Absorption of iron is one of the first steps in iron metabolism. Metabolism is a process of chemical interactions that generate energy from food that you eat. Iron metabolism is the part of the process that manages iron in the body. Abnormal iron metabolism can result in too much or too little iron in the body, which can cause poor health or even death.
Iron enters the stomach where it is exposed to stomach acid and changed into a form that allows it to be absorbed. The portion of the small intestine called the duodenum is the chief area where iron absorption takes place. There may be a second minor absorption site near the end of the small intestinal tract.
Once iron is absorbed it is carried (transported) by a protein called transferrin. Each molecule of transferrin can carry two atoms of iron. When working normally, transferrin binds to iron, and transports it to all tissues, vital organs, and bone marrow, so that normal metabolism, DNA synthesis, and red blood cell production can take place. Recently scientists have discovered that transferrin does not work completely alone in the transport of iron. Ceruloplasmin a major copper-containing protein in plasma is also involved in iron transport. Iron needs adequate amounts of copper to reach some of its intended destinations, such as the brain.
Transferrin is the major transporter of iron and ideally should be about 25-35% saturated with iron. Transferrin molecules that are heavily loaded (saturated) lose the ability to hold onto (bind) iron. Unbound or free iron is highly destructive and dangerous. Unbound iron can trigger free radical activity, which can cause cell death, and destroy DNA. Unbound iron is sometimes called uncontrolled iron.
IMAGE: Anemia, chapter 1 ferritin (color) or image #1007
One place that transferrin carries iron to is ferritin. Ferritin is a protein that acts like a large holding vessel. Ferritin contains iron that we don't presently need. It is sometimes called an iron storage protein. Ferritin is produced by nearly every cell of the body. The brain contains huge amounts of ferritin, so does the liver. Ferritin is a very large molecule; one ferritin molecule alone can hold up to 4, 500 atoms of iron.
Elevated serum ferritin can be a sign that the person has inflammation due to disease, or that potential disease causing factors such as iron overload may be present.
Like transferrin, ferritin can also become unstable, and ineffective. Think of ferritin like a big sink; when this sink gets full, ferritin and its iron can be changed into something called hemosiderin.
Hemosiderin is a yellowish-brown substance that contains ferric oxide (rust). A small amount of hemosiderin in tissues is probably normal and may not be harmful, but when large amounts of the substance is allowed to collect in organs, it then becomes a threat to good health. Hemosiderin can accumulate in cells of the heart, liver, lungs, pancreas, central nervous system, thyroid, reproductive organs, skin, adrenals, pituitary and thyroid gland. When the build up of hemosiderin is great, the organ cannot function properly. For example, when beta cells (insulin producing cells of the pancreas) are loaded with hemosiderin, these cells become unable to produce or store adequate amounts of the hormone insulin, which results in diabetes.
FREE Iron and Harmful Invaders:
Free iron can provide nourishment for bacteria such as Yersinia, Listeria and Vibrio. These bacteria are harmless for people with normal iron levels, but when transferrin is highly saturated with iron Yersinia, Listeria, and Vibrio, contained in raw shellfish such as oysters, can lead to life-threathening infection (blood poisoning or septicemia). Death by septicemia can occur within hours if a person has very high body iron levels. People with high iron should always take care not to eat raw shellfish or walk barefoot on a beach where they might step on an infected shells.
Some microorganisms are skilled in other ways in obtaining iron from human hosts. Staph, for example can break open red blood cells and extract the iron it needs. Another pathogen, the protozoan that causes malaria, can get into the red blood cell to obtain iron necessary to thrive. And finally, there are bacteria such as the one that causes tuberculosis that grow best inside macrophages that are iron loaded.
IMAGE: Anemia, chapter 12 # 12003 macrophage
Macrophages are white blood cells that protect us against disease; they scavenge for harmful invaders that enter our bloodstream. When the macrophage is called into action, it engulfs the bacteria or harmful debris and traps it so that it cannot thrive and spread disease in the human host. Iron-loaded macrophages are helpless to defend us against opportunistic infection and disease. Overwhelmed with an iron these macrophages can migrate to other parts of the body and release free iron to that organ. An example is iron-loaded alveolar (lung macrophages) that migrate to the bladder and increase the risk of bladder disease. For this reason, people who smoke are at risk for many diseases, especially cancer. Cancer cells thrive on iron.
IRON THAT IS NOT ABSORBED:
For those with normal iron metabolism, unabsorbed iron, about 90% of iron ingested through diet and supplements, is taken up by specific cells in the intestinal tract, called enterocytes. These cells become engorged with iron, die, drop off, and are excreted in feces.