Too little iron harms your health, but too much may harm your gut. Learn why iron is such a crucial nutrient to get right.

Updated October 2026: This article is being revised to reflect current research. Some outdated sections have been removed or corrected in the meantime. Be sure to subscribe to be notified when the full update is published.

Iron-deficiency is a major global health issue, affecting more than two billion people worldwide.¹ Iron is a vital component of hemoglobin and myoglobin, the proteins that transport oxygen in the blood and muscle. It’s also involved in energy production and replication in cells. Iron deficiency has severe consequences, including anemia and impaired cognitive development and growth in children.²

But excess iron is also a problem. The body only requires a small amount of iron to function properly, and excess iron can build up and cause oxidative stress in organs. Iron overload can damage the liver, heart, and pancreas, and higher iron stores have been associated with conditions ranging from diabetes to neurodegenerative disease.³

Iron, iron, everywhere: widespread iron fortification

Iron fortification has become the norm in the United States since World War II, when rationing meant little access to iron-rich proteins. Fortification has helped reduce iron deficiency, though it hasn’t eliminated it: about one in three US women of reproductive age is still iron deficient. But because fortification is applied across the whole food supply, it also delivers large amounts of iron to people who don’t need it. Since 1950, iron intake has risen by about 50 percent in the U.S. due to food fortification.⁴

Here are some of the most commonly consumed fortified cereals, and the amount of iron they contain per cup, according to the USDA national nutrient database:⁵

Special K: 8.7 mg/cup

Cheerios: 9.0 mg/cup

Kix: 9.6 mg/cup

Fiber one: 16 mg/cup

Frosted mini wheats: 16 mg/cup

Raisin bran: 18 mg/cup

According to the Institute of Medicine, children, men, and postmenopausal women only need about 8 milligrams of iron per day, while premenopausal women need 18 milligrams per day.⁶ The tolerable upper intake level is 45 milligrams per day for adults and 40 milligrams per day for children under 14, which means a big bowl of Frosted Mini Wheats or Raisin Bran gets you close to the limit before you’ve eaten anything else. That limit was set based on digestive side effects from supplemental iron, not food. When iron regulation is working normally, the body absorbs less iron as body stores rise, which protects most people from clinical overload. But that protection breaks down in hereditary hemochromatosis (which is common and underdiagnosed), and is weakened by metabolic dysfunction and regular alcohol use. Men are also more prone to gradually accumulating iron, since they lack the regular iron losses of menstruation, and there’s ongoing research into whether iron stores at the high end of normal contribute to metabolic disease.

Iron fortificants are a type of iron called non-heme iron, which is less absorbable than the heme iron found in meat, poultry, and fish. Most foods fortified with iron, like cereal grains, are also rich in phytates, which further prevent iron absorption. Even low levels of phytate (about 5% of the amounts in cereal and grains) can reduce iron absorption by 50-65%.⁷

But the iron your body doesn’t absorb doesn’t simply disappear — it continues on to the colon, where it becomes available to gut microbes.

Iron and the gut microbiome

Unabsorbed iron wreaks havoc on the gut. Iron in the body is tightly bound, limiting its supply to pathogenic microbes. During infection, the immune system even tries to “lock up” iron inside of cells and reduce the amount of iron in the blood. But there is no similar system for “locking up” dietary iron in the gut. Iron is a growth-limiting nutrient for many gut pathogens, and an iron-rich environment increases their virulence and exaggerates infection.⁸ Most beneficial bacteria, on the other hand, require little iron.

Several studies have tested iron fortification and supplementation as a way to treat iron-deficiency anemia in children in Africa and Pakistan.^9–11 In these settings, where infectious disease is common, the added iron came with major consequences for gut health:

• Increased hospitalization for episodes of bloody diarrhea^9,11
• 5-fold increase in Enterobacteria¹⁰
• 5-fold decrease in beneficial Lactobacilli¹⁰
• Higher abundance of pathogenic E. coli¹¹
• Increase in gut inflammation^9,11

Excess iron has also been shown to accumulate in the intestinal mucosa and cause increased free radical damage in the large intestine.¹² This may increase intestinal permeability, potentially allowing pathogenic bacteria to leak into the bloodstream and cause systemic inflammation.¹³

While we certainly need to correct iron deficiency, these studies beg the question: Is widespread iron fortification creating an epidemic of gut dysbiosis, gut inflammation, and leaky gut?

Know your iron status

• Both too little and too much iron can be detrimental. It’s worth knowing where you stand before making changes to your diet or supplements.
• Ask your doctor for an iron panel (including ferritin and transferrin saturation) along with a CBC, especially if you menstruate, are pregnant, have a gut condition, or have a family history of hemochromatosis. Iron markers can be tricky to interpret, particularly when inflammation is present, so work with your clinician to make sense of your results and decide on treatment.
• If you have unexplained iron deficiency, ask about evaluation for gastrointestinal blood loss. Unexplained iron deficiency warrants an upper endoscopy and colonoscopy in men and postmenopausal women, who do not have regular menstrual loss as a potential cause.
• If you are deficient, don’t avoid iron. Treating iron deficiency matters, and there are ways to do it that are gentler on the gut. (I’ll be discussing these more in a future article update!)

That’s all for now! If you liked this article, be sure to subscribe so you never miss a post.

Sources:

1. WHO | Micronutrient deficiencies. WHO Available at: http://www.who.int/nutrition/topics/ida/en/. (Accessed: 17th January 2018)
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4. Nutrient Content of the US Food Supply | Center for Nutrition Policy and Promotion. Available at: https://www.cnpp.usda.gov/USfoodsupply. (Accessed: 17th January 2018)
5. Food Composition Databases Show Foods List. Available at: https://ndb.nal.usda.gov/ndb/search/list. (Accessed: 17th January 2018)
6. Office of Dietary Supplements – Nutrient Recommendations : Dietary Reference Intakes (DRI). Available at: https://ods.od.nih.gov/Health_Information/Dietary_Reference_Intakes.aspx. (Accessed: 17th January 2018)
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20. Paganini, D. et al. Consumption of galacto-oligosaccharides increases iron absorption from a micronutrient powder containing ferrous fumarate and sodium iron EDTA: a stable-isotope study in Kenyan infants. Am J Clin Nutr ajcn145060 (2017). doi:10.3945/ajcn.116.145060