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(Nutrition) Ch. 13: The Trace Minerals

Terms in this set (61)

- body conserves iron - bc it's difficult to excrete iron once in body, balance is maintained primarily through absorption
- more iron is absorbed when stores are empty and less is absorbed when stores are full
- special proteins help body absorb iron from food - ferritin captures iron from food and stores it in cells of small intestine; when body needs iron, ferritin releases some iron to iron transport protein transferrin
- if body doesn't need iron, it's carried out when intestinal cells are shed and excreted in feces; intestinal cells replaced every 3-5 days
- by holding iron temporarily, these cells control iron absorption by either delivering iron when the day's intake falls short or disposing of it when intakes exceed needs
- iron occurs in 2 forms in foods: as heme iron which is only found in flesh of animals and as nonheme iron, found in both plant derived and animal derived food
- on avg, heme iron represents about 10% of iron a person consumes in a day - it is so well absorbed it contributes significant iron
- about 25% of heme iron and 17% of nonheme iron is absorbed, depending on dietary factors and body's iron stores
- in iron deficiency, absorption increases and in overload, absorption declines
- heme iron has high availability and is not influenced by dietary factors but several dietary factors influence nonheme iron absorption
- figure 13-3 (p. 411)
- meat, fish, poultry contain heme iron and a peptide (MFP factor) that promotes absorption of nonheme iron
- vitamin c also enhances nonheme iron absorption
- some acids and sugars also enhance absorption
- some dietary factors bind w nonheme iron, inhibiting absorption - phytates in legumes, whole grains, and rice; veggie proteins in soybeans, other legumes, and nuts; calcium in milk; polyphenols in tea, coffee, grain products, oregano, and red wine
- enhancers, inhibitors, and combined effects make it difficult to estimate iron absorption - most factors exert strong influence individually but not combined in meals - impact of combined effects diminishes when a diet is evaluated over several days
- when multiple meals are analyzed together, 3 factors appear to be most relevant: MFP factor and vitamin c as enhancers and phytates as inhibitors
- 18% of dietary iron absorbed from mixed diets and 10% from vegetarian diets
- iron absorption also depends on an individual's health, stage in life cycle, and iron status
- body makes more ferritin to absorb more iron from small intestine and more transferrin to carry more iron around body
- transferrin delivers iron to bone marrow and other tissues- bone marrow uses large quantities of iron to make new RBCs while other tissues use less
- surplus iron is stored in ferritin, primarily in the liver, but also in bone marrow and spleen
- when dietary iron has been plentiful, ferritin is constantly and rapidly made and broken down, providing an ever-ready supply of iron
- when iron concentrations become abnormally high, the liver converts some ferritin into another storage protein called hemosiderin - it releases iron more slowly than ferritin does
- storing excess iron in hemosiderin protects body against damage that free iron can cause - free iron acts as a free radical, attacking cell lipids, DNA, and protein
- avg RBC lives 4 months then spleen and liver remove it from the blood, take it apart, and prepare the degradation products for excretion or recycling
- the iron is salvaged: liver attaches it to transferrin, which transports it back to bone marrow to be reused in making new RBCs - even though RBCs only live for 4 months, iron recycles through each new generation of cells
- figure 13-5 (p. 412)
- body loses some iron daily via Gi tract and through blood if bleeding occurs
- tiny amounts of iron lose in urine, sweat, and shed skin
- women and men lose about 1 milligram of iron per day with women losing addition iron in menses; menstrual losses vary but over a month, they average about 0.5 mg per day
- central to regulation of iron balance is hormone hepcidin (produced by liver) - helps to maintain blood iron within normal range by limiting absorption from small intestine and controlling release from liver, spleen, and bone marrow - production increases in iron overload and decreases in deficiency