Minerals · 6 min read ·

Iron: mainly for a confirmed deficiency

Without iron, the blood cannot carry oxygen, but too much of it is harmful. Who benefits from a supplement and how to take it.

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In a nutshell

1How much iron

EFSA recommendation 11 / 16 mg EFSA: 11 mg of iron a day for men and women after menopause, 16 mg for women before menopause, because they lose more of it through menstruation.1

Iron is found in meat, fish, cereals, pulses, nuts, egg yolk, dark green leafy vegetables and potatoes. It is poorly and variably absorbed, depending on the diet and on the body’s stores.1 More in the article Bioavailability.

2Who benefits from a supplement

Moderate evidence 18 studies A review of 18 randomised trials with 1,170 adults with iron deficiency without anaemia: supplements eased fatigue as rated by the people themselves but did not improve measured physical performance.3
Moderate evidence VO₂max A summary of studies in endurance athletes with iron deficiency without anaemia: iron treatment improved iron stores, haemoglobin and maximal oxygen uptake.4

3How to take it

Weak evidence — a small study +34% In a study of 40 women with low iron stores, the same number of 60 mg doses taken every other day led to about a third more iron absorbed in total (175 mg vs 131 mg) than taking them every day. Splitting the dose into two a day did not improve absorption.5
Moderate evidence — a randomised trial 440 people In a study of 440 people with iron-deficiency anaemia, vitamin C added to iron tablets did not speed up the recovery of haemoglobin or iron stores.6

4Beware of excess

EFSA safe level 40 mg EFSA (2024) set a safe level of total iron intake of 40 mg a day for adults, including pregnant and breastfeeding women. Above it, the risk does not necessarily rise straight away, but safety is no longer established.2

The body has no active way to excrete iron; it controls the amount only through absorption.1 So only supplement it when a blood test shows a deficiency; in hereditary haemochromatosis, iron builds up and causes harm.

5Forms and what blocks absorption

In the EU, supplements may contain iron in many forms — including ferrous sulphate, fumarate, gluconate or bisglycinate, ferric diphosphate or saccharate, and elemental iron.7

Moderate evidence 17 studies A summary of 17 randomised trials: ferrous bisglycinate raised haemoglobin in pregnant women more than other forms of iron and caused digestive problems about 64% less often. For other groups there were too few studies for a clear conclusion.8
Weak evidence — absorption studies 79–94% Black tea drunk with a meal reduced iron absorption from bread by 79–94%, and cocoa by 71%. Drinks containing polyphenols generally reduced absorption by 50–90%, depending on how much polyphenol they contained.9
Weak evidence — absorption studies −50–60% As little as 165 mg of calcium from milk, cheese or a supplement reduced iron absorption from a meal by 50–60%, and it also significantly reduced the absorption of iron from meat (haem iron).10

So it is better to take iron apart from tea, cocoa and dairy products.9,10

What to take from this

Sources

Written on , based on studies available at that date. Newer research may change the conclusions.

  1. EFSA (2017). Dietary Reference Values for nutrients: Summary report. EFSA Supporting Publications 14(12):e15121. doi.org/10.2903/sp.efsa.2017.e15121
  2. EFSA NDA Panel (2024). Scientific opinion on the tolerable upper intake level for iron. EFSA Journal 22:e8819. doi.org/10.2903/j.efsa.2024.8819
  3. Houston B. L. et al. (2018). Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials. BMJ Open 8:e019240. doi.org/10.1136/bmjopen-2017-019240
  4. Burden R. J. et al. (2015). Is iron treatment beneficial in, iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. Br J Sports Med 49:1389–1397. doi.org/10.1136/bjsports-2014-093624
  5. Stoffel N. U. et al. (2017). Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol 4:e524–e533. doi.org/10.1016/S2352-3026(17)30182-5
  6. Li N. et al. (2020). The efficacy and safety of vitamin C for iron supplementation in adult patients with iron deficiency anemia: a randomized clinical trial. JAMA Netw Open 3:e2023644. doi.org/10.1001/jamanetworkopen.2020.23644
  7. Directive 2002/46/EC on food supplements, Annex II — forms of vitamins and minerals permitted in supplements (consolidated version 26 Nov 2025). eur-lex.europa.eu
  8. Fischer J. A. J. et al. (2023). The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev 81:904–920. doi.org/10.1093/nutrit/nuac106
  9. Hurrell R. F., Reddy M., Cook J. D. (1999). Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr 81:289–295. doi.org/10.1017/S0007114599000537
  10. Hallberg L. et al. (1991). Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. Am J Clin Nutr 53:112–119. doi.org/10.1093/ajcn/53.1.112

This is general information and does not replace a doctor. Iron supplements are dangerous for small children — keep them out of their reach. If you have anaemia, a doctor should look for the cause.