Iron – Strong Blood, Strong Body

Iron is an essential trace mineral and the most abundant transition metal in the human body. Its primary role is oxygen transport: iron sits at the center of hemoglobin in red blood cells, binding oxygen in the lungs and releasing it to tissues, and at the center of myoglobin in muscle fibers, where it holds oxygen for use during contraction. Beyond oxygen transport, iron is needed for the mitochondrial electron transport chain, DNA synthesis, and neurotransmitter production. Iron deficiency is the most common nutritional deficiency worldwide, affecting well over a billion people — and it’s also frequently misunderstood, because the body’s regulation of iron is complex, and several distinct conditions can look alike on standard blood tests.

Heme vs Non-Heme Iron

This is the most important iron distinction for anyone managing intake through diet or supplements, and it’s missing from most basic mineral guides.

Heme iron comes from animal foods — meat, poultry, fish, and seafood — and is the form found in hemoglobin and myoglobin. It’s absorbed through its own pathway at about 15–35% efficiency, and that rate is much less affected by other foods in the same meal. (Meat and fish also contain some non-heme iron.)

Non-heme iron comes from plant foods — legumes, grains, vegetables, nuts, and seeds — as well as eggs, dairy, and iron-fortified foods. It’s absorbed at only about 2–20%, and that rate is highly sensitive to the rest of the meal. Vitamin C, meat and fish eaten in the same meal, and certain organic acids all enhance absorption. Phytates (in grains and legumes), polyphenols (in tea, coffee, and red wine), calcium, and oxalates all reduce it.

The practical consequence: vegetarians and vegans absorb considerably less iron from their food, and the US recommendations advise them to get about 1.8 times as much dietary iron as people who eat meat. Plant-heavy diets can contain plenty of iron on paper while delivering much less to the body.

How the Body Regulates Iron

The body has no active way to excrete iron — it loses iron only through bleeding, menstruation, and small daily losses from shed gut and skin cells and sweat. So iron balance is controlled almost entirely by adjusting how much is absorbed.

The master regulator is hepcidin, a hormone made by the liver. When iron stores are adequate, hepcidin rises and blocks both iron absorption from the gut and iron release from storage. When stores are low, hepcidin falls and absorption opens up. But hepcidin also rises with inflammation and infection — which is why people with chronic inflammatory conditions often develop anemia even when their diet is good, and why oral iron works poorly during illness. The body is actively resisting iron uptake.

This matters for athletes too: exercise triggers a temporary rise in hepcidin that peaks around 3–6 hours after training, reducing iron absorption during that window. The timing of iron supplements around training therefore matters (see below).

RDA (Recommended Dietary Allowance)

* Adequate Intake (AI) for infants. Source: US National Academies, via the NIH Office of Dietary Supplements. The RDAs for vegetarians are 1.8 times higher. The daily value used on food labels is 18mg.

Note the sharp drop in women’s needs after menopause — from 18mg to 8mg — as menstrual losses stop, and the large increase during pregnancy to 27mg a day. Older babies (7–12 months) also need a lot of iron for their size, because their birth stores run out and breast milk contains little — so iron-rich complementary foods are important from around 6 months.

ODA (Optimal Daily Allowance)

Meet the RDA for your sex and life stage — mainly from food — and supplement only if a blood test shows you need it. Iron is one of the few nutrients where “more” is genuinely risky, because the body can’t excrete the excess. Men and post-menopausal women need 8mg a day and usually get more than that from food, so they generally shouldn’t take iron supplements unless a doctor finds a deficiency. Women of reproductive age (18mg), pregnant women (27mg), vegetarians and vegans (about 1.8 times the RDA), and endurance athletes — especially female endurance athletes — are the groups most likely to need extra attention, ideally guided by ferritin tests.

Found In (per 100g)

Iron content alone doesn’t tell the full story — how much is absorbed matters as much as how much a food contains. The heme iron in meat, liver, and shellfish is absorbed far better than the non-heme iron in plants. Spinach is a good example: it looks iron-rich, especially cooked, but its oxalates and polyphenols mean very little of its iron is absorbed, and raw spinach contains much less iron per 100g than cooked. Beef liver is another excellent source (about 5mg per 85g serving, according to the NIH), and many breakfast cereals are fortified with up to 100% of the daily value per serving. Eating plant sources of iron with vitamin C-rich foods — such as peppers, citrus, or tomatoes — and avoiding tea or coffee with meals helps you absorb more.

Helps With

• Transporting oxygen in the blood via hemoglobin in red blood cells
• Storing oxygen in muscle via myoglobin, for use during exercise
• Aerobic energy production — iron is part of all four complexes of the mitochondrial electron transport chain
• DNA synthesis and cell division
• Immune function — both iron deficiency and iron excess impair immunity
• Neurotransmitter production — iron is needed by the enzymes that make dopamine, serotonin, and noradrenaline
• Brain development and cognitive function, particularly in infants and children
• Thyroid hormone production — the enzyme that builds thyroid hormones needs iron

For Athletes and Active People

Iron is arguably the trace mineral with the most direct impact on athletic performance, through its central role in oxygen delivery and aerobic energy production.

The ferritin threshold: ferritin reflects your iron stores. Standard lab reference ranges (often starting around 12–15 ng/mL) were designed to identify clinical deficiency, not to support performance, and many doctors now treat ferritin below about 30 ng/mL as iron deficiency in adults. Many athletes report fatigue, reduced endurance, and poor recovery with ferritin in the 15–30 ng/mL range, and sports medicine practitioners commonly aim for ferritin above about 35–50 ng/mL in competitive athletes. In iron-deficient athletes without anemia, iron supplementation has improved endurance in some studies. This is worth knowing if your results came back “normal” but performance feels compromised — though ferritin also rises with inflammation, so it should be interpreted alongside other markers.

Foot-strike haemolysis: in runners, the repeated impact of each footstrike destroys some red blood cells as they pass through the capillaries of the feet — more so on hard surfaces and with high mileage. Red blood cells are also broken down in other sports through muscle contraction and other mechanisms. These losses are usually modest, but they add to the other iron losses athletes face.

Sports anemia vs true iron deficiency: endurance training increases plasma volume — the liquid part of the blood expands, which benefits heart output and heat loss. But this dilutes red blood cells, so hemoglobin can look lower on blood tests even though total red blood cell mass is normal or even increased. This “dilutional pseudoanemia” or sports anemia needs no treatment. It’s important to distinguish it from true iron deficiency (low ferritin, low transferrin saturation), which does need treatment.

Hepcidin and training timing: hepcidin rises after exercise and peaks around 3–6 hours later, suppressing iron absorption. Research from the Australian Institute of Sport suggests the best time to take iron is in the morning — either before training or within about 30 minutes after a morning session, before hepcidin has risen — rather than in the afternoon or evening, when hepcidin levels are naturally higher. Training twice a day makes timing harder, which is one reason endurance athletes are at higher risk of iron depletion.

Female endurance athletes face several risk factors at once: menstrual losses, high training volumes, and often low energy intake (intentional or not), on top of the effects of hepcidin. Iron deficiency is one of the most common nutritional deficiencies in this group, and one of the most frequently missed causes of underperformance.

Insufficient Doses

Deficiency risk is higher in:

• Women of reproductive age — menstrual blood loss, especially heavy periods, is the most common cause of iron deficiency
• Pregnant women — iron needs rise sharply, especially later in pregnancy
• Babies and toddlers — especially after 6 months, when birth stores run low and breast milk alone doesn’t provide enough
• Vegetarians and vegans — because non-heme iron is absorbed less well
• Endurance athletes, especially women — through foot-strike haemolysis, sweat losses, hepcidin effects, and small amounts of gut bleeding during prolonged exercise
• Frequent blood donors
• People with chronic gut blood loss — from ulcers, regular aspirin or NSAID use, polyps, bowel cancer, or inflammatory bowel disease
• People with malabsorption — from coeliac disease, Crohn’s disease, Helicobacter pylori infection, long-term acid-reducing medicines, or weight-loss surgery
• People with chronic inflammatory conditions — “anemia of inflammation” can mimic iron deficiency anemia but has a different cause and treatment

An important warning: in men and post-menopausal women, iron deficiency without an obvious cause should always be investigated by a doctor, because it can be the first sign of hidden bleeding in the gut — including from bowel cancer. Don’t simply self-treat with supplements.

Symptoms of deficiency (from mild to severe):

• Fatigue and reduced stamina — the earliest and most consistent symptom
• Reduced exercise tolerance and VO₂max
• Pale skin, pale inner eyelids, and pale nail beds
• Shortness of breath on exertion
• Rapid or irregular heartbeat
• Headaches and dizziness
• Cold hands and feet
• Brittle or spoon-shaped nails, and hair thinning
• Difficulty concentrating and impaired cognitive function
• Restless legs syndrome — strongly linked with low iron
• Pica — cravings for non-food substances, most commonly ice (pagophagia), clay, or starch; unusual, but highly characteristic of iron deficiency

Iron deficiency develops in stages: first iron stores (ferritin) fall, then transport iron (transferrin saturation) drops, and finally hemoglobin falls, causing anemia. Symptoms often appear at the ferritin stage — before standard anemia tests show anything abnormal.

Excessive Doses

Upper limit (UL): 45mg a day for adults and teenagers aged 14 and over, and 40mg a day for children under 14, from food and supplements combined. The UL is based mainly on digestive side effects — doctors often prescribe higher doses to treat deficiency, under supervision.

Hereditary haemochromatosis: one of the most common genetic conditions in people of Northern European descent — about 1 in 200 to 300 inherit two copies of the main HFE gene variant. It causes iron to build up in the liver, heart, pancreas, joints, and pituitary gland. Not everyone with the gene variant develops problems — men are affected more often and earlier, while women are partly protected by menstrual losses — but in those who do, it’s often undiagnosed until organ damage occurs. Symptoms include fatigue, joint pain, abdominal pain, an enlarged liver, diabetes, heart rhythm problems, and bronze skin discoloration. Treatment is straightforward: regular removal of blood (therapeutic phlebotomy). People with haemochromatosis should avoid iron supplements and vitamin C supplements, which increase iron absorption. This is a key reason why people who aren’t deficient shouldn’t take iron “just in case.”

Acute iron poisoning: a medical emergency, most often when young children swallow adult iron tablets — historically a leading cause of fatal poisoning in young children. Symptoms can begin at around 20mg of elemental iron per kg of body weight, and doses above about 60mg/kg can cause serious, potentially fatal poisoning. Keep iron supplements locked away from children, and seek emergency help immediately if a child swallows them.

Side effects of supplemental iron in adults:

• Digestive upset — nausea, stomach pain, constipation, and dark or black stools (normal with iron supplements)
• Increased oxidative stress — free iron drives the Fenton reaction, which produces highly reactive hydroxyl radicals
• Reduced zinc and copper absorption at high doses
• Possible links with colorectal cancer — high intakes of heme iron from red and processed meat are associated with higher colorectal cancer risk; evidence for iron supplements is mixed

Different Types

• Ferrous Bisglycinate (Glycinate) — iron bound to the amino acid glycine; well absorbed (less affected by dietary inhibitors in some studies) and causes noticeably fewer digestive side effects. A good choice for many people, though more expensive than ferrous sulfate.
• Ferrous Sulfate — the most common and cheapest form, about 20% elemental iron. Effective, but often causes nausea, constipation, and stomach cramps, especially on an empty stomach. The standard medically prescribed form.
• Ferrous Fumarate — about 33% elemental iron, so more iron per tablet than sulfate. Similar digestive side effects.
• Ferrous Gluconate — about 12% elemental iron; somewhat gentler than sulfate, with less iron per tablet.
• Carbonyl Iron — very fine particles of metallic iron that dissolve slowly in the stomach; gentler on the gut and less toxic in accidental overdose.
• Heme Iron Polypeptide (HIP) — made from animal hemoglobin and absorbed through the heme pathway, so it’s less affected by dietary inhibitors and causes fewer digestive side effects. Each tablet contains relatively little iron, and it isn’t suitable for vegetarians or vegans.
• Liquid Iron (such as Floradix or Spatone) — usually provides less iron per dose, in forms that tend to be gentle on the stomach. Popular in pregnancy and with people who struggle with tablets.
• Intravenous (IV) Iron — given in hospital or clinic for people who can’t tolerate or absorb oral iron, have severe deficiency or ongoing blood loss, or have conditions such as inflammatory bowel disease or chronic kidney disease. Restores iron stores much faster than tablets.

Which form for your goal:

How to Take It

Take iron on an empty stomach if you can — absorption is noticeably higher than with food. If it causes nausea or stomach upset, taking it with a small amount of food (not dairy, tea, or coffee) reduces side effects without dramatically reducing absorption. Ferrous bisglycinate is usually better tolerated on an empty stomach than ferrous sulfate.

Vitamin C: vitamin C clearly helps you absorb non-heme iron from meals, so eating vitamin C-rich foods with plant sources of iron is worthwhile. With iron supplements, the benefit is less clear — in a 2020 trial in adults with iron deficiency anemia, adding vitamin C to iron tablets didn’t speed recovery compared with iron alone. Taking iron with orange juice or a vitamin C tablet is reasonable but optional.

Keep apart from: take iron at least 2 hours apart from tea, coffee, dairy, calcium supplements, antacids, and zinc or magnesium supplements, which reduce absorption. Long-term acid-reducing medicines (proton pump inhibitors and H₂ blockers) lower stomach acid all day, so timing doesn’t avoid their effect — if you take them and need iron, discuss it with your doctor.

Alternate-day dosing: studies show that each iron dose raises hepcidin for about a day, reducing absorption from the next day’s dose. Taking iron every other day, as a single morning dose, improves the proportion absorbed and causes fewer side effects, and may be as effective as daily dosing for rebuilding stores. It’s worth trying, especially if daily iron upsets your stomach — ask your doctor which schedule suits you.

For athletes: take iron in the morning, either before training or within about 30 minutes after a morning session, before the post-exercise rise in hepcidin.

Dark or black stools are normal while taking iron supplements. Recheck ferritin after about 8–12 weeks to see how you’re responding, and don’t continue high-dose iron indefinitely without monitoring.

Interactions

Vitamin C enhances the absorption of non-heme iron from food. Proton pump inhibitors and H₂ blockers reduce stomach acid, which impairs iron absorption. Iron binds tetracycline and quinolone antibiotics (such as ciprofloxacin), reducing the absorption of both — take the antibiotic at least 2 hours before or 4–6 hours after iron. Iron also reduces the absorption of levothyroxine (thyroid medicine) — take them at least 4 hours apart — and of levodopa, used for Parkinson’s disease.

Synergy

• Works with copper — copper-dependent enzymes (ceruloplasmin and hephaestin) are needed to release iron from storage and from gut cells and load it onto transferrin for transport.
• Works with vitamin C to enhance non-heme iron absorption from food.
• Works with vitamin B12 and folate in red blood cell production — iron deficiency causes small, pale red blood cells, while B12 or folate deficiency causes large ones, and deficiencies can occur together.
• Works with vitamin A, which helps mobilize stored iron for making red blood cells.

Antagonistic Interactions

• Calcium reduces iron absorption from the same meal — keep calcium supplements apart from iron, though the long-term effect of dietary calcium on iron status appears small.
• Zinc and iron supplements reduce each other’s absorption when taken together, especially without food — take them separately.
• Phytates in grains and legumes bind non-heme iron and reduce absorption from those meals.
• Polyphenols in tea, coffee, and red wine reduce non-heme iron absorption — avoid drinking them with iron-rich meals or supplements.
• Hepcidin, raised by inflammation, infection, or recent exercise, blocks iron absorption and release from stores regardless of intake.

Food values: USDA FoodData Central — dark chocolate, 70–85% cacao (SR Legacy, FDC ID 170273); eastern oysters, wild, cooked, moist heat (SR Legacy, 171980); pumpkin seeds (pepitas), raw (Foundation Foods, 2515380); lamb liver, braised (SR Legacy, 172532); blue mussels, cooked (SR Legacy, 174217); cashews, raw (Foundation Foods, 2515374); oats, dry (SR Legacy, 173904); white bread, commercial (Foundation Foods, 2758993); spinach, boiled (SR Legacy, 168463); lentils, cooked (SR Legacy, 172421); sardines, canned in oil (SR Legacy, 175139); chickpeas, cooked (SR Legacy, 173757); tofu, firm (SR Legacy, 172475); ground beef, 80% lean, cooked (SR Legacy, 171797); egg, hard-boiled (SR Legacy, 173424); chicken breast, braised (Foundation Foods, 331960). Beef liver figure from the NIH Office of Dietary Supplements. RDA and upper limits from the US National Academies, via the NIH Office of Dietary Supplements. % Daily Values use the FDA’s current Daily Value of 18mg.