The absorption and utilization of any nutrient can be quite complicated. Here’s an excerpt on the absorption of iron from the Massachusetts Institute of Technology:

“There are two types of iron: heme and nonheme. Heme iron, found in animal products like meat, is easily absorbed while nonheme iron is found in plants and is less easily absorbed. Copper is needed for normal iron metabolism. Vitamin A assists with iron in eliminating anemia. Zinc and iron taken together may limit the absorption of zinc. Calcium and iron combined in a meal may decrease the absorption of iron. Other products that inhibit absorption are: phosphates (sodas), polyphenols (red wine, purple grape juice, coffee, tea, spices, some fruits, some vegetables), wheat bran, phytates (phytic acids found in legumes, grains and rice can decrease absorption by 50%), tannins (teas, coffees), manganese, cobalt, copper, cadmium (in cigarette smoke), calcium, legume protein (soybeans, lentils, black beans, mung beans, and split peas), lignin (fibrous tissued vegetables) and soy protein. Nonheme iron is more affected by the above inhibitors than heme iron.

Nonheme iron helpers that increase absorption: Vitaminc C, citric (fruit), malic, tartaric (tart fruit and baking powder) and lactic acids, alcohol, fructose, sorbitol, meat, fish and poultry.”

Source: Optimizing your Diet

Note for readers:

Who might need an iron supplement and how much can be quite complicated. When to take an iron supplement will also depend on individual circumstances. These are good questions to ask the physician responsible for your care.