Q: Do the oxalates in some vegetables and phytates in grains block absorption of their calcium and other minerals?
A: Oxalates and phytates, sometimes called anti-nutrients, do reduce absorption of calcium, iron, and other minerals, but they don’t block them completely. Keep in mind, however, that your body uses only a portion of the nutrients you consume, and that many factors influence absorption, including the presence of other nutrients and compounds. This is the principle of bioavailability.
Oxalates are highest in certain dark leafy greens such as spinach and beet greens; kale and turnip greens have less. Plant foods high in oxalates are often also high in iron or calcium. Though some of the mineral is chemically bound by oxalates, much of it still gets through and is absorbed.
If you want to avoid high-oxalate greens, eat kale instead of spinach or beet greens, for instance. But the best way to get calcium and other minerals is simply to eat a varied diet, including dark greens. Note: People who form calcium oxalate kidney stones (the most common type) may have to limit their intake of oxalates.
With phytates, it’s a similar story. They are found in whole grains, nuts, seeds, and legumes. Whole grains are higher in phytates than refined white flour, but the mineral content of whole grains is also higher, so you still end up ahead. In breads, leavening counteracts phytates and increases bioavailability.