A lack of iron in the blood is the most common cause of anemia. Blood loss often is the underlying issue, but in other cases, your body may not absorb enough iron. Bone marrow requires iron to make RBCs, since iron plays a role in the structure of hemoglobin.
Iron deficiency may also result from stomach ulcers or other issues that cause a slow but chronic bleeding, such as cancers of the colon or uterus, hemorrhoids, intestinal polyps, and so on. You can increase your iron intake by eating plenty of leafy green vegetables, animal proteins, or using iron supplements.