Health Reference LibraryDoes iron compete with calcium for absorption?
This entry is part of the Nutri Tailor Health Reference Library — cited research on supplements, nutrients and adjacent areas of health.
Summary
Yes, at the same sitting. Calcium reduces the amount of iron your gut absorbs from that dose, and the effect is large: in the 1991 isotope study, 165 mg of calcium cut non haem iron absorption by 50 to 60 percent, and the effect levelled off between 300 and 600 mg. A glass of milk with an iron tablet is enough to do it. The reassuring half is that a 1998 trial of 1200 mg of calcium a day found no change in iron stores over six months in people who were not deficient, because the body adjusts over time. The rule: take iron and calcium supplements two hours apart, and keep to it while correcting a low ferritin.
How it works
Calcium and iron are handed across the gut wall by the same cells, and calcium appears to interfere with the final transfer step out of those cells into the blood. The effect is dose dependent up to about 300 mg of calcium in one meal, then it plateaus, which is why a 500 mg calcium tablet does no more harm than a calcium rich meal. It affects both plant iron and, to a lesser degree, the haem iron in meat. Over weeks the gut compensates, which is why long term iron status in people eating plenty of dairy is little different.
Effective dose
Any calcium supplement is above the plateau, so the practical question is timing not dose. Dietary calcium is harder to plan around, which is why iron is best taken away from meals when the stomach allows, or with a low calcium meal. The iron dose itself is the bigger lever: NICE describes a daily ferrous salt for diagnosed deficiency, continued three months after recovery, with the GP setting the dose.
Timing
Two hours between an iron supplement and a calcium supplement, either way round. Iron first thing with water, or mid morning, then calcium with lunch, is a schedule most people can keep. Alternate day iron dosing absorbs more per tablet and makes the spacing easier. Keep tea and coffee an hour clear on the far side too.
Safety profile
Never take iron without a blood test showing you need it; the same tiredness occurs in iron overload, where extra iron is harmful. Keep iron out of reach of children. Calcium above 1500 mg a day from supplements can cause stomach pain and diarrhoea.
Interactions
Calcium is the strongest of the common inhibitors; tea and coffee polyphenols, and the phytates in whole grains, are the others. Vitamin C increases non haem iron absorption at the same meal. Zinc and iron compete only at supplement doses on an empty stomach.
| Interaction | Issue | Guidance | Citation |
|---|---|---|---|
| Iron and supplemental calcium | Supplemental calcium reduces non-haem iron absorption | Separate iron supplements from calcium tablets by around 2 hours | NIH ODS — Iron Fact Sheet for Health Professionals |
| Iron and dietary calcium | Dietary calcium reduces non-haem iron absorption in a dose-dependent fashion, plateauing around 300mg per meal | If iron-deficient, take iron away from milk-, yoghurt-, and cheese-rich meals | NIH ODS — Iron Fact Sheet for Health Professionals |
| Iron and vitamin C | Vitamin C enhances non-haem iron absorption (single-meal effect; long-term clinical benefit less reliable) | Take iron with a vitamin C source such as orange juice | NIH ODS — Iron Fact Sheet for Health Professionals |
Guideline positions
Calcium and iron absorption, dose dependent inhibition, stable isotope study (1991). Effect of calcium supplementation on iron status over six months (1998). Review of dietary inhibitors of iron absorption (2010). NICE Clinical Knowledge Summary: anaemia, iron deficiency. British Society of Gastroenterology guideline on iron deficiency anaemia (2021).
Practical framework
If you are correcting a deficiency, the two hour gap is worth the effort; if you are iron replete and eating normally, it barely matters. Nutri Tailor stocks iron bisglycinate with vitamin C, 15 mg of iron as bisglycinate with 400 mg of vitamin C per capsule, gentle enough to take mid morning on its own, made in the UK. Retest ferritin after eight weeks; how long iron takes to work covers the timeline. To read your own result against the UK ranges, use the free blood test result interpreter. This is a summary of published research, not personal health advice. Talk to your GP or a registered nutritional therapist before changing what you take, especially if you are pregnant, have a diagnosed condition or are under ongoing care.
Common misconceptions
That a little milk in your tea blocks your iron: the tea does more than the milk. That calcium destroys iron: it delays absorption of that dose, and the body adapts over weeks. That you need to cut dairy to correct iron deficiency: you need to separate the supplement, not the food.
Who this matters for
This entry is relevant for the following groups and situations:
- Pregnancy
- Breastfeeding
- Adults over 65
- Post-menopause
- Inflammatory bowel disease
Recent updates
Reviewed and corrected against sources on , ,
Sources
- Hallberg L, Brune M, Erlandsson M, Sandberg AS, Rossander-Hultén L 1991. Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. American Journal of Clinical Nutrition. PMID: 1984335 · DOI: 10.1093/ajcn/53.1.112
- Minihane AM, Fairweather-Tait SJ 1998. Effect of calcium supplementation on daily nonheme-iron absorption and long-term iron status. American Journal of Clinical Nutrition. PMID: 9665102 · DOI: 10.1093/ajcn/68.1.96
- Hurrell R, Egli I 2010. Iron bioavailability and dietary reference values. American Journal of Clinical Nutrition. PMID: 20200263 · DOI: 10.3945/ajcn.2010.28674f
- Snook J, Bhala N, Beales ILP, Cannings D, Kightley C, Logan RPH, Pritchard DM, Sidhu R, Surgenor S, Thomas W, Verma AM 2021. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut. PMID: 34497146 · DOI: 10.1136/gutjnl-2021-325210
- NIH Office of Dietary Supplements. NIH Office of Dietary Supplements — Iron Fact Sheet for Health Professionals. NIH Office of Dietary Supplements (US government).