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I have often read that calcium and iron-rich foods (or supplements) should not be ingested within the same meal, as those two minerals impede each other's absorption trough the gut. But several vegetables are recommended as high sources of both minerals. Can the body absorb and use these minerals in these foods?

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Stomach acid helps to get iron absorbed from food. Calcium supplements in the form of calcium carbonate will lower stomach acid levels and thereby impede iron absorption. One can avoid that problem by using calcium citrate supplements. The calcium that's naturally present in foods is not in the form of calcium salts like calcium carbonate or calcium citrate. Instead, the calcium in vegetables sits inside an organic molecule, a so called molecular complex. In some cases this leads to poor absorption for calcium, e.g. in case of spinach the calcium is bound to oxalate and only 5% of the calcium is absorbed. The calcium in kale, in contrast, has a 50% bioavailability. The calcium in typical dairy products have about 30% biovailability.

Another thing to keep in mind when you try to get your minerals like calcium and iron from vegetables instead of dairy and meat products, is to make sure you eat a lot of vegetables. While only a few slices of cheese and small piece of meat is sufficient to get to the RDA for calcium and iron, the volume of vegetables you need to eat to meet the RDA is huge by ordinary standards. The best way to go about this is to include vegetables in your lunch and even in your breakfast if you struggle to eat large volumes in single meals.

  • So, just to be sure I understand you correctly, it IS possible to get both calcium AND iron from food items, as long as the calcium is not in the form of calcium carbonate. Is calcium carbonate naturally found in food items like vegetables, or is it just in supplements made from mineral sources? – G. Laurin Oct 22 '17 at 22:42
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    " huge by ordinary standards" what does this mean? By western standards? Most vegetarians have no problem getting their adequate minerals from their diets. – Graham Chiu Oct 22 '17 at 22:50
  • @G.Laurin Calcium carbonate is not naturally found in vegetables, it is present in supplements and in some fortified foods. – Count Iblis Oct 22 '17 at 23:43
  • @GrahamChiu Yes, but we need to consider vegans here as the problem is also getting enough calcium from vegetables. Getting the RDA for calcium from only vegetables (so, including the calcium added in tofu is cheating) is quite difficult, but it has been suggested that this may be due to the RDA being chosen too high. See e.g. this video where a large amount of healthy vegan foods gets you to just below 800 mg of calcium. – Count Iblis Oct 23 '17 at 0:02
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    Sorry, that answer doesn't pass my sniff test. – Graham Chiu Oct 23 '17 at 2:37
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There doesn't seem to be a problem when you look at iron status so there may be other mechanisms at play that compensate for the effect of calcium on iron absorption.

Studies on human subjects have shown that calcium (Ca) can inhibit iron (Fe) absorption, regardless of whether it is given as Ca salts or in dairy products. This has caused concern as increased Ca intake commonly is recommended for children and women, the same populations that are at risk of Fe deficiency. However, a thorough review of studies on humans in which Ca intake was substantially increased for long periods shows no changes in hematological measures or indicators of iron status. Thus, the inhibitory effect may be of short duration and there also may be compensatory mechanisms.

https://www.ncbi.nlm.nih.gov/pubmed/21462112

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