Iron from tablets is only partly absorbed, and how you take it decides how much reaches your blood. Three things matter: how often you take it, what else is in your stomach and whether vitamin C is nearby.
Why not more often than needed
Iron from a tablet raises hepcidin, a hormone that “closes” the gut to iron for about a day. In a study by Moretti and colleagues (2015), absorption the next day fell by 35–45% after doses of 60 mg or more. And a sixfold larger dose (240 mg instead of 40) yielded only three times as much absorbed iron — 18.1 mg instead of 6.7 mg.
Alternate days beat every day
In a study by Stoffel and colleagues (2017), women with low iron stores took 14 doses of 60 mg either daily or on alternate days. On alternate days they absorbed 21.8% instead of 16.3%, 175 mg of iron in total instead of 131 mg. Splitting the daily dose in two didn’t help: it pushed hepcidin higher.
These studies were done in young women without anaemia, so agree on a treatment plan for anaemia — dose, frequency and duration — with your doctor.
What gets in the way
- Calcium. 300–600 mg of calcium in the same meal cut iron absorption by 50–60%, even of heme iron from meat (Hallberg et al., 1991). So keep iron and calcium supplements or dairy apart.
- Tea. Compared with water, black tea cut iron absorption by 79–94% (Hurrell et al., 1999).
- Coffee. A cup of coffee with a meal cut absorption by 39%, while coffee an hour before the meal had no effect (Morck et al., 1983). More in Tea after buckwheat.
What helps
Vitamin C keeps iron from forming insoluble compounds and converts it to a form the gut absorbs more easily (Hallberg et al., 1989). So it’s handy to take an iron tablet with orange juice, or with kiwi or bell pepper.
In short
- One dose a day, not two; ask your doctor about alternate-day dosing.
- Apart from calcium and dairy — a few hours between them.
- No tea or coffee together with iron.
- Together with vitamin C.
Don’t take iron “just in case”: get a blood test first. The safe upper limit for adults is 45 mg a day including food, and treatment doses are often higher, which is why a doctor prescribes them.
In the CookCount diary you can add an iron tablet in the Supplements section. Use the amount of iron itself: “ferrous sulphate 200 mg” usually also says “iron 65 mg” — enter 65. The calculator adds the iron to your food and warns you if a day goes over 45 mg. For other minerals, see Magnesium, zinc, calcium.