 |  Perspectives in Nutrition, 5/e Gordon M. Wardlaw,
Ohio State University
Margaret W. Kessel,
Ohio State University
The Fat-Soluble Vitamins
Chapter 9 Summary- Vitamins are essential organic (carbon-containing) compounds needed for
important metabolic reactions in the body. They are not a source of energy.
Instead, they promote many energy-yielding and other reactions in the body,
thereby promoting the growth, development, and maintenance of various body
tissues. Vitamins A, D, E, and K are fat soluble, whereas the B-vitamins and
vitamin C are water soluble. Fat-soluble vitamins are excreted less readily
from the body and are less susceptible to cooking loss than are water-soluble
vitamins.
- Some fat-soluble vitamins pose a potential threat for toxicity. Vitamins
A and D can readily accumulate in the body to toxic concentrations. The water-soluble
vitamins niacin, vitamin B-6, and vitamin C can also induce toxic signs and
symptoms, but only at doses much higher than their RDAs.
- Fat-soluble vitamins are absorbed along with dietary fat. They travel by
way of the lymphatic system into general circulation, carried by chylomicrons,
one type of lipoprotein. In disease states in which fat digestion is limited,
fat-soluble vitamin status may be compromised, especially with vitamins A,
E and K.
- Vitamin A consists of a family of retinoid compounds: retinal, retinol,
and retinoic acid. A plant derivative known as beta-carotene, along with some
other carotenoids, yields vitamin A after metabolism by the intestine or liver.
Vitamin A contributes to the maintenance of vision, the proper development
of cells (especially mucus-forming cells), and immune function. Vitamin A
is found in foods of animal origin, such as liver, fish oils, and fortified
milk. Carotenoids are obtained from plants and are especially plentiful in
dark green and orange vegetables and in some fruits. These likely contribute
to protection from oxidizing agents for the body.
- Americans at risk for poor vitamin A status are people exhibiting limited
fat absorption and alcoholics. Vitamin A can be quite toxic when taken at
3 times or more the RDA, but only with preformed vitamin A. Use is especially
dangerous during pregnancy because it can lead to fetal malformations.
- For most people, vitamin D is more correctly viewed as a hormone rather
than a vitamin because sufficient amounts of it can be produced by the body.
Provitamin D is synthesized in the skin from a derivative of cholesterol in
a process that depends on ultraviolet light. With adequate sun exposure, no
dietary intake of vitamin D is needed. The provitamin, whether produced in
the skin or obtained from the diet, is metabolized in the liver and kidneys
to yield 1,25(OH)2D (or calcitriol), the active hormonal form of vitamin D.
1,25(OH)2D is important for calcium absorption from the intestine and, with
other hormones, it helps regulate bone metabolism. Vitamin D is found in fish
oils and fortified milk. Vitamin D can be very toxic when taken in supplement
form, especially in infancy, when an intake just three times or more than
the Adequate Intake can be toxic. Anyone who feels a need to use a vitamin
D supplement containing more than two times the Adequate Intake, such as an
older person, should consult a physician first.
- Vitamin E functions as an antioxidant. By donating electrons to electron-seeking
compounds (oxidizing agents), it neutralizes their action. One group of electron-seeking
compounds, known as free radicals, can cause widespread destruction, both
to cell membranes and to DNA. Vitamin E is one of several components in the
body's defense system against oxidizing agents, which reduces damage to cells.
Vitamin E is plentiful in plant oils. The more plant oils one consumes, the
more vitamin E one needs, but this need is usually met by the same plant oils.
To date, the use of megadose supplements of vitamin E by healthy adults to
limit cardiovascular disease and cancer risk in people at high risk has not
shown to be effective in most major trials.
- Vitamin K contributes to the body's blood-clotting ability by facilitating
the conversion of precursor proteins to active clotting factors, such as prothrombin,
which promotes blood coagulation. Vitamin K also plays a role in bone metabolism.
Some of the vitamin K absorbed each day likely comes from bacterial synthesis
in the intestine; most comes from foods, primarily green leafy vegetables
and vegetable oils. Vitamin K is readily excreted in the body, but the usual
daily intake from diet alone meets one's needs.
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2002 McGraw-Hill Higher Education