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Chapter 10
Fat-Soluble Vitamins
Key Terms
1.
cornea: The transparent outer surface of the eye.
2.
osteoporosis: A bone disease characterized by a decrease in bone mineral density and the
appearance of small holes in bones due to loss of minerals.
3.
xerophthalmia: A condition caused by vitamin A deficiency that dries the cornea and
mucous membranes of the eye.
4.
provitamins: Inactive forms of vitamins that the body can convert into active useable
forms. Also referred to as vitamin precursors.
5.
rickets: A bone disease in children that results from vitamin D deficiency.
6.
osteomalacia: A disease in adults that results from vitamin D deficiency; it is marked by
softening of the bones, leading to bending of the spine, bowing of the legs, and increased
risk for fractures.
7.
retinoids: Compounds in foods that have chemical structures similar to vitamin A. These
include the active forms of vitamin A (retinol, retinal, and retinoic acid) and the main
storage forms of retinol (retinyl esters).
8.
bleaching process: A complex light-stimulated reaction in which rod cells lose color as
rhodopsin is split into vitamin A (retinal) and opsin.
9.
cones: Cells in the retina that are sensitive to bright light and translate it into color
images.
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10.
iodopsin: Color-sensitive pigment molecules in cone cells that consist of opsinlike
proteins combined with retinal.
11.
epithelial tissues: Closely packed layers of epithelial cells that cover the body and line its
cavities.
12.
dark adaptation: The process that increases the rhodopsin concentration in the eyes,
allowing them to detect images in the dark better.
13.
carotenoids: A group of yellow, orange, and red pigments in plants. Some of these
compounds are precursors of vitamin A.
14.
menaquinones: The form of vitamin K that comes from animal sources or is produced by
intestinal bacteria. Also known as vitamin K2.
15.
retina: A paper-thin tissue that lines the back of the eye and contains cells called rods and
cones.
16.
teratogen: Any substance that causes birth defects.
18.
retinal: The aldehyde form of vitamin A; one of the retinoids; the active form of vitamin
A in the retina. It is interconvertible with retinol.
19.
provitamin A: Carotenoid precursors of vitamin A in foods of plant origin, primarily
deeply colored fruits and vegetables.
20.
retinol activity equivalent (RAE): A unit of measurement of the vitamin A content of a
food. One RAE equals 1 microgram of retinol.
Fill-in-the-Blank
1.
The millions of cells that line and protect the external and internal surfaces of the body
are epithelial cells. These form epithelial tissues such as skin and mucous membranes.
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2.
The form of vitamin K that comes from plant sources is phylloquinone; it is also known
as vitamin K1.
3.
The active form of vitamin D, calcitriol, is an important regulator of blood calcium
levels.
4.
Menadione is a medicinal form of vitamin K; also known as vitamin K3.
5.
A protein that combines with retinal to form rhodopsin in rod cells is opsin.
6.
The inability of the eyes to adjust to dim light or to regain vision quickly after exposure
to a flash of bright light is called night blindness.
7.
Tocopherol is the chemical name for vitamin E. There are four kinds (alpha, beta,
gamma, delta), but only alpha is active in the body.
8.
Formative cells whose daughter cells may differentiate into other cell types are stem
cells.
9.
Light-sensitive cells in the retina that react to dim light and transmit black-and-white
images are rod cells.
10.
Calcitonin is a hormone secreted by the thyroid gland in response to elevated blood
calcium. It stimulates calcium deposition in bone and calcium excretion by the kidneys,
thus reducing blood calcium.
11.
An outdated system to measure vitamin activity, international units (IU) does not
consider differences in bioavailability.
12.
The alcohol form of vitamin A is retinol.
13.
The parathyroid hormone (PTH) is a hormone secreted by the parathyroid glands in
response to low blood calcium. It stimulates calcium release from bone and calcium
absorption by the intestines, while decreasing calcium excretion by the kidneys.
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14.
Tocotrienol and tocopherols are collectively known as vitamin E.
15.
The acid form of vitamin A is retinoic acid.
Fill-in-the-Blank Summaries
Vitamin A
The body uses three active forms of vitamin A, known collectively as the retinoids. While all
forms have essential functions, retinol is the key player in the vitamin A family. In wellnourished people, the liver stores more than 90 percent of the body’s vitamin A. The
remainder is stored in adipose tissue, lungs, and kidneys. Vitamin A is important to
vision. In the eye, retinal combines with opsin to form a pigment called rhodopsin,
which makes it possible to see in dim light. Vitamin A is also involved in color vision as
part of the pigment in cone cells. A lack of vitamin A affects rod cells first, so as a
vitamin A deficiency worsens, night blindness emerges before color blindness. A large
proportion of the body’s vitamin A is in the form of retinoic acid. It is involved in cell
differentiation, the process in which stem cells develop into specific types of cells with
unique functions. The best sources of provitamin A carotenoids are dark green and
yellow-orange vegetables. Vitamin A deficiency is the leading cause of childhood
blindness in the world.
Vitamin D
Vitamin D is considered both a vitamin and a hormone. The body can synthesize all of the
vitamin D it needs with just sufficient amounts of sunlight. Adequate vitamin D prevents
a childhood disease called rickets. The liver and adipose tissue store vitamin D. When it
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is needed, the liver and kidneys convert stored vitamin D to calcitriol, which works with
parathyroid hormone and calcitonin from the thyroid gland to regulate blood calcium
levels. In addition to fortified products, vitamin D is also found in fish oils. In the United
States, rickets is sometimes seen in children who have fat malabsorption conditions.
Osteomalacia is a related skeletal problem seen in adults with a vitamin D deficiency
that results in softened bones and bending of the spine.
Short Answer
1.
What are the major roles of vitamins in the body?
Antioxidants: Vitamin E, vitamin C, carotenoids
Coenzymes: The eight B-vitamins
Bone health: Vitamin D, vitamin K
Vision: Vitamin A
Blood clotting: Vitamin K
2.
Which vitamin prevents free radical damage?
Vitamin E helps prevent free radical damage to polyunsaturated fatty acids in cell
membranes.
3.
Although vitamins are not an energy source, how do they contribute to energy
production?
Many vitamins play crucial roles in regulating the chemical reactions that allow us to
extract energy from those nutrients.
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4.
When consumed in excess amounts, fat-soluble vitamins generally pose a greater risk of
toxicity than water-soluble vitamins. In relationship to vitamin storage, explain why this
is true.
Fat-soluble vitamins, which are stored in the liver and fatty tissues of the body, are
generally excreted much more slowly than water-soluble vitamins.
5.
How does retinol become a functioning part of the retina and in turn contribute to our
ability to see in dim light?
Retinol is carried in the blood to the retina, where it is converted to retinal. Retinal in turn
combines with the protein opsin to form a pigment known as rhodopsin. Rhodopsin is
abundant in rod cells and makes it possible to see in dim light.
6.
Although there is no evidence that carotenoids are essential nutrients in the technical
sense, they do have important roles that contribute to our health and well-being. Identify
the role(s) of carotenoids.
Carotenoids have roles in fighting free radicals, bolstering immune function, enhancing
vision, and preventing cancer.
7.
Identify vitamin E’s most prominent role.
Vitamin E functions as an antioxidant, protecting cell membranes in all parts of the body
from the damaging effects of oxidation.
8.
Without this vitamin to promote blood clotting, a single cut would eventually lead to
death by blood loss.
Vitamin K
9.
Identify sources of vitamin K.
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We obtain vitamin K from two sources: food (primarily in green vegetables and in some
vegetable oils; animal foods, in general, contain limited amounts of vitamin K), and
bacteria living in our colons.
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