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food source · 5
←Beef
Most of the iron in beef is heme iron, which is absorbed several times more efficiently than the non-heme iron in plants. Phytate and tea polyphenols in the same meal barely hold it back; only calcium may lower it somewhat.
Mechanism · Two kinds of iron, two doors
The iron in plants is an unframed ion that has to be reduced to ferrous iron before it can enter a gut cell, and on the way phytate and tea polyphenols can lock it up. The iron in beef is framed inside a molecule called a porphyrin ring; gut cells take it in ring and all, then open the ring inside the cell and add the iron to the same pool. So vitamin C helps the plant iron on the same plate get absorbed, not the iron in this meat.
Numbers · How much is absorbed, and when less is
Estimated across a whole diet, about 14–18% of iron is absorbed from a mixed diet with meat, seafood and vitamin C, and about 5–12% from a vegetarian diet (Hurrell and Egli 2010). Both figures were estimated for people whose iron stores are empty and are used to set dietary recommendations; they are not the absorption from any single meal. Efficient absorption does not mean more is better: the body has almost no active way to get rid of iron, and in someone whose iron is already sufficient, the gut absorbs less on its own.
←Lamb
Like beef, lamb is red meat, and most of its iron is heme iron, which is absorbed several times more efficiently than the iron in plants. That is lamb's most solid nutritional strength.
Mechanism · Heme iron enters ring and all
The iron in lamb, beef and blood is mainly heme iron: the iron is framed by a porphyrin ring, and the gut has a dedicated route that takes in the whole molecule, so the rest of the meal matters little. The non-heme iron in plants has to be reduced before it is absorbed and is easily locked up by tea polyphenols and the phytate in grains. Calcium is the exception: it may lower the absorption of both kinds somewhat.
Numbers · Efficiency is not a reason to eat more
Estimated across a whole diet, about 14–18% of iron is absorbed from a mixed diet with meat, seafood and vitamin C, and about 5–12% from a vegetarian diet (Hurrell and Egli 2010); both figures were estimated for people whose iron stores are empty. So from the same amount of iron, far more gets in from red meat than from vegetables such as spinach. That is a point about efficiency, not a case for more red meat: in someone whose iron is already sufficient, the gut absorbs less on its own.
Liver is very rich in iron. Part of it is heme iron, which is absorbed several times more efficiently than the iron in plants and is barely held back by tea drunk with the meal; the stored part sits in ferritin. Cooked pork liver has about 17.9 mg of iron per 100 g.
Mechanism · Iron guarded by the ring
The liver is one of the busiest organs at handling heme, so part of its iron is heme iron guarded by the porphyrin ring; the iron it keeps in store is mostly packed into ferritin. The polyphenols in tea cannot grab this kind of iron. The greens and beans on the same plate still bring unguarded non-heme iron, and that is the share vitamin C helps.
Numbers · How much iron a serving holds
Cooked pork liver has about 17.9 mg of iron per 100 g and raw liver about 23.3 mg, several times as much as red meat. For people short of iron it is one of the most efficient food sources; for people who already have enough, that density is a burden. Pregnancy is the exception, because liver is so high in vitamin A.
The iron in lentils is non-heme iron, so how much is absorbed depends on the meal: phytate and calcium hold it back, and vitamin C in the same meal clearly helps it get absorbed.
Mechanism · Vitamin C helps on the spot
Non-heme iron has to be reduced to ferrous iron before DMT1, the transporter on gut cells, will take it in. Vitamin C reduces ferric iron to ferrous iron and holds it in a soluble complex, so it does not turn into a compound that cannot be absorbed (Hallberg 1989). This has to happen in the gut during the same meal: a vitamin C tablet a few hours later does nothing for the iron in this bowl of soup.
Numbers · Single-meal swings even out
Cooked lentils have about 3.3 mg of iron per 100 g, near the top among plant foods, but it is non-heme iron, so how much is absorbed depends on the meal: vitamin C pulls it up, and tea, phytate and calcium push it down. Swings like these within one meal become much smaller across several days of a varied diet (Hurrell and Egli 2010). Soaking the lentils and pouring off the water removes part of the phytate.
The iron in black beans is non-heme iron that phytate holds back; a food rich in vitamin C in the same meal clearly helps it get absorbed, by the same mechanism as in lentils.
Mechanism · The same door as lentils
Like the iron in lentils, the iron in black beans is non-heme iron: phytate locks it up, and vitamin C in the same meal reduces it to ferrous iron in the gut and escorts it in. Tomato or green pepper in the soup helps the iron in this meal's beans; it does not build up a body that absorbs iron better later.
Numbers · The iron needs help from the sides
Cooked black beans have about 2.1 mg of iron per 100 g. The beans themselves have almost no vitamin C, so it has to come from the side dishes. Dry beans should be soaked well and cooked until completely soft; changing the water after soaking removes some of the gas-producing oligosaccharides and helps lower the phytate too.
synergy · 4
←IBD
The ESPEN guideline recommends oral iron first-line when inflammatory bowel disease is clinically inactive and the iron deficiency or anemia is mild; when the disease is active, oral iron was not tolerated before, or hemoglobin is below 100 g/L, intravenous iron is first-line. Forcing oral iron during a flare leaves unabsorbed iron in an inflamed gut.
Vitamin C helping non-heme iron into gut cells is one of the classic same-meal pairings in nutrition: eat fruit or vegetables with beans, leafy greens or fortified grains, and more of the iron is absorbed.
Low riboflavin (vitamin B2) can hamper the absorption of iron and its release from storage for use; in populations where riboflavin is commonly low, anemia may improve more slowly when only iron is given. The evidence for this link comes mainly from animal studies and a few human studies.
Iron and folate are both essential raw materials for red blood cells, but their deficiencies push in opposite directions: low iron makes red cells small, while low folate or vitamin B12 stalls DNA synthesis and makes them large. When both are low, the mean cell volume (MCV) can land back in the normal range and hide both problems.
cofactor · 8
Before iron can leave the gut and the body's stores and be loaded onto its transport protein in the blood, several copper-containing enzymes have to oxidize it. When copper runs short, iron can get stuck where it is and go unused, causing an anemia that looks like iron deficiency.
Vitamin C converts the ferric iron in plants into ferrous iron, which is easier to absorb, and holds onto the iron so it does not turn into a deposit the gut cannot take up. With vitamin C in the same meal, non-heme iron absorption rises markedly.
The key enzyme that builds iodine into thyroid hormone, thyroid peroxidase (TPO), itself contains iron (as heme). So even with enough iodine, iron deficiency can leave thyroid hormone production short. In pregnancy, iron deficiency can drag the thyroid down with it, and too little thyroid hormone in pregnancy may affect the baby's brain development.
Vitamin C in the same meal converts the ferric iron in plant foods into ferrous iron, which is easier to absorb, so fruit and vegetables do more than bring a little iron of their own: they set how much of the non-heme iron on the same table gets absorbed.
About two thirds of the body's iron sits in the heme at the center of hemoglobin, the atom that grabs oxygen in the lungs and lets go of it in the tissues. When iron runs short, the marrow builds red cells that are small and pale, and oxygen delivery falls. Stored iron (ferritin) runs out first; hemoglobin falls last.
Loading oxygen in the lungs and unloading it in the tissues depends on two properties of hemoglobin: once it binds one oxygen, the next binds more easily, and it releases oxygen more readily in tissues rich in carbon dioxide and slightly acidic (the Bohr effect). Iron sits at the center of hemoglobin, so with iron deficiency the lungs are fine; there are simply fewer carriers.
↔Zinc
The last step in making heme is an enzyme placing iron into a porphyrin ring. When iron is short, or lead blocks this enzyme, it inserts zinc instead, forming zinc protoporphyrin (ZPP), so a raised ZPP is a blood marker of both iron deficiency and lead exposure. Another enzyme early in heme synthesis, ALA dehydratase, depends on zinc, and lead can push the zinc off it.
Red meat supplies heme iron, which the gut absorbs with its porphyrin ring attached, several times more efficiently than non-heme iron and far less affected by tea or phytate, although calcium can lower both kinds. The upside is well-absorbed iron; the other side is that heme may promote oxidative reactions in the gut, which is thought to be one of the mechanisms behind the link between red meat and colorectal cancer.
antagonism · 4
Calcium lowers the absorption of iron eaten in the same meal, and both non-heme and heme iron are affected. People who need more iron can take calcium tablets or large servings of dairy at a different time from their iron supplement or iron-rich meal.
↮Zinc
Zinc and iron compete with each other for absorption in the gut. When large doses of zinc and iron supplements are taken together on an empty stomach, each is absorbed less; at the amounts in ordinary meals the effect is much smaller. People who need both can take them at different times.
Manganese and iron share the same gut transporter (DMT1). In iron deficiency the body makes more of this transporter, so more manganese is absorbed too; where drinking water is high in manganese, iron-deficient children may therefore take in too much manganese.
What blocks non-heme iron is the polyphenols in tea and coffee, not caffeine itself: black tea drunk with a meal cut that meal's non-heme iron absorption by 79–94% (Hurrell 1999, against water with the same bread meal). People who need more iron can simply move tea and coffee to between meals.
depletes · 2
Blood carries about 0.5 mg of iron per mL, and a period typically loses 30–40 mL, so 15–20 mg of iron leaves with it. That monthly exit is the main reason the iron recommendation for women of reproductive age is nearly double a man's (it is set near the heavy end of the range, not at the average). More than 80 mL counts as heavy menstrual bleeding, and the iron lost more than doubles.
Mechanism · Why the iron ledger is hard to balance
The debit side is simple: 15–20 mg of iron leaves with each period. The credit side is the hard one: iron is not absorbed in proportion to what you eat; it is held back by **absorption**. Plant-source non-heme iron is absorbed poorly and swings hard with the meal: black tea drunk with a meal cut that meal's non-heme iron absorption by 79–94% (Hurrell 1999, against water with the same bread meal); coffee suppresses it too, and calcium and phytate each take a share. Animal-source heme iron is absorbed several times more efficiently and far more steadily, though calcium can lower it as well. That is why the US Recommended Dietary Allowance is **18 mg/day** for women aged 19–50 against 8 mg for men: the near-doubling exists to cover that monthly exit. And that figure is set **near the heavy end of the range, not at the average**.
Clinical · Stored iron runs out first
The body does not wait for hemoglobin to fall before it feels this. The monthly exit draws first on **stored iron**, the part held in ferritin. Clinically, serum ferritin is the most sensitive measure of stored iron: in the absence of inflammation, a level below about 30 µg/L means the stores are empty (lab reports often write it as ng/mL; the number is the same). So someone with a perfectly normal blood count can already have run out of stored iron, and fatigue, hair shedding and falling endurance often arrive before anemia does. So checking iron cannot mean checking hemoglobin alone.
Red flag · Heavy bleeding needs its cause found
Losing more than **80 mL** meets the definition of heavy menstrual bleeding, and the iron lost more than doubles; at that level, diet cannot catch up. The practical trouble is that nobody can measure 80 mL at home. Useful signs instead: needing to change a pad or tampon every one to two hours, having to get up at night to change, passing large clots, or bleeding for longer than seven days. When these show up, the question to ask is **why is the bleeding this heavy**, not **how much iron should I take**; the second is like pouring water into a tap nobody has turned off. See a doctor to find the cause.
Blood volume expands by about 50% in pregnancy, and the fetus and placenta build their own blood on top of that, which pushes the US iron recommendation from 18 mg a day to 27 mg. That is not a matter of eating a bit more; the whole balance sheet has changed.
regulates · 7
Around 6 months, the iron a baby is born with runs out and breast milk is low in iron, so the first job of solid food is iron: heme iron from meat is well absorbed, and iron-fortified cereal counts too. Before age 1, cow's milk should not be the main milk, because it slows the absorption of non-heme iron.
Pregnancy raises blood volume and iron needs together; the US Recommended Dietary Allowance rises from 18 mg to 27 mg a day. How much iron the gut absorbs is regulated by the hormone hepcidin, and only a modest share of dietary iron is absorbed anyway, so the WHO antenatal care guideline recommends a daily iron supplement in pregnancy rather than relying on food alone.
How an iron supplement is fitted into the day changes how much is absorbed: Moretti 2015 found in young women with low iron stores that a single fasting morning dose of 60 mg or more raises hepcidin, the hormone that closes the gut door to iron, for about 24 hours, and absorption of the next-day dose falls by 35-45%. The authors concluded that alternate-day dosing may be better, though its long-term effects still need study.
Iron supplements do not gain from split dosing: dividing the same amount into two doses a day does not increase absorption and instead pushes up hepcidin, the hormone that suppresses iron absorption. In a small study by Stoffel 2017 in iron-depleted women, taking iron once every other day gave a higher absorbed fraction than taking it every day; so iron is generally taken at most once a day, and whether to switch to alternate days is a question for your doctor.
Iron deficiency is one of the common triggers of telogen effluvium, in which many hairs enter the resting phase together and shed a few months later. This kind of shedding is usually reversible once the trigger is removed, so with diffuse shedding, having your ferritin checked is more useful than buying an anti-hair-loss shampoo.
Picky eaters eat little meat, so iron and zinc are easy to run short of: in the UK ALSPAC cohort, persistently picky children ate about 40% less carcass meat and had lower iron and zinc intakes. Toddlers grow fast and need plenty of iron, and after age 1 cow's milk should stay under 500 mL a day. Only a blood test shows whether a child is short of iron; whether to supplement is for a pediatrician to decide.
←After birth and breastfeeding
Milk iron does not change with the mother's intake, so the iron question after birth is the mother's own: blood lost at delivery drains her stores, while in the first 8 weeks of exclusive breastfeeding periods rarely return, saving the iron that bleeding would take. Where anemia in pregnancy is common, the World Health Organization says iron may be given for 6 to 12 weeks after birth, a recommendation based on low-certainty evidence.