Principles of Biochemistry Volume 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Human Nutrition
Iron and copper are essential for the synthesis of heme proteins

Iron deficiency is among the most widespread nutritional disorders in North America, particularly prevalent among children, adolescent girls, and women of childbearing age. Iron is absorbed exclusively as Fe²⁺ ions; both its absorption and excretion are extremely slow processes governed by numerous complex factors. Only a small fraction of the iron present in food is actually assimilated, and its bioavailability varies significantly depending on the food source. Iron is absorbed most efficiently from meat, whereas cereals and grains exhibit a much lower absorption rate. Milk contains very little iron.

Iron is essential for the synthesis of iron-porphyrin Proteins, including Hemoglobin, Myoglobin, Cytochromes, and cytochrome oxidase (Fig. 26-25). In the bloodstream, iron is transported as a complex with the plasma protein transferrin, whereas in Tissues it is stored as ferritin, a protein complex containing iron hydroxide and phosphate. Ferritin is found in large quantities in the Liver, Spleen, and Bone Marrow. Iron is not excreted in the urine; instead, it is eliminated via Bile and feces, as well as through Blood loss. Due to twofold or even threefold iron losses during menstruation, women require higher daily Dietary intake of iron than men. Although bread and other grain products are routinely fortified with extra iron, this approach does not always solve Structure/149.html">The problem of iron deficiency, as many young women and girls eliminate bread from their diets to control their weight. Iron deficiency leads to iron-deficiency anemia, a condition characterized by a normal erythrocyte count accompanied by a reduced hemoglobin content within those Cells.

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Fig. 26-25. Absorption, storage, and utilization of iron in the synthesis of cytochromes, myoglobin, and hemoglobin. Upon intestinal absorption, iron is immediately bound to proteins; it is transported in the blood as transferrin and stored within cells as ferritin. When the storage capacity of ferritin is exceeded, excess iron accumulates in the Cell/35.html">Mitochondria of certain tissues as insoluble hemosiderin granules.

Copper ions are another essential dietary requirement, as they facilitate proper iron utilization—particularly in the synthesis of cytochrome oxidase, which contains both iron and copper. Copper is also vital for the normal development of Connective Tissues and Blood Vessels. The daily human requirement for copper is 2.5–5.0 mg. Rich dietary sources of copper include meat, seafood, vegetables, and nuts, whereas dairy products contain virtually no copper.