Generally calcium inhibits the absorption of iron, but to what extent? One atom of calcium will not block 100% of iron absorption, that clearly isn't feasible. On the other hand, if someone ate 10 pounds of calcium, they wouldn't absorb any iron. How can we quantitatively determine the inhibition of iron absorption for a given calcium intake?


1 Answer 1


In short: Calcium can inhibit absorption of iron, but this does not necessary have any significant effect on the long-term iron status in the human body.

Iron (Linus Pauling Institute):

The presence of calcium decreases iron absorption from both nonheme (i.e., most supplements and food sources other than meat, poultry, and seafood) and heme sources (meat). However, calcium supplementation up to 12 weeks has not been found to change iron nutritional status, probably due to a compensatory increase in iron absorption. Individuals taking iron supplements should take them two hours apart from calcium-rich food or supplements to maximize iron absorption.

In various studies, calcium from food or supplements in usual amounts decreased iron absorption by about 50%, but the calcium dose/iron absorption relationship cannot be simply quantified, because it is affected by several factors, for example:

  • Phytates in food inhibit the absorption of both calcium and iron (BiomedCentral).
  • Calcium may inhibit the absorption of nonheme iron from plant foods less than heme iron from meat (academic.oup.com).
  • Different calcium supplements may differently affect the absorption of different iron supplements.

The inhibitory effect of calcium on iron absorption is not linear and one may not be able to express it as a simple formula. Below are some examples from studies.

  1. A review of studies, American Journal of Clinical Nutrition, 2010

The reported dose-effect relation between the amount of calcium given and the degree of inhibition of iron absorption differs from other factors influencing iron absorption. This dose-effect pitfall is illustrated by 2 studies...[Read more in the linked article ].

  1. A review of studies, PubMed, 2010

The quantitative effect, although dose dependent, was modified by the form in which Ca was administered and by other dietary constituents (such as phosphate, phytate and ascorbic acid) known to affect Fe bioavailability. The results of most multiple-meal human studies suggest that Ca supplementation will have only a small effect on Fe absorption unless habitual Ca consumption is very low.

  1. A study, PubMed, 1991

When taken without food, calcium carbonate did not inhibit the absorption of ferrous sulphate with doses of either 300 mg Ca and 37 mg Fe or 600 mg Ca and 18 mg Fe. However, at the latter levels, calcium citrate and calcium phosphate reduced iron absorption significantly by 49% and 62%, respectively. All calcium supplements inhibited absorption of the iron supplement when taken with food. The absorption of dietary nonheme iron was also inhibited by all three supplements. This inhibition was less pronounced from a meal of high iron availability and low calcium content (28%) than from a breakfast meal of low iron availability and high calcium content (55%).

  1. A study, American Journal of Clinical Nutrition, 2005

Calcium supplementation reduced heme and total iron without significantly affecting nonheme-iron absorption, regardless of meal bioavailability.

  • $\begingroup$ This doesn't really answer anything because it says nothing about the timing or ratio. Obviously if you don't take calcium supplements at the same time as you ingest iron, there is no problem, but when taken together calcium definitely inhibits the absorption of iron, you will be hard-pressed to find a certified nutritionist who says otherwise. That is why the article still recommends taking them two hours apart. The question still remains: by what percent does a given mass of calcium decrease iron absorption by? This can only happen if you ingest them within two hours of each other. $\endgroup$
    – Vane Voe
    Apr 6, 2019 at 23:08
  • $\begingroup$ @VaneVoe, if you are asking how calcium supplements inhibit the absorption of iron supplements, please make this clear in your question. Food can affect iron absorption in so many ways that it makes your question unanswerable. Even with supplements alone, it may not be possible to answer with a certain "percent." They are individual differences in iron absorption, current iron stores in the body and different Ca and Fe supplement forms that affect absorption $\endgroup$
    – Jan
    Apr 8, 2019 at 8:22
  • $\begingroup$ You're the one who chose the focus on supplements, I am asking about per-weight inhibition, you chose to ignore that. $\endgroup$
    – Vane Voe
    Apr 9, 2019 at 7:22

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