Download Chapter 12 Summary

Chapter 12 Summary
Trace minerals are inorganic atoms or compounds the body needs in very small quantities. The
bioavailability of trace minerals is influenced by many factors, such as nutritional status and other
components of foods. In general, trace minerals serve functional roles as cofactors of enzymes or
components of nonenzyme molecules such as hemoglobin. Some trace minerals also provide structure to
the body’s mineralized tissues.
Two forms of iron are found in foods: heme iron and nonheme iron. Heme iron is more readily absorbed
in the intestine, whereas nonheme iron bioavailability is influenced by many factors. Good sources of
heme iron include meat and seafood. Nonheme iron is found in some vegetables, enriched cereal
products, and fortified foods. Regulation of iron homeostasis occurs primarily in the small intestine, such
that poor iron status increases iron absorption. Iron is a component of hemoglobin and myoglobin and is
needed for oxygen transport, and thus aerobic metabolism. Iron is also associated with the electron
transport chain and is involved in protecting the body from toxins and free radicals. Iron deficiency can
cause fatigue, decreased work performance, and impaired intellectual abilities. Severe iron deficiency
causes microcytic hypochromic anemia. Iron toxicity resulting from genetic disorders and
overconsumption of iron supplements or medicine can be fatal.
Copper is similar to iron in its regulation and use in the body. Homeostasis of this mineral occurs via
alterations in absorption, such that it increases during copper deficiency. Excess copper is excreted in the
bile. Copper is a cofactor for enzymes involved in redox reactions and is important for energy
metabolism, neural function, and antioxidant reactions. Good sources of copper include liver, shellfish,
mushrooms, and nuts. Copper deficiency, which is rare, causes connective tissue and bones to weaken,
anemia, and neural problems. Copper toxicity results in gastrointestinal distress and perhaps liver
damage.
Iodine is found in fortified salt (iodized salt), shellfish, and dairy products. Its bioavailability is typically
high, and excess iodine is excreted in the urine. Iodine is a component of the thyroid hormones thyroxine
(T4) and triiodothyronine (T3),which are important for growth, development, brain function, and energy
metabolism. However, the presence of goitrogens in some vegetables can decrease incorporation of
iodine into T3 and T4. The many forms of iodine deficiency are collectively called iodine deficiency
disorders (IDDs). Cretinism is the form of IDD that affects infants born to iodine-deficient mothers and
causes mental retardation, poor growth, and delayed development. IDD in children and adults can cause
goiters, a condition characterized by an enlarged thyroid gland. Goiters can be serious if they obstruct a
person’s trachea. Iodine toxicity causes both hypo- and hyperthyroidism.
Food sources of selenium include nuts, seafood, and meats. Cereal products can also provide selenium,
although the amount is dependent on soil selenium levels. Selenium absorption is efficient, allowing most
dietary selenium to enter the blood. Excess selenium is excreted in the urine. Selenium is a component of
at least 14 proteins, called selenoproteins, some of which are involved in redox reactions, protecting the
body from oxidative damage. Others regulate thyroid metabolism and activation of vitamin C. Selenium
deficiency can cause Keshan disease, especially in children, resulting in sometimes fatal heart problems.
Selenium toxicity causes a garlic-like odor of the breath, gastrointestinal upset, and brittle teeth and
fingernails.
Chromium is found in whole-grain products, fruits, and vegetables, and its bioavailability is increased by
vitamin C. Excess chromium is excreted in the urine. Chromium is needed for the function of the
hormone insulin and for optimal growth and development of children. Although chromium
supplementation can increase growth of muscles and decrease adipose tissue in animals, these effects
have not been seen in humans. Nonetheless, chromium supplementation is common in athletes.
Chromium deficiency is rare but causes high blood glucose concentrations, decreased insulin sensitivity,
and weight loss. Chromium toxicity from foods is rare, although it is sometimes caused by environmental
exposure, resulting in severe skin irritations and increased risk for lung cancer.
Manganese is found in whole-grain products, nuts, legumes, and some fruits and vegetables. Water can
also be a good source. Very little manganese is absorbed, and excess is excreted in the bile. Manganese
works with several enzymes in the body involved in energy metabolism, bone formation,
gluconeogenesis, and antioxidant function. Manganese deficiency causes dry skin, weak bones, and poor
growth. Toxicity from foods is rare, but environmental exposure can cause serious problems.
In general, legumes, grains, and nuts are good sources of molybdenum. Molybdenum is a cofactor for
enzymes involved in the metabolism of sulfur-containing amino acids and the purines important for
protein, DNA, and RNA structures. Thus this mineral is needed for protein synthesis and cellular growth.
Deficiency is extremely rare but causes headaches, abnormal cardiac function, and visual difficulties.
Toxicity has never been documented in humans but causes reproductive failure in laboratory animals.
Zinc is found in shellfish, fortified cereals, meat, legumes, and chocolate, and its absorption is influenced
by many factors. For example, phytates decrease absorption, and animal sources are more bioavailable
than plant sources. Zinc absorption is highly regulated, increasing during zinc deficiency and decreasing
during toxicity. Acrodermatitis enteropathica is a genetic defect resulting in decreased zinc absorption.
This disease is caused by the production of a faulty protein needed for zinc transport into the enterocyte.
Zinc is a cofactor for enzymes needed for RNA synthesis and is a vital component of proteins (zinc
fingers) that modulate gene expression. It is also important for cell maturation and immune function.
Zinc deficiency causes poor appetite, growth failure, skin irritations, diarrhea, delayed physical
maturation, and infertility. Zinc toxicity is rare but can influence immune function, lipid metabolism, and
appetite.
Fluoride is not an essential nutrient. However, increased fluoride intake decreases tooth decay and
strengthen bones. Fluoride is not found naturally in many foods. Instead, we tend to consume it from the
water we drink. Fluoride increases the strength of both teeth and bones by becoming incorporated in the
mineral matrix of these tissues. In addition, fluoride stimulates osteoblasts, which build bone. Topical
application of fluoride (such as in toothpaste) decreases bacterial production of acid in the mouth.
Because these acids cause cavities, the use of fluoridated toothpaste enhances dental health. Fluoride
toxicity can discolor teeth (dental fluorosis) and weaken bones (skeletal fluorosis).
In addition to the trace minerals known to be essential for life, many others may influence health. These
include nickel, aluminum, silicon, vanadium, arsenic, and boron. Although scientists do not know
whether these minerals are important for human health, there is evidence that they influence the health of
other animals. Further research is needed to understand whether these minerals are vital for humans or
whether they, like fluoride, are involved in nonessential but important functions.