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Calcium is the only known substance to inhibit absorption of both non-heme and heme iron, according to the Iron Disorders Institute. Calcium in amounts 300-600 mg – or about one cup of skimmed milk – is enough to hinder the process. Eggs contain phosvitin, a protein, which impairs the absorption of iron. Oxalates have the same effect. The presence of oxalates in spinach explains why the iron in spinach is not absorbed. They are also found in beets, chocolate, tea, strawberries, oregano, basil, and parsley.