Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
911
24
Vitamins
Distinguish between fat- and water-soluble vitamins, and
list the vitamins in each group.
For each vitamin, list important sources, body functions,
and important consequences of its deficit or excess.
Vitamins
(
vita
5
life) are potent organic compounds needed
in minute amounts for growth and good health. Unlike other
organic nutrients, vitamins are not used for energy and do not
serve as building blocks, but they are crucial in helping the body
use those nutrients that do. Without vitamins, all the carbohy-
drates, proteins, and fats we eat would be useless.
Most vitamins function as
coenzymes
(or parts of coenzymes),
which act with an enzyme to accomplish a particular chemical
task. For example, the B vitamins act as coenzymes when glucose
is oxidized for energy. We will describe the roles of some vitamins
in the metabolism discussion later in this chapter.
Most vitamins are not made in the body, so we must ingest
them in foods or vitamin supplements. Te exceptions are vita-
min D made in the skin, and small amounts of B vitamins and
vitamin K synthesized by intestinal bacteria. In addition, the
body can convert
beta-carotene
(kar
9
o-tēn), the orange pigment
in carrots and other foods, to vitamin A. (For this reason, beta-
carotene and substances like it are called
provitamins
.)
Vitamins are found in all major food groups, but no one food
contains all the required vitamins. A balanced diet is the best
way to ensure a full vitamin complement.
ing stress, enhance protein breakdown and conversion of
amino acids to glucose.
Dietary Requirements
Besides supplying essential amino acids, dietary proteins fur-
nish the raw materials for making nonessential amino acids
and various nonprotein nitrogen-containing substances. Te
amount of protein a person needs reflects his or her age, size,
metabolic rate, and current state of nitrogen balance. As a rule
of thumb, nutritionists recommend a daily intake of 0.8 g per
kilogram of body weight. A small serving of fish and a glass of
milk supply about 30 g of protein.
Most Americans eat far more protein than they need. Pro-
longed high protein consumption may lead to bone loss. Tis
may occur because metabolizing sulfur-containing amino acids
makes the blood more acidic and calcium is pulled from the
bones to buffer these acids.
Check Your Understanding
1.
What are the six major nutrients?
2.
Why is it important to include cellulose in a healthy diet even
though we do not digest it?
3.
How does the body use triglycerides? Cholesterol?
4.
John eats nothing but baked bean sandwiches. Is he
getting all the essential amino acids he needs in this
restricted diet?
For answers, see Appendix H.
Corn and
other grains
Beans
and other
legumes
Tryptophan
Methionine
Valine
Threonine
Phenylalanine
Leucine
Isoleucine
Lysine
Vegetarian diets providing the eight essential amino
acids for humans
(b)
Essential amino acids
(a)
Valine
Threonine
Phenylalanine
(Tyrosine)
Leucine
Isoleucine
Lysine
Methionine
(Cysteine)
Tryptophan
Histidine
(Infants)
Arginine
(Infants)
Total
protein
needs
Figure 24.2
Essential amino acids.
For
protein synthesis to occur, 10 essential amino
acids must be readily available and in the
correct proportions.
(a)
Relative amounts
of the essential amino acids and total
proteins needed by adults. Notice that the
essential amino acids represent only a small
percentage of the total recommended protein
intake. Histidine and arginine, graphed with
dashed lines, are essential in infants but not
in adults. Amino acids shown in parentheses
are not essential but can substitute in part for
methionine and phenylalanine.
(b)
Vegetarian
diets must be carefully constructed to provide
all essential amino acids. A meal of corn and
beans fills the bill: Corn provides the essential
amino acids not in beans and vice versa.
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