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At the beginning of the 20th century, the thrill of the discovery of the first vitamins
captured the world’s imagination as seemingly miraculous cures took
place. In the usual scenario, a whole group of people was unable to walk (or were
going blind or bleeding profusely) until an alert scientist stumbled onto the substance
missing from their diets.*1 The scientist confirmed the discovery by feeding vitamindeficient
feed to laboratory animals, which responded by becoming unable to walk (or
going blind or bleeding profusely). When the missing ingredient was restored to their
diet, they soon recovered. People, too, were quickly cured from such conditions when
they received the vitamins they lacked.
In the decades that followed, advances in chemistry, biology, and genetics allowed
scientists to isolate the vitamins, define their chemical structures, and reveal their
functions in maintaining health and preventing deficiency diseases. Today, research
hints that certain vitamins may be linked with the development of two major
scourges of humankind: cardiovascular disease (CVD) and cancer. Many other conditions,
from infections to cracked skin, bear relation to vitamin nutrition, details that
unscrupulous sellers of vitamins often use to market their wares (see the Controversy
section).
Can foods rich in vitamins protect us from life-threatening diseases? What about
vitamin pills? For now, we can say this with certainty: the only disease a vitamin will
cure is the one caused by a deficiency of that vitamin. As for chronic disease prevention,
research is ongoing, but evidence so far supports the conclusion that vitamin-rich
foods, but not vitamin supplements, are protective. (The DRI recommended intakes for
vitamins are listed on the inside front cover pages.)
Definition and Classification
of Vitamins
LO 7.1 List the fat-soluble and water-soluble vitamins, and describe how solubility
affects the absorption, transport, storage, and excretion of each type.
A child once defined a vitamin as “what, if you don’t eat, you get sick.” Although the
grammar left something to be desired, the definition was accurate. Less imaginatively,
a vitamin is defined as an essential, noncaloric, organic nutrient needed in tiny
amounts in the diet. The role of many vitamins is to help make possible the processes
by which other nutrients are digested, absorbed, and metabolized or built into body
structures. Although small in size and quantity, the vitamins accomplish mighty
tasks.
As each vitamin was discovered, it was given a name, and some were given letters
and numbers—vitamin A came before the B vitamins, then came vitamin C, and so
forth. This led to the confusing variety of vitamin names that still exists today. This
chapter uses the names in Table 7–1; alternative names are given in Tables 7–6 and
7–7 at the end of the chapter.
The Concept of Vitamin Precursors
Some of the vitamins occur in foods in a form known as precursors. Once inside the
body, these are transformed chemically to one or more active vitamin forms. Thus,
to measure the amount of a vitamin found in food, we often must count not only the
amount of the true vitamin but also the vitamin activity potentially
available from its precursors. Tables 7–6 and 7–7 specify
which vitamins have precursors.
Two Classes of Vitamins: Fat-Soluble
and Water-Soluble
The vitamins fall naturally into two classes: fat-soluble and
water-soluble (listed in Table 7–1). Solubility confers on vitamins
many of their characteristics. It determines how they
are absorbed into and transported around by the bloodstream,
whether they can be stored in the body, and how easily they are
lost from the body.
Like other lipids, fat-soluble vitamins are mostly absorbed
into the lymph, and they travel in the blood in association with
protein carriers.2 Fat-soluble vitamins can be stored in the liver
or with other lipids in fatty tissues, and some can build up to
toxic concentrations. The water-soluble vitamins are absorbed directly into the bloodstream,
where they travel freely. Most are not stored in tissues to any great extent;
rather, excesses are excreted in the urine. Thus, the risks of immediate toxicities are
not as great as for fat-soluble vitamins.
Table 7–2, p. 236, outlines the general features of the fat-soluble and water-soluble
vitamins. The chapter then goes on to provide important details first about the fatsoluble
vitamins and then about the water-soluble ones. At the end of the chapter, two
tables sum up the basic facts about all of them.
The Fat-Soluble Vitamins
LO 7.2 Discuss the significance of the fat-soluble nature of some vitamins to
human nutrition.
The fat-soluble vitamins—A, D, E, and K—are found in the fats and oils of foods and
require bile for absorption. Once absorbed, these vitamins are stored in the liver and
fatty tissues until the body needs them. Because they are stored, you need not eat
foods containing these vitamins every day. If an eating pattern provides sufficient
amounts of the fat-soluble vitamins on average over time, the body can survive for
weeks without consuming them. This capacity to be stored also sets the stage for toxic
buildup if you take in too much. Excesses of vitamins A and D from supplements and
highly fortified foods are especially likely to reach toxic levels.
Deficiencies of the fat-soluble vitamins occur when the diet is consistently low in
them. We also know that any disease that produces fat malabsorption (such as liver
disease, which prevents bile production) can cause the loss of vitamins dissolved in
undigested fat and so bring on deficiencies. In the same way, a person who uses mineral
oil (which the body cannot absorb) as a laxative risks losing fat-soluble vitamins
because they readily dissolve into the oil and are excreted. Deficiencies are also likely
when people follow eating patterns that are extraordinarily low in fat because a little
fat is necessary for absorption of these vitamins.
Fat-soluble vitamins play diverse roles in the body. Vitamins A and D act somewhat
like hormones, directing cells to convert one substance to another, to store this,
or to release that. They also directly influence the genes, thereby regulating protein
production. Vitamin E flows throughout the body, guarding the tissues against harm
from destructive oxidative reactions. Vitamin K is necessary for blood to clot and for
bone health. Each is worth a book in itself.
Vitamin A
LO 7.3 Summarize the physiological roles of vitamin A and its precursor betacarotene,
name the consequences of deficiencies and toxicities, and list
the major food sources of both forms.
Vitamin A has the distinction of being the first fat-soluble vitamin to be recognized.
Today, after a century of scientific investigation, vitamin A and its plant-derived precursor,
beta-carotene, are still very much a focus of research.
Three forms of vitamin A are active in the body. One of the active forms, retinol,
is stored in specialized cells of the liver. The liver makes retinol available to the bloodstream
and thereby to the body’s cells. The cells convert retinol to its other two active
forms, retinal and retinoic acid, as needed.
Foods derived from animals provide forms of vitamin A that are readily absorbed
and put to use by the body. Foods derived from plants provide beta-carotene, which
must be converted to active vitamin A before it can be used as such.3
Roles of Vitamin A and Consequences
of Deficiency
Vitamin A is a versatile vitamin, with roles in gene expression, vision, maintenance of
body linings and skin, immune defenses, growth of bones and of the body, and normal
development of cells.4 It is of critical importance for both male and female reproductive
functions and for normal development of an embryo and fetus.5 In short, vitamin
A is needed everywhere (its chief functions in the body are listed in the Snapshot on
page 242 and in Table 7–6). The following sections provide some details.
Eyesight The most familiar function of vitamin A is to sustain normal eyesight.
Vitamin A plays two indispensable roles: in the process of light perception at the retina
and in the maintenance of a healthy, crystal-clear outer window, the cornea (see
Figure 7–1).
When light falls on the eye, it passes through the clear cornea and strikes the cells
of the retina, bleaching many molecules of the pigment rhodopsin that lie within
those cells. Vitamin A is a part of the rhodopsin molecule. When bleaching occurs,
the vitamin is broken off, initiating the signal that conveys the sensation of sight to
the optic center in the brain. The vitamin then reunites with the pigment, but a little
vitamin A is destroyed each time this reaction takes place, and fresh vitamin A must
replenish the supply.
Night Blindness If the vitamin A supply begins to run low, a lag occurs before
the eye can see again after a flash of bright light at night (see Figure 7–2, p. 238).
This lag in the recovery of night vision, termed night blindness, often indicates a
vitamin A deficiency.6 A bright flash of light can temporarily blind even normal, wellnourished
eyes, but if you experience a long recovery period before vision returns,
your health-care provider may want to check your vitamin A intake.
Xerophthalmia and Blindness A more profound deficiency of vitamin A is
exhibited when the protein keratin accumulates and clouds the eye’s outer vitamin
A–dependent part, the cornea. The condition is known as keratinization, and if the
deficiency of vitamin A is not corrected, it can worsen to xerosis (drying) and then
progress to thickening and permanent blindness, xerophthalmia.7 Tragically, a
half million of the world’s vitamin A–deprived children become blind each year from
this often preventable condition; about half die within a year after losing their sight.
Vitamin A supplements given early to children developing vitamin A deficiency can
reverse the process and save both eyesight and lives.8 Better still, a child fed a variety
of fruits and vegetables regularly is virtually assured protection.
Gene Regulation Vitamin A also exerts considerable influence on an array of
other body functions through its interaction with genes—hundreds of genes are regulated
by the retinoic acid form of vitamin A.9 Genes direct the synthesis of proteins,
including enzymes, that perform the metabolic work of the tissues. Hence, through its
influence on gene expression, vitamin A affects the metabolic activities of the tissues,
and, in turn, the health of the body.
Researchers have long known that the presence of genetic equipment needed to
make a particular protein does not guarantee that the protein will be made, any more
than owning a car guarantees you a ride across town. To get the car rolling, you must
also have the right key to start up its engine and to turn it off at the appropriate times.
Some dietary components, including the retinoic acid form of vitamin A, are now
known to act like such keys—they help to activate or deactivate genes responsible for
the production of proteins that perform essential body functions.
Cell Differentiation Vitamin A is needed by all epithelial tissue (external skin
and internal linings). The cornea of the eye, already mentioned, is such a tissue; so are
skin and all of the protective linings of the lungs, intestines, vagina, urinary tract,
and bladder. These tissues serve as barriers to infection and other threats.
An example of vitamin A’s health-supporting work is the process of cell differentiation,
in which each type of cell develops to perform a specific function. For example,
when goblet cells (cells that populate the linings of internal organs) mature, they
specialize in synthesizing and releasing mucus to protect delicate tissues from toxins
or bacteria and other harmful elements. In the body’s outer layers, vitamin A helps to
protect against skin damage from sunlight.
If vitamin A is deficient, cell differentiation is impaired, and goblet cells fail to
mature, fail to make protective mucus, and eventually die off. Goblet cells are then displaced
by cells that secrete keratin, mentioned earlier with regard to the eye. Keratin
is the same protein that provides toughness in hair and fingernails, but in the wrong
place, such as skin and body linings, keratin makes the tissue surfaces dry, hard, and
cracked. As dead cells accumulate on the surface, the tissue becomes vulnerable to
infection (see Figure 7–3). In the cornea, keratinization leads to xerophthalmia; in the
lungs, the displacement of mucus-producing cells makes respiratory infections likely;
in the urinary tract, the same process leads to urinary tract infections.
Immune Function Vitamin A has gained a reputation as an “anti-infective” vitamin
because so many of the body’s defenses against infection depend on an adequate
supply.10 Much research supports the need for vitamin A in the regulation of the genes
involved in immunity. Without sufficient vitamin A, these genetic interactions produce
an altered response to infection that weakens the body’s defenses.
When the defenses are weak, especially in vitamin A–deficient children, an illness
such as measles can become severe. A downward spiral of malnutrition and infection
can set in. The child’s body must devote its scanty store of vitamin A to the immune
system’s fight against the measles virus, but this destroys the vitamin. As vitamin
A dwindles further, the infection worsens. Measles takes the lives of more than 450
of the world’s children every day.11 Even if the child survives the infection, blindness
is likely to occur. The corneas, already damaged by the chronic vitamin A shortage,
degenerate rapidly as their meager supply of vitamin A is diverted to the immune
system.