Iron is a mineral that is critical in the body because it is a constituent of hemoglobin, the protein that carries oxygen from the lungs and delivers it to tissues. You need enough iron to make healthy red blood cells. A lack of iron causes a condition called iron deficiency anemia. This condition makes you tired because tissues do not receive enough oxygen. Women who are pregnant and those who have heavy menstrual cycles have greater requirements for iron. The best sources of iron include fortified breakfast cereal, oysters, white beans, dark chocolate, and beef liver. Smaller amounts are found in spinach, lentils, kidney beans, sardines, and chickpeas. If you take an iron supplement, take it with a little vitamin C or vitamin C-rich food because this nutrient boosts the absorption of the mineral.