You've heard that spinach has a lot of iron, right? But what you may not know is that spinach is a poor way to get iron because of its content of phytate.
Some of the iron in spinach is bound to phytate. Most of the iron you get is absorbed in the small intestine's duodenum. It comes into the mucosal cell as either a free ion or as heme. If iron is attached to phytates, however, its resistant to disassociation in the gut.
One way to help improve the absorption is by cooking the spinach to break down ligands attached to the iron. And by combining protein with your spinach, you can cause the stomach to release more hydrochloric acid, lowering the pH and helping free up some more iron.
When people have stomach problems that inhibit their ability to release hydrochloric acid (such as when people become older), it's known that a lot of iron is not absorbed at all. In these cases, it may be important to increase the amount of iron in the diet (specifically heme iron from animal foods since its easiest to absorb), even supplement with iron.