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Supplements BODY HANDBOOK
Supplements · §512
Iron
Iron is the rare supplement where taking it without knowing your numbers can hurt you — the body has no way to throw iron out, so excess just accumulates. Get your ferritin tested; if it is low, an alternate-day single dose absorbs better than the daily pill most prescribers still remember to write. The payoff for genuinely deficient adults is dramatic — energy back, hair settling, the next workout not feeling like a hangover — but for someone whose stores are already full, the same pill builds toward iron overload without delivering anything.
Decide · Course Evidence Moderate Chapter Supplements

Ferritin — one blood test — tells you whether iron is the right move. Under thirty, the gains land within weeks: energy first, then attention, then hair months later. Above fifty, you'll spend money building toward an iron overload you can't undo. The protocol the evidence now backs — a single morning dose every other day — is also gentler on the stomach than the daily-with-meals advice most prescribers grew up with.

The version of you that's been writing off afternoons as personality — sleepy after lunch, unable to push through the three-o'clock slump, breathless on stairs that didn't used to count — may not have a personality problem at all. In a randomised trial of fatigued women whose ferritin was under fifty, four weeks of oral iron cut fatigue by twenty-nine percent against thirteen percent on placebo Verdon 2003. Among the women whose ferritin was under fifteen in a separate trial, eighty-two percent said they felt better, versus forty-seven percent on placebo Krayenbuehl 2011.

The decision turns on one blood test. Ferritin is the protein your body stores iron inside, and the blood level tracks the size of the store. Under thirty micrograms per litre is the cutoff most current guidelines use to call iron deficiency, regardless of sex or whether your haemoglobin looks normal Snook 2021. That picture — depleted ferritin with a blood count that still reads normal — is iron deficiency without anaemia, the version most often missed. Above fifty, you almost certainly do not need a supplement — and taking one anyway adds iron you cannot get rid of. The grey zone between is where the literature still argues.

For people who do land low, what comes back is consistent across studies: energy first, attention next, hair and nails over months. Cognitive tasks — attention, processing speed, working memory — sharpen within weeks; the brain runs on iron-dependent enzymes, and depleted stores blunt the dopamine system the prefrontal cortex relies on Pasricha et al. 2021. Hair shedding from telogen effluvium slows once stores rebuild; new growth takes another two to three months to reach the scalp Trost et al. 2006. Restless legs at night respond when stores are low even if the rest of the bloodwork looks fine — brain iron tracks separately from blood iron, and the cutoff in restless-legs medicine is a higher ferritin under seventy-five Trenkwalder et al. 2017.

Why every other day beats every day

The body has no way to throw iron out. Sweat, shed skin, and sloughed-off gut cells together lose about one to two milligrams a day — that is the entire excretion budget. Everything else stays. So the body controls iron by controlling how much it absorbs from food and supplements, and the hormone running that control is hepcidin.

The mechanism is simple. Hepcidin locks the door iron uses to leave the gut cell and enter the bloodstream. When your stores are high, hepcidin rises and absorption falls. When stores are low, hepcidin falls and absorption rises. The hormone updates fast — and that update window is the lever that decides how to take iron.

A single oral iron dose of sixty milligrams of elemental iron or more triggers a hepcidin spike that lasts about twenty-four hours Moretti et al. 2015. Take a second dose four hours later and the gut absorbs thirty-five to forty-five percent less of it — your morning pill closed the door on your evening one. The "ferrous sulfate three times a day with meals" protocol older guidelines still recommend is fighting itself.

The clinical conclusion is now picked up in current guidelines Snook et al. 2021: one dose, single morning, every other day. Same total iron absorbed from fewer pills, with less of the nausea that makes people quit halfway through.

The protocol

Order a ferritin and a complete blood count before the first pill. If ferritin is under thirty — or under seventy-five and you have restless legs at night — supplement. If it is between thirty and fifty and you are symptomatic (fatigue, hair shedding, exertional breathlessness with no other explanation), supplement and retest at six weeks. If it is above fifty, the answer is no.

If you take a proton-pump inhibitor — omeprazole, esomeprazole, lansoprazole, any of the long-term reflux pills — tell whoever is prescribing your iron. Those drugs cut the stomach acid that converts iron to the absorbable form, and oral repletion is slower and often incomplete while you are on them Snook et al. 2021. Intravenous iron — a single thousand-milligram infusion of ferric carboxymaltose or an equivalent — is the alternative when oral does not work, does not absorb, or you need stores rebuilt fast (heart failure with iron deficiency, inflammatory bowel disease, severe anaemia close to a planned operation).

Where this goes wrong

The pattern of how people get iron supplementation wrong is consistent enough to list:

  • Self-prescribing without a ferritin. Fatigue is non-specific. Roughly one in three fatigued adults turns out to be iron-deficient — the rest have something else going on, and a few of them have a genetic condition that makes the supplement actively dangerous (see the next section). Without the lab number, you are guessing.
  • Daily dosing. Hepcidin closes the door for about twenty-four hours after each dose, so daily delivery overlaps its own absorption window. Every other day is better-absorbed and gentler on the gut Moretti et al. 2015.
  • Coffee or tea with the pill. Tannins bind iron in the gut. One cup of coffee within an hour of dosing can cut absorption by half — repletion that should take three months drags into six for the cost of a habit you could shift by an hour.
  • Calcium at the same time. Milk, yogurt, and calcium supplements compete for the same transporter. Less dramatic than coffee, still measurable.
  • Stopping when the blood count looks good. Haemoglobin normalises in four to six weeks. Ferritin takes three to six months. Quit when the haemoglobin is back and stores are still empty, and symptoms return within a year Camaschella 2015.
  • Pushing through high-dose daily ferrous sulfate. The nausea and constipation are real and account for most quit-rates. Cutting to every other day or switching to bisglycinate solves more cases than "tough it out" ever did.

When iron is the wrong move

The safety asymmetry that defines this entry: the body has no way to remove iron once it is in. Excess vitamin C floods out in urine. Excess B-vitamins do the same. Iron does not. So the people who should not supplement are not a marginal group — they are anyone whose stores are normal or high to begin with, and a smaller group whose biology removes the absorption brake entirely.

Beyond haemochromatosis: people on chronic transfusion programmes, with thalassemia, with sideroblastic anaemia, or with active untreated infections should not take iron without a haematologist's input. And the practical floor that applies to every household: iron pills are among the most common pediatric poisonings. The dose that helps a deficient adult is acutely toxic to a small child. Lock the bottle the way you would paracetamol.

What comes back, and when

The timeline, for an adult with low ferritin who starts the alternate-day protocol:

  • Weeks one and two. Haemoglobin starts climbing, typically about ten grams per litre per month if absorption is working. In the most-deficient, a subtle energy lift is already detectable.
  • Weeks four to eight. Fatigue noticeably softens Verdon et al. 2003. Your partner stops asking if you're coming down with something. The afternoon you used to write off as a wash is back on the table. Climbing a flight of stairs stops being a small event.
  • Months two to four. Attention and processing speed sharpen — work that took an hour takes forty minutes again. For the restless-legs subset, the night-time twitch settles Trenkwalder et al. 2017. Hair shedding in the shower drain visibly drops; new growth is in the scalp but too short to see.
  • Months three to six. Ferritin rebuilds into a comfortable range. Brittle nails stop splitting. The new hair pushes past the original line and the part visibly thickens.
  • Year one and beyond. For menstruating women, the deficit can come back unless heavy bleeding or diet shifted. An annual ferritin is a cheap check; another short course if it drifts down.

The honest version: in someone who was genuinely deficient, this is among the most reliably-felt nutritional interventions there is. In someone who wasn't, you will feel nothing — and your stores will keep climbing toward a problem you cannot undo.

Adjacent topics worth knowing about: iron deficiency in pregnancy (different target, different dose, different protocol), heavy menstrual bleeding as the upstream driver of recurrent deficiency in women, the vegetarian iron equation (plant-source iron is less absorbable and competes with phytates), and the heart-failure iron deficiency syndrome where intravenous iron has its own outcome evidence. Each warrants its own entry.

Related in the handbook
Worsens / aggravates
  • — Tannins in coffee and tea block iron absorption, so keep your iron pill an hour clear of your morning cup.
Helps
  • — Low ferritin with normal hemoglobin is the classic case where iron helps — energy back within weeks.
  • — Restless legs is one of the clearest reasons to fix low iron — replacing it is the first treatment to try.
  • — Take iron with a vitamin-C source and you absorb noticeably more of it; a free upgrade.
  • — Heavy periods from fibroids are a leading reason women run low. Fix the bleeding and the iron, not just one.
Diagnosis
  • — Never start iron blind — a ferritin test tells you whether you need it, since the body can't dump excess.
Related
  • — If your ferritin is low, a weekly portion of liver delivers heme iron your gut absorbs far better than plant sources.
  • — Heavy periods are the single most common reason iron stores run dry in menstruating adults — treat the bleeding, not just the iron.
  • — The reason to test before supplementing: in hemochromatosis, extra iron speeds a slow poisoning.
  • — Before supplementing iron, the MCV/RDW grid tells you whether iron is actually what's behind the low count.
  • — If your iron is low and your plate is plant-heavy, phytic acid may be locking the iron away; prep matters.
  • — Take iron well away from thyroid medication, or it'll blunt the dose.
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