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?
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.
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.