (as animals consume plant foods with non-heme iron) and fortified foods.

Iron is stored in the body as ferritin (in the liver, spleen, muscle tissue, and bone marrow) and is delivered throughout the body by transferrin (a protein in blood that binds to iron). A doctor may sometimes check blood levels of these two components if anemia is suspected.

Recommended Amounts

RDA:  The Recommended Dietary Allowance (RDA) for adults 19-50 years is 8 mg daily for men, 18 mg for women, 27 mg for pregnancy, and 9 mg for lactation.  The higher amounts in women and pregnancy are due to blood loss through menstruation and because of the rapid growth of the fetus requiring extra blood circulation during pregnancy. Adolescents 14-18 years actively growing also need higher iron: 11 mg for boys, 15 mg for girls, 27 mg for pregnancy, and 10 mg for lactation. The RDA for women 51+ years drops to 8 mg with the assumption that cessation of menstruation has occurred with menopause. It may be noted that menopause occurs later for some women, so they should continue to follow the RDA for younger women until menopause is confirmed.

UL:  The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause harmful effects on health. The UL for iron is 45 mg daily for all males and females ages 14+ years.  For younger ages, the UL is 40 mg.

Food Sources

Meats, poultry, and seafood are richest in heme iron. Fortified grains, nuts, seeds, legumes, and vegetables contain non-heme iron. In the U.S. many breads, cereals, and infant formulas are fortified with iron.

Heme iron is better absorbed by the body than non-heme iron. Certain factors can improve or inhibit the absorption of non-heme iron. Vitamin C and heme iron taken at the same meal can improve the absorption of non-heme iron. Bran fiber, large amounts of calcium particularly from supplements, and plant substances like phytates and tannins can inhibit the absorption of non-heme iron.

Sources of heme iron:

• Oysters, clams, mussels
• Beef or chicken liver
• Organ meats
• Canned sardines
• Beef
• Poultry
• Canned light tuna

Sources of non-heme iron:

• Fortified breakfast cereals
• Beans
• Dark chocolate (at least 45%)
• Lentils
• Spinach
• Potato with skin
• Nuts, seeds
• Enriched rice or bread

What about iron supplements?

Iron is available in supplement form. Some cereals and multivitamin/mineral supplements are fortified with 100% of the RDA for women for iron (18 mg). Over-the-counter high-dosage iron supplements prescribed for those with iron-deficiency anemia or who are at high risk for it may contain 65 mg or more. Commonly reported side effects of using high-dosage iron supplements include constipation and nausea.

Confusion with iron supplements

There are several types of iron available as over-the-counter supplements, e.g., ferrous sulfate, ferrous fumarate, ferrous gluconate. Confusion is also caused by two number amounts listed on the label, a higher number and a lower number. What is the difference among supplement forms and which number should you refer to for the right amount to take?

Elemental versus chemical form of iron. If two iron amounts are listed on the label, the larger number is the chemical compound form because iron is bound to salts (e.g., ferrous sulfate), whereas the smaller number refers only to the amount of iron in the compound, also called the elemental iron. Elemental iron is the more important number because this is the amount available for the body to absorb. However, a physician may not specify in a prescription if the iron amount is the chemical form or the elemental iron. For example, a ferrous sulfate iron supplement may list a total of 325 mg of ferrous sulfate on the front of the label but 65 mg of elemental iron in smaller print on the back. If a physician prescribed 65 mg of iron, would you take five pills to equal 325 mg, or just one pill, assuming the prescription referred to elemental iron?

Different types. All types of supplemental iron help to increase red blood cell production but vary in cost and amounts of elemental iron. Ferrous gluconate is usually sold in liquid form and some clinical studies have shown that it is better absorbed than ferrous sulfate tablets. However, ferrous gluconate contains less elemental iron than ferrous sulfate, so a greater dosage may be needed to correct a deficiency. It is also more expensive than ferrous sulfate. Newer slow-release forms of iron have been introduced, which may help reduce gastrointestinal side effects, but they are more expensive and usually contain less iron.

Any confusion with iron supplement types and amounts can be resolved by asking your doctor to specify both the elemental amount and the chemical compound amount. You can also ask a store pharmacist for assistance in interpreting a doctor’s prescription or to recommend an appropriate amount if you do not have a prescription.

References
Le CH. The prevalence of anemia and moderate-severe anemia in the US population (NHANES 2003-2012). PLoS One. 2016 Nov 15;11(11):e0166635.
Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc : a Report of the Panel on Micronutrients. Washington, DC: National Academy Press; 2001.
National Institutes of Health Office of Dietary Supplements: Iron Fact Sheet for Health Professionals https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/. Accessed 9/2/2019.
Powers JM, Buchanan GR. Disorders of Iron Metabolism: New Diagnostic and Treatment Approaches to Iron Deficiency. Hematology/Oncology Clinics. 2019 Jun 1;33(3):393-408.

Last reviewed March 2023

source: nutritionsource.hsph.harvard.edu