Iron supports your energy levels mainly by helping your blood carry oxygen. It is a core part of hemoglobin, the protein in red blood cells that moves oxygen from the lungs to every tissue, and of myoglobin, which supplies oxygen to working muscles. When iron stores run low, less oxygen reaches your cells, and tiredness, weakness and trouble concentrating are among the first things people notice.
Key Takeaways
- Iron is part of hemoglobin and myoglobin, and cells use it inside mitochondria to build enzymes involved in turning food into usable energy.
- Adults need 8 mg of iron a day, except women aged 19 to 50, who need 18 mg; the need rises to 27 mg in pregnancy. People who eat no meat need about 1.8 times more.
- Heme iron from meat, seafood and poultry is absorbed better than nonheme iron from plants; vitamin C and meat improve nonheme iron absorption.
- In adults with low iron stores but normal hemoglobin, a review of 18 trials found iron lowered self-reported fatigue but did not improve measured physical capacity.
- Iron supplements commonly cause constipation, nausea and stomach upset. The upper limit for adults is 45 mg a day from food and supplements.
- Get a blood test, such as serum ferritin, before taking iron on your own, because extra iron is not helpful when your stores are already full.
Evidence grade: Strong. Iron's role in oxygen transport and its daily intake values are established NIH reference values, and a systematic review of randomized trials supports a modest effect on self-reported fatigue in adults with low iron stores.
How Iron Supports Energy
Oxygen transport
Most of the 3 to 4 grams of iron in an adult body sits in hemoglobin, which picks up oxygen in the lungs and releases it in the tissues. Iron is also part of myoglobin, which supplies oxygen inside muscle and supports muscle metabolism. Much of the rest is stored as ferritin in the liver, spleen and bone marrow.
Energy metabolism inside cells
Iron's role goes beyond the blood. Cells send iron to their mitochondria, the structures that produce most of the body's usable energy. There it is used to make heme and iron-sulfur clusters. These cofactors let enzymes carry out the oxidation-reduction reactions that turn food into usable energy. Iron is also needed for growth, neurological development and the making of some hormones.
Why low iron feels like low energy
Low iron develops in stages. First, stored iron (measured as ferritin) drops. Next, the supply to new red blood cells falls, though hemoglobin often still looks normal. Only in the last stage does hemoglobin drop, which is called iron-deficiency anemia. The NIH Office of Dietary Supplements lists weakness, fatigue, difficulty concentrating, gastrointestinal disturbances and reduced exercise or work performance among the effects of that last stage. MedlinePlus adds shortness of breath, headache, dizziness and irritability. These signs have many causes, so they are a reason to get checked, not to start iron on your own.
How Much Iron Do You Need?
The Food and Nutrition Board sets Recommended Dietary Allowances (RDAs) for iron. Women of childbearing age need more than men because they lose iron through menstrual blood. These amounts cover iron from food, fortified foods and supplements combined.
| Age | Male | Female | Pregnancy | Lactation |
|---|---|---|---|---|
| 7–12 months | 11 mg | 11 mg | ||
| 1–3 years | 7 mg | 7 mg | ||
| 4–8 years | 10 mg | 10 mg | ||
| 9–13 years | 8 mg | 8 mg | ||
| 14–18 years | 11 mg | 15 mg | 27 mg | 10 mg |
| 19–50 years | 8 mg | 18 mg | 27 mg | 9 mg |
| 51+ years | 8 mg | 8 mg |
People who follow vegetarian diets need about 1.8 times these amounts, because iron from plant foods is less available to the body. If you eat a plant-based diet, our guide to nutrients that can run short in vegan diets covers iron alongside vitamin B12, zinc and others.
Who Is More Likely to Run Low on Iron?
Most people in the United States get enough iron from food. The NIH Office of Dietary Supplements identifies groups who are more likely to fall short:
- Pregnant women, whose blood volume expands to support the fetus and placenta. See our overview of key nutrients during pregnancy.
- Infants and young children, especially those born early or small.
- Women with heavy menstrual bleeding, who lose much more iron each cycle.
- Frequent blood donors. About 25% to 35% of regular donors develop low iron.
- People with certain digestive conditions or digestive surgery. ODS names celiac disease, ulcerative colitis, Crohn's disease, gastrectomy and intestinal resection, which can limit absorption or cause blood loss in the digestive tract.
Iron and Your Digestive System
Iron and digestion are linked in three separate ways.
- The gut controls how much iron you absorb. Stomach acid helps you absorb nonheme iron from food. Conditions or surgery that affect the stomach or intestines can lower absorption or cause slow blood loss, and either can drain iron stores over time.
- Low iron itself can affect the gut. Gastrointestinal disturbances are among the effects ODS lists for the advanced stage of iron deficiency.
- Iron supplements often upset the stomach. High-dose iron can cause stomach upset, constipation, nausea, abdominal pain, vomiting and diarrhea. A 2015 meta-analysis of 43 trials with 6,831 adults found that ferrous sulfate, the most common form, more than doubled the odds of digestive side effects compared with placebo (odds ratio 2.32). The researchers found no clear link between the dose and the odds of side effects.
If you have ongoing digestive symptoms along with signs of low iron, a clinician can look for the cause.
Best Food Sources of Iron
Heme iron is found in meat, seafood and poultry; nonheme iron is found in plants, fortified grains and meat. In the United States, about half of dietary iron comes from bread, cereal and other grain products, because wheat flour is fortified. The amounts below come from the NIH Office of Dietary Supplements, based on USDA FoodData Central. The Daily Value on labels is 18 mg.
| Food | Serving | Iron (mg) | Type |
|---|---|---|---|
| Breakfast cereal, fortified with 100% of the DV | 1 serving | 18 | Nonheme |
| Oysters, eastern, cooked | 3 ounces | 8 | Heme and nonheme |
| White beans, canned | 1 cup | 8 | Nonheme |
| Beef liver, pan fried | 3 ounces | 5 | Heme and nonheme |
| Lentils, boiled | ½ cup | 3 | Nonheme |
| Spinach, boiled | ½ cup | 3 | Nonheme (low availability) |
| Chickpeas, boiled | ½ cup | 2 | Nonheme |
| Beef, braised bottom round | 3 ounces | 2 | Heme and nonheme |
| Chicken, roasted | 3 ounces | 1 | Heme and nonheme |
Spinach contains a fair amount of iron, but also polyphenols that limit how much you absorb. For meal ideas built around these foods, see our guides to breakfast foods for sustained energy and snacks that give you energy without the crash.
How to Absorb More Iron From Food
From a mixed diet that includes meat, seafood and vitamin C, people absorb about 14% to 18% of the iron. From vegetarian diets it is about 5% to 12%. A few habits can tip the balance:
- Add vitamin C. Citrus, strawberries, tomatoes and potatoes help you absorb nonheme iron.
- Combine plant iron with a little meat, fish or poultry. MedlinePlus notes this can improve absorption of iron from beans and greens up to three times.
- Time your tea. Black tea contains compounds that bind iron. Phytate in grains and beans, and polyphenols in some plant foods, also lower nonheme iron absorption.
- Space out calcium supplements. Calcium may reduce absorption of both heme and nonheme iron, so experts suggest taking calcium and iron supplements at different times of day.
ODS notes that in a typical Western diet these enhancers and blockers have only a small effect on most people's iron status.
Do Iron Supplements Improve Energy?
Iron supplements help when the body is short of iron. They are not a general energy booster. The research points the same way:
- A 2012 randomized trial in 198 menstruating women with unexplained fatigue, ferritin below 50 mcg/L and normal hemoglobin gave 80 mg of elemental iron a day or placebo for 12 weeks. Fatigue scores fell 47.7% with iron versus 28.8% with placebo; quality of life did not differ.
- A 2018 systematic review of 18 randomized trials in 1,170 adults with low iron but no anemia found iron reduced self-reported fatigue. It did not improve objective measures of physical capacity, such as maximal oxygen uptake.
These studies enrolled people who already had low iron stores. They do not show that extra iron raises energy in people whose iron is normal. Our guide to mitochondrial health covers some of the other factors in cellular energy.
Forms of iron in supplements
The Supplement Facts panel lists elemental iron, so you do not need to do the math yourself. Iron-only supplements often provide 65 mg of iron, well above the Daily Value. Multivitamins designed for women usually contain 18 mg, while men's and seniors' formulas often contain less or none.
| Form | Elemental iron by weight | Notes from NIH ODS |
|---|---|---|
| Ferrous fumarate | 33% | Ferrous salt; more soluble and better absorbed than ferric forms |
| Ferrous sulfate | 20% | The most studied form; linked to digestive side effects |
| Ferrous gluconate | 12% | Used at 37.5 mg/day in a blood-donor recovery trial |
| Iron amino acid chelates, carbonyl iron, heme iron polypeptides, polysaccharide-iron complexes | Varies | Might cause fewer digestive side effects than ferrous or ferric salts |
Who should test before supplementing?
Ideally, anyone considering an iron-only product. Serum ferritin is the most efficient and cost-effective test for low iron and can detect low stores before hemoglobin falls; a value below 30 mcg/L suggests low iron. Inflammation can raise ferritin, so clinicians often pair it with hemoglobin and other tests. Testing matters most for older adults (in one study of 1,106 adults aged 67 to 96, 13% had high iron stores), people with a family history of iron buildup (see Safety below), and anyone who is pregnant, gives blood often or has heavy periods, so a clinician can match the dose to their needs.
Safety and Interactions
- Upper limit. The Tolerable Upper Intake Level for iron is 45 mg a day for anyone 14 and older, and 40 mg for infants and children. It covers food and supplements together and was set based on digestive side effects. A clinician may prescribe more for a diagnosed deficiency.
- Digestive side effects. Doses of 45 mg a day or more can cause nausea, constipation, stomach upset, abdominal pain, vomiting and diarrhea. Taking iron with food can help reduce these effects.
- Zinc. Supplements with 25 mg of iron or more can lower zinc absorption.
- Iron buildup. People with hereditary hemochromatosis absorb too much iron from food. Liver specialists advise them to avoid iron and vitamin C supplements. Talk to your doctor before taking iron if this runs in your family.
- Children. Accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6, and FDA requires a warning on solid iron supplements. Store all iron products out of children's reach, and call a doctor or poison control center if a child swallows one.
- Medications. Talk to your doctor or pharmacist if you take levodopa, because iron can reduce how much of it the body absorbs. Talk to them too if you take levothyroxine: iron can reduce its absorption, and its labels advise taking it at least 4 hours apart from iron. If you take a proton pump inhibitor such as omeprazole or lansoprazole, ask too, because lower stomach acid can reduce iron absorption.
Frequently Asked Questions
How does iron affect energy levels?
Iron is part of hemoglobin, which carries oxygen in red blood cells, and myoglobin, which supplies oxygen to muscles. Cells also use iron to make enzymes that turn food into energy. When iron stores are low, less oxygen reaches the tissues, and fatigue and weakness are common results.
Can low iron cause digestive problems?
Gastrointestinal disturbances are among the effects the NIH lists for advanced iron deficiency. The link often runs the other way too: some digestive conditions reduce iron absorption or cause blood loss. Iron supplements themselves frequently cause constipation, nausea or stomach upset, so ongoing symptoms are worth discussing with a clinician.
Which form of iron is easiest on the stomach?
The NIH Office of Dietary Supplements says iron amino acid chelates, carbonyl iron, heme iron polypeptides and polysaccharide-iron complexes might cause fewer digestive side effects than ferrous or ferric salts. Taking iron with food can also help. Ferrous sulfate is the form most clearly linked to digestive side effects in trials.
Can iron supplements give you more energy?
In adults with low iron stores, trials show a modest drop in self-reported fatigue, but no improvement in measured physical capacity. There is no evidence that extra iron raises energy when your iron is already normal. A blood test tells you which group you are in.
How much iron is too much?
The upper limit for adults and teens 14 and older is 45 mg a day from food and supplements combined, unless a clinician prescribes more. Most food alone does not reach that level, but iron-only supplements often contain 65 mg per tablet.
Sources
- NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals.
- NIH Office of Dietary Supplements. Iron: Fact Sheet for Consumers.
- MedlinePlus Medical Encyclopedia. Iron in diet. National Library of Medicine.
- Paul BT, Manz DH, Torti FM, Torti SV. Mitochondria and iron: current questions. Expert Rev Hematol. 2017.
- Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. 2012.
- Houston BL, et al. Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials. BMJ Open. 2018.
- Tolkien Z, Stecher L, Mander AP, Pereira DI, Powell JJ. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. 2015.
- U.S. Department of Agriculture, Agricultural Research Service. FoodData Central.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
