Iron as a Nutrient: Oral vs IV Replacement

Why iron matters at all

Iron is not a bit-player nutrient; it is the workhorse of oxygen chemistry. Most of the body's iron sits inside haemoglobin, where a single iron atom at the heart of each haem group is what actually binds and releases oxygen — four per haemoglobin molecule, billions per red cell. More iron works in myoglobin, the oxygen store of muscle, and in a long list of enzymes: the cytochromes of the mitochondrial electron-transport chain that make ATP, catalase, and ribonucleotide reductase, which builds the DNA of every dividing cell. Iron is useful precisely because it flips easily between its ferrous (Fe2+) and ferric (Fe3+) states, shuttling electrons — and that same reactivity is why free iron is dangerous and the body chaperones every atom on carrier proteins. Run low and the first casualty is oxygen delivery: iron-deficiency anaemia, the single commonest nutritional deficiency in the world, hitting menstruating women, growing children, and pregnancy hardest. The full anaemia work-up — ferritin, transferrin saturation, the microcytic film — belongs to the Hematology section; here the question is narrower: how do you put the iron back?

The gut is the gatekeeper: haem vs non-haem

Dietary iron comes in two forms that the gut treats very differently. Haem iron — the iron already tucked inside haemoglobin and myoglobin in red meat, poultry and fish — is absorbed whole and efficiently, largely indifferent to what else is in the meal. Non-haem iron — the ionic iron in plants, pulses, fortified cereals and every iron tablet — is far more fussy. It must be in the reduced ferrous (Fe2+) state to cross the enterocyte, and its uptake is throttled up or down by the company it keeps. Vitamin C (ascorbic acid) reduces ferric to ferrous and boosts absorption sharply — the reason a citrus drink with the tablet is genuine advice, cross-linked to the Vitamin C chapter. A stomach acidic with gastric acid does the same. Working against you: phytates in wholegrains and legumes, tannins and polyphenols in tea and coffee, calcium (including a dairy meal or a calcium supplement), and — importantly for prescribers — anything that raises gastric pH, so long-term antacids and proton-pump inhibitors blunt iron uptake, a link back to the Gastrointestinal section where PPIs are covered in full.

Hepcidin: the master switch you can't out-swallow

The body has one hormone that decides whether iron gets in at all — and inflammation can slam it shut. However much iron you swallow, a liver hormone called hepcidin holds the final veto. Hepcidin acts on ferroportin, the single doorway that lets iron leave the enterocyte and the macrophage to enter the blood; when hepcidin rises, it degrades ferroportin and the door shuts, so dietary iron is taken up by gut cells and then simply shed as they slough off. Iron overload raises hepcidin (a sensible brake); true iron deficiency lowers it, flinging the door open. The clinical trap is inflammation: infection, chronic disease, cancer and inflammatory states drive hepcidin up regardless of iron stores, locking iron away inside macrophages. This is the anaemia of chronic disease — and it is why oral iron often fails there: the tablets cannot force a door that hepcidin is holding shut. Recognising this before reaching for iron matters, and the fuller hepcidin and ferritin story sits in the Hematology section. Elegantly, hepcidin also explains a dosing trick: a single iron dose itself nudges hepcidin up for roughly a day, briefly reducing the absorption of the next dose.

Oral iron: the ferrous salts and their price

Oral iron is, and should be, the first-line replacement for uncomplicated iron deficiency: cheap, effective, and safe. The workhorses are the ferrous salts — ferrous sulfate, ferrous fumarate and ferrous gluconate — all in the absorbable Fe2+ form. They differ mainly in how much elemental iron each carries per tablet (roughly: sulfate around 65 mg, fumarate around 65 mg, gluconate around 35 mg per standard tablet), so the salts are not interchangeable milligram-for-milligram — what counts is the elemental iron dose, not the weight of the salt. The classic problem is not efficacy but tolerability. Unabsorbed iron irritates the gut directly, and the toll is predictable: nausea, epigastric pain, constipation (sometimes diarrhoea), and harmless but alarming black stools. These effects are dose-related, and they are the main reason patients quietly abandon treatment — adherence, not potency, is where oral iron usually fails.

The modern fix flows straight from hepcidin physiology. For years the reflex was to push more — three tablets a day. The evidence now points the other way. Because each dose spikes hepcidin for about 24 hours, closing the door on the next dose, giving iron on alternate days (and as a single daily dose rather than divided) actually raises the fraction absorbed and, by more than halving the pill burden on the gut, improves tolerability too — often delivering as much or more absorbed iron with far fewer side effects. Practical prescribing follows: use a modest elemental-iron dose, take it once daily or every other day, on an empty stomach if the patient can tolerate it (or with vitamin C to boost uptake), and separate it from tea, dairy, calcium and antacids. Set expectations, too: the reticulocyte bump comes within a week and haemoglobin climbs over weeks, but you must keep treating for perhaps three months after the haemoglobin normalises to refill the body's iron stores — stopping the moment the blood count looks better simply invites relapse.

IV iron: bypassing the gut

When the oral route can't deliver, intravenous iron steps past the gut and hepcidin altogether, handing iron directly to the plasma carriers and refilling stores in one or two visits. Modern preparations — ferric carboxymaltose, iron sucrose and ferric derisomaltose — wrap the iron in a carbohydrate shell that releases it slowly and safely. The indications are specific, not a convenience upgrade: genuine intolerance of or failure to respond to oral iron; malabsorption (coeliac disease, after bariatric or gut surgery); ongoing losses that outpace oral intake; chronic kidney disease and dialysis (where hepcidin is high and erythropoiesis-stimulating agents need iron to work — cross-linked to Hematology); inflammatory bowel disease, where oral iron both absorbs poorly and inflames the gut further (a link to the Gastrointestinal section); and situations demanding rapid repletion, such as correcting anaemia before planned surgery, or treating iron deficiency in heart failure, where IV iron improves symptoms and is now part of guideline care (cross-linked to the Cardiovascular section).

Bypassing the gut buys speed, but brings its own two cautions. The first is the infusion reaction. Historic high-molecular-weight iron dextrans caused feared anaphylaxis, and that reputation still haunts IV iron — but modern preparations are dramatically safer, and serious hypersensitivity is now rare. Infusions are still given where reactions can be managed, and a mild self-limiting flushing or chest-tightness (the "Fishbane reaction") is recognised and not true anaphylaxis. The second, easily missed, is hypophosphataemia: certain preparations — ferric carboxymaltose most notably — drive phosphate loss through the kidney (by raising FGF23), and repeated or high-dose infusions can produce a persistent, sometimes symptomatic low phosphate with bone pain and weakness. It is worth checking phosphate after repeated dosing, and choosing an alternative preparation in at-risk patients — the mechanism and the consequences of low phosphate are covered in the Phosphate chapter.