Spinach is notable for its relatively high iron content in comparison with other vegetables. Many vegetables contain between 1 and 3 milligram of iron per 100 gram but spinach contains 4 milligram per 100 grams.
Try to use spinach fresh and cook while still wet without adding any extra water; cover the pan and shake it from time to time. Spinach cooks down tremendously. Fresh spinach should be crisp and dark green.
Spinach beet looks like very similar to spinach and is often sold as such but it is actually a completely different plat, related to beetroot but grown for the leaves rather than the root. The leaves are larger and the stems coarser; remove these when washing. Otherwise treat exactly like spinach.