Iron is absorbed less well if the body already has good stores of iron ~ the body protects itself from iron overload.
The type of food containing iron also affects the efficiency of iron absorption. Foods of animal origin containing iron in the heme form are absorbed better than foods of plant origin which contain iron in the non-heme form.
Vitamin C taken at the same time will increase absorption of iron but the tannins found in tea, calcium, polyphenols and phytates found in some plant leaves interfere with iron absorption in the human intestine. The iron is bound up by these other chemicals, and it cannot be separated out by the digestive enzymes.
People who eat only food that has a plant origin such as vegans and vegetarians are often advised to take iron supplements as it is difficult to obtain sufficient iron from their diet. Women are particularly susceptible to iron-deficiency anaemia because they lose blood, and therefore iron stores, each month during menstruation.