Definition
Anemia can be caused by iron deficiency, folate deficiency, vitamin B12 deficiency, and other causes. Iron deficiency anemia is due to a shortage of iron. It is characterized by the production of red blood cells that are smaller than normal (microcytic) and appear pale or light colored (hypochromic) when viewed under a microscope. For this reason, the anemia that occurs with iron deficiency is also called hypochromic microcytic anemia.
Description
Iron deficiency anemia is the most common type of anemia throughout the world. In the United States, iron deficiency anemia occurs to a lesser extent than in developing countries because of the higher consumption of red meat and the practice of food fortification (addition of iron to foods by manufacturers). In the United States, iron deficiency anemia is caused by a variety of factors, including excessive losses of iron in menstrual fluids and excessive bleeding into the gastrointestinal tract. In developing countries located in tropical climates, the most common cause of iron deficiency anemia is infestation with hookworm.
Causes and symptoms
Infancy is a period of increased risk for iron deficiency. A human infant is born with a built-in supply of iron, which can be tapped during periods of drinking low-iron milk or formula. Both human milk and cow milk contain rather low levels of iron (0.5-1.0 mg iron/liter). However, about 50% of the iron in human milk is absorbed by an infant, while only 10% of the iron in cow milk is absorbed. During the first six months of life, growth of an infant is made possible by milk in the diet and by the infant's built-in supply. Premature infants have a lower supply of iron. For this reason, it is recommended that pre-term infants (beginning at two months of age) be given oral supplements of 7 mg iron/day, in the form of ferrous sulfate. Iron deficiency can develop when infants are fed formulas that are based on cow milk that has not been fortified. For example, unfortified cow milk is given free of charge to mothers in Chile. This practice prevents general malnutrition, but results in the development of mild iron deficiency.
The normal rate of blood loss in the feces is 0.5-1.0 ml per day. About 60% of persons with cancer of the colon and rectum experience further blood loss in the range of 10 ml/day, which can lead to iron deficiency anemia. The fecal occult blood test is widely used to screen for the presence of cancer of the colon or rectum. In the absence of testing,colorectal cancer may be first detected because of the resulting iron deficiency anemia.
Infection with hookworm can also cause iron deficiency anemia. The hookworm is a parasite that thrives in warm climates, including in the southern United States. A hookworm enters the body through the skin, very commonly through bare feet. The hookworm then migrates to the small intestines where it attaches itself to the villi (small, finger-like structures found on the walls of the intestines, which are used for the absorption of nutrients). Hookworms damage the villi, resulting in blood loss. Further, they produce anticoagulants which promote continued bleeding. Each hookworm can cause the loss of up to 0.25 ml of blood per day.
Bleeding and blood loss through the gastrointestinal tract can also be caused by hemorrhoids, anal fissures, irritable bowel syndrome, aspirin-induced bleeding, blood clotting disorders, and diverticulosis (a condition caused by an abnormal opening from the intestine). Several genetic diseases are characterized by bleeding disorders. These includehemophilia A, hemophilia B, and von Willebrand's disease. Of these, only von Willebrand's disease leads to gastrointestinal bleeding.
The symptoms of iron deficiency anemia include weakness and fatigue. These symptoms result from the lack of function of red blood cells, and the reduced ability of red blood cells to carry iron to exercising muscles. Iron deficiency can also affect other tissues, including the tongue and fingernails. Prolonged iron deficiency can result in changes of the tongue, which may become smooth, shiny, and reddened, a condition known as glossitis. Fingernails may grow abnormally and acquire a spoon-shaped appearance.
Decreased iron intake is a contributing factor in iron deficiency and the resulting iron deficiency anemia. The iron content of some common foods is:
- whole wheat bread (43 mg/kg)
- spinach (33 mg/kg)
- beef (28 mg/kg)
- raisins (20 mg/kg)
- eggs (20 mg/kg)
- lima beans (15 mg/kg)
- potatoes (14 mg/kg)
- canned tuna (13 mg/kg)
- chicken (11 mg/kg)
- peanut butter (6.0 mg/kg)
- tomatoes (3.0 mg/kg)
- cabbage (1.6 mg/kg)
- apples (1.5 mg/kg)
- corn oil (0.6 mg/kg)
It is readily apparent that apples, tomatoes, and corn oil are relatively low in iron, while whole wheat bread, spinach, and beef are relatively high in iron. The assessment of whether a food is low or high in iron can also be made by comparing the amount of that food eaten per day with the recommended dietary allowance (RDA) for iron. The RDA for iron for an adult male is 10 mg/day, while that for an adult woman is 15 mg/day. The RDA during pregnancy is 30 mg/day. The RDA for infants of 0-0.5 years of age is 6 mg/day, while that for infants of 0.5-1.0 year of age is 10 mg/day. RDA values are based on the assumption that a person eats a mixture of plant and animal foods.
The above list of iron values alone may be deceptive, because bioavailability varies. Bioavailability means the percent of iron in the food that is absorbed via the gastrointestinal tract to the bloodstream. Non-absorbed iron is lost in the feces. The bioavailability of iron in fruits, vegetables, and grains is very low, but is much higher in meats. The bioavailability of iron in plants ranges from only 1-10%, while that in meat, fish, chicken, and liver is 20-30%. The most readily absorbable source of iron is human milk, which has a 50% bioavailability.
Interactions between various foods also influence the absorption of dietary iron. Vitamin C, for example, increases the absorption of dietary iron. Thus, if rice is consumed with a vitamin C-rich food such as orange juice, then the absorption of the rice's iron is enhanced. The increased use of formulas fortified with both iron and vitamin C has led to a marked reduction in anemia in infants and young children in the United States. In contrast, if rice is consumed with tea, certain chemicals (tannins) in the tea reduce the absorption of iron. Another potent inhibitor of iron absorption is phytic acid, a chemical that occurs naturally in legumes, cereals, and nuts.
