717
CHAPTER EIGHTEEN
Nutrition and Metabolism
or certain drugs. Certain diseases, lead to mineral toxicity.
Hemochromatosis, for example, is an inherited form of “iron
overload.” Injuries may also lead to mineral toxicity, such as
severe trauma leading to hyperkalemia (high potassium).
PRACTICE
53
How do minerals dif
er From other nutrients?
54
What are the major Functions oF minerals?
Major Minerals
Calcium and phosphorus account for nearly 75% by weight
of the mineral elements in the body; thus, they are
major
minerals
(macrominerals). Other major minerals, each of
which accounts for 0.05% or more of body weight, include
potassium, sulfur, sodium, chlorine, and magnesium.
Descriptions of the major minerals follow:
1.
Calcium.
Calcium (Ca) is widely distributed in cells and
body fl
uids, even though 99% of the body’s supply is in
the inorganic salts of the bones and teeth. It is essential
for nerve impulse conduction, muscle F
ber contraction,
and blood coagulation. Calcium also decreases the
permeability of cell membranes and activates certain
enzymes.
The amount of calcium absorbed varies with a
number of factors. ±or example, the proportion of
calcium absorbed increases as the body’s need for
calcium increases. Vitamin D and high protein intake
promote calcium absorption; increased motility of the
digestive tract or an excess intake of fats decreases
absorption. Daily intake of 800 mg is sufF
cient to cover
adult requirements even with variations in absorption.
Only a few foods contain signiF
cant amounts of
calcium. Milk and milk products and F
sh with bones,
such as salmon or sardines, are the richest sources.
Leafy green vegetables, such as mustard greens, turnip
greens, and kale, are good sources, but because one
must consume large amounts of these vegetables to
obtain sufF
cient minerals, most people must regularly
consume milk or milk products to get enough calcium.
Calcium toxicity is rare, but overconsumption of
calcium supplements can deposit calcium phosphate
in soft tissues. Calcium deF
ciency in children causes
stunted growth, misshapen bones, and enlarged wrists
and ankles. In adults, such a deF
ciency may remove
calcium from the bones, thinning them and raising risk
of fracture. Calcium is required for normal closing of the
sodium channels in nerve cell membranes, so too little
calcium (hypocalcemia) can cause tetany. Extra calcium
demands in pregnancy can cause cramps.
2.
Phosphorus.
Phosphorus (P) accounts for about 1% of
total body weight, most of it in the calcium phosphate
of bones and teeth. The remainder serves as structural
components and plays important roles in nearly all
metabolic reactions. Phosphorus is a constituent of
Millions oF Americans regularly take vitamin supplements. Consumer
spending on vitamins and minerals is well into the billions oF dollars
annually. This practice has led to clinical signs oF excess vitamin and
mineral toxicity. Iron-containing vitamins are the most toxic, espe-
cially in acute pediatric ingestions.
18.7
MINERALS
Carbohydrates, lipids, proteins, and vitamins are organic
compounds. Dietary
minerals
are inorganic elements essen-
tial in human metabolism. Plants usually extract these ele-
ments from the soil, and humans obtain them from plant
foods or from animals that have eaten plants.
Characteristics of Minerals
Minerals contribute about 4% of body weight and are most
concentrated in the bones and teeth. The minerals
calcium
and
phosphorus
are very abundant in these tissues.
Minerals are usually incorporated into organic mole-
cules. Examples include phosphorus in phospholipids, iron
in hemoglobin, and iodine in thyroxine. However, some
minerals are part of inorganic compounds, such as the cal-
cium phosphate of bone. Other minerals are free ions, such
as sodium, chloride, and calcium ions in the blood.
Minerals compose parts of the structural materials of all
cells. They also constitute portions of enzyme molecules, con-
tribute to the osmotic pressure of body fl uids, and play vital
roles in nerve impulse conduction, muscle F ber contraction,
blood coagulation, and maintenance of the pH of body fl u-
ids. The physiologically active form of minerals is the ionized
form, such as Ca
+2
.
Homeostatic mechanisms regulate the concentrations of
minerals in body fl uids. This ensures that excretion of min-
erals matches intake. Mineral toxicity may result not only
from consumption of too much of a mineral, but also from
overexposure to industrial pollutants, household chemicals,
FIGURE 18.17
Vitamin C de±
ciency causes bleeding gums and
other symptoms oF scurvy.
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