About three-quarters of the iron in our bodies is active. Of that, about 70 percent is in hemoglobin, 5 percent is in myoglobin (muscle oxygenating protein), and the rest is part of iron cofactors and enzymes such as catalases, peroxidases, and the cytochromes. Some is also in transition, attached to transferrin, which transports iron to the bone marrow, liver, and other tissues for its functions in processing hemoglobin, myoglobin, and various enzymes. Fortunately, the body conserves iron very well, though this increases the possibility of toxicity. Toxicity has not been a great concern until recently, when the possibility of liver irritation and the increased risk of heart disease in men and postmenopausal women due to the oxidant effect of iron was suggested. About 1 percent of red blood cells are recycled each day (their average life span is 120 days), and we use the iron from them (about 30-50 mg. daily) to manufacture new cells. The recycled iron provides about 90 percent of the iron required to make new cells and to carry out other functions; therefore, we need only a little more for full functioning, unless, of course, there is blood loss.

Iron lost from the body must be replaced through dietary iron, but this often takes time and requires a regular source from food or supplements. A pint of blood contains about 200 mg. of iron. Even though iron absorption increases with increased need, it can still take several months to replenish the iron lost when we donate blood. About 30-40 mg. of iron will be lost during an average female menstrual cycle; this is why menstruating women need a consistently higher iron intake than men, a minimum of 18 mg. per day. During breast-feeding, the nursing mother will lose about 1-2 mg. per day. In pregnancy, the mother transfers 500-1000 mg. of iron to her growing baby, most of that (500-700 mg.) during the last few months. Since there are usually less than 500 mg. stored in the bone marrow and other tissues, the mother needs a regular, good dietary and supplemental intake of iron, or she will become very depleted and will be less able to obtain the extra oxygen she requires during pregnancy, labor, and delivery of her baby. After delivery, iron depletion could cause her to feel run-down and to have difficulty caring for her infant.

Many factors can increase iron absorption from the intestines and improve our chances of maintaining adequate body levels. Absorption improves when there is increased need for iron, as during growth periods, pregnancy, and lactation or after blood loss. Acids in the stomach, such as hydrochloric acid, and ascorbic acid (vitamin C) in the small intestine help change any ferric iron to the more easily absorbable ferrous form. Citrus fruits and many vegetables contain vitamin C and therefore help our iron absorption. The animal flesh foods have the more easily absorbed "heme," or blood, iron and also provide amino acids, which stimulate production of hydrochloric acid in the stomach. Cooking with an iron skillet will add iron to the food and make more of it available for absorption. Copper, cobalt, and manganese in the diet also improve iron absorption.

Likewise, many factors can reduce the body's iron absorption. Low stomach acid or taking antacids or other alkalis will diminish iron absorption. Rapid gastric motility reduces the chance to absorb iron, which is a slow process anyway. Phosphates, found in meats and soft drinks; oxalates, in spinach, chard, and other vegetables; and phytates, in the whole grains, all can form insoluble iron complexes or salts that will not be absorbed. Soy protein is being researched, as it may also reduce iron absorption. The caffeine and tannic acid in coffee and tea lower absorption of iron. Low copper in the gut and in the body reduces iron absorption, and high calcium can compete with iron. Supplementing calcium with iron may create a more alkaline digestive medium, which further reduces iron absorption. Iron absorption usually decreases with age as well.