|
DESCRIPTION
Vitamin A is a stable, fat-soluble
vitamin that is involved in a wide variety of bodily functions.
As a constituent of the visual pigment
rhodopsin, it plays an important role in the chemical initiation of visual
excitation.
Vitamin A also has an effect on the
development and maintenance of epithelial cells and the development of bone. It
is necessary for the differentiation of underlying cutaneous cells into mucous
epithelial cells, and hence is an important factor in determining the integrity
of mucous membranes, such as that in the respiratory tract, urinary tract, and
digestive tract.
It is essential for the proper synthesis
of mucoproteins and glycoproteins found in mucous membrane secretions which act
to inhibit infection by maintaining epithelial integrity.
By virtue of its roles in epithelial cell
differentiation and the effects of retinoids on cancer malignancies, vitamin A
has been considered to serve an anticancer function as well.
Method of
Action
Vitamin A occurs in several forms;
retinol, retinal (or retinene), and retinoic acid. It can be metabolically
converted to retinal ester. Sources of vitamin A are the carotenes, yellow-red
crystalline compounds synthesized by plants.
Vitamin A is split by various enzymes in
the pancreatic juice to free retinol. The retinol form is then absorbed by the
mucosal cells of the intestine, where it is incorporated in small transport
particles of fat called chylomicrons. These particles are released into the
lymphatic system and eventually are carried to and stored in the liver. Factors
that affect fat absorption also affect vitamin A absorption because vitamin A
is a fat-soluble vitamin.
Vitamin A is mobilized from the liver
bound to the retinol-binding protein (RBP) which serves to transport vitamin A
into the circulatory system for subsequent use.
Vitamin A is an essential component of
photoreception. As a visual pigment, it is combined with a protein opsin and is
itself what is called the II-cis form. This form changes to the II-trans
retinaldehyde upon light exposure, releasing the protein opsin and invoking
visual excitation response. Vitamin A also plays an important role in the
development of bone and maintenance of epithelial cells.
Beta-carotene, a vitamin A precursor that
is yellow in color and a constituent pigment of green, orange, and yellow
vegetables, is one of the most important sources of vitamin to humans.
Beta-carotene yields two molecules of vitamin A per molecule of beta-carotene
metabolized, but this conversion is only about 50% efficient in the body.
Properties and
Uses
One form of vitamin A, retinoic acid, has
proven to be effective in some cases for the treatment of acne and various skin
disorders when applied topically.
Vitamin A has been shown to be of value
in protection against cancer. This has been related to the function of
retinoids in promoting normal differentiation of epithelial cells during
premalignant periods.
Vitamin A supplementation has proved
effective in reversing symptoms of night blindness and corneal damage. It has
been considered an anti-infective vitamin because deficiency is directly linked
to high incidence of respiratory ailments. Adequate vitamin A intake will
decrease the added risk of respiratory infection by maintaining the body's
stores at a protective level.
Vitamin A seems to stimulate the immune
system and the production of white blood cells. Often when some kind of
infection like the flu or a cold hits an intake of 100,000 to 150,000 IU for
not more than three days will effectively turn on the immune system to the
extent that the infection is stopped. The implications of this for other health
problems should be obvious adequate supplementation of vitamin A on a daily
basis will keep the immune system up and ready to fight back, although it is
certainly no miracle cure for all the ills and abuses of the body.
Consequences
of Deficiency
Vitamin A deficiencies occur from lack of
adequate dietary intake or upon interference with its absorption, storage, or
transport. Vitamin A metabolism is determined by available supply. With a
decreased supply, the body slows the rate of vitamin A metabolism in an effort
to conserve the current supply.
Severe deficiency symptoms include skin
changes, night blindness and corneal ulcerations. In the most advanced states
of deficiency, the mucous membranes of the respiratory system, gastrointestinal
system, and urinary tract may be affected.
Vitamin A deficiency can also cause
xerophthalmia or xerosis conjunctivae. These rare conditions may also be caused
by weight loss from a disease such as cancer or malabsorption. Xerophthalmia
can eventually lead to blindness after keratomalacia. Eye disorders caused by
vitamin A deficiency are more rapid and severe in children. Skin changes also
occur in vitamin A deficiency. Goose flesh or xeroderma (alligator skin) are
characteristic changes. Hair follicles become plugged with keratin, making skin
rough, dry and scaly. Other symptoms of vitamin A deficiency include anorexia,
inhibited growth, decreased immunity to infection, and skeletal abnormalities.
Toxicity
Levels
Hypervitaminosis has been observed in
individuals ingesting 50,000 IU of vitamin A daily for several years. Symptoms
of vitamin A toxicity include: bone pain and fragility, coarsening or loss of
hair, dry and fissured skin, irritability, fatigue, severe headaches, anorexia,
abnormal bone growth, liver damage and abdominal discomfort.
Toxicity in children can result from
prolonged intake of vitamin A 25,000 IU daily. Some symptoms include:
drowsiness, vomiting, loss of hair, double vision, skin rashes, and skin
cracking.
Toxicity symptoms, in most instances,
will subside within a few days after curtailed ingestion of vitamin A.
The above figures are several years old,
and were considered safe by all. However, new figures show lower doses
involving toxicity of Vitamin A. The new figures consider that toxicity may
occur from animal foods and/or supplements with intakes of over 50,000 IU/day
in adults and 15,00 IU per day in infants and young children. Intakes of as
little as 3,000 IU/day may increase the risk of birth defects in pregnant
women.
Recommended
Dietary Allowances
Vitamin A needs are modified by a number
of variables. Although vitamin A is generally ample in most diets, many factors
may alter the need in a given individual such as the amount stored in the
liver, the form in which it is taken (as carotene or vitamin A), illness, and
gastrointestinal or hepatic defect. Potential deficiency must be considered in
any chronic disease of the liver, pancreas, or intestinal tract.
To cover such variables, the RDA standard
recommends a margin of safety above the minimal needs. Traditionally, vitamin A
has been measured in international units (IU). One IU is equivalent to the
biologic activity of 0.6 mcg of beta-carotene or 0.3 mcg of retinol. The
biologic activity of a vitamin is measured in rats according to its ability to
forestall the development of disease associated with a deficiency of that
specific vitamin.
The RDA standard uses the term retinol
equivalents (RE), a more accurate term because individual absorption and
conversion of provitamin A carotenoids are variable factors. With this variance
included in the newer term, a means is provided for clearer information
exchange. Complete transition to retinol equivalents from IU as a means of
measuring vitamin A activity will await the listing of various food composition
tables in terms of contents of retinol, beta-carotene, and other provitamin A
carotenoids separately on the basis of weight. In the interim, the RDA standard
will be given in both RE and IU.
Research used to indicate that no more
than 50,000 IU of vitamin A on a daily basis can be utilized by the body,
except in therapeutic situations, in which case 100,000 IU was recommended.
Samples of meals served in 50 colleges,
analyzed for nutrient content, were found to have a vitamin A value that
averaged between 6,819 to 19,424 IU per person daily.
(U.S. RDAs)
A term that once indicated suggested
intake levels for nutrients. U.S. RDAs simplified the RDAs of the National
Academy of Sciences by providing a single recommended allowance for the general
healthy population. With few exceptions, these allowances were based on the
highest RDA for each nutrientthe amounts required for young adult males. Since
these values were excessively high for children, women and the elderly, U.S.
RDAs have now been replaced by RDIs which represent average RDAs. ~see Daily
Values (DVs), Daily Reference Values (DRVs), Recommended Dietary Allowances
(RDAs), Reference Daily Intakes (RDIs)
The current listing of the RDAs for
Vitamin A are:
· Adult Males - 1,000 mg or 40,000
IU
· Adult Females - 800 mg 0r 32,000
IU
· Children, 7 to 10 Years - 700 mg
or 28,000 IU
· Infants - 375 mg or 15,000 IU
· Pregnant and Lactating Women - 65
mg or 2,600 IU
Food Sources
High: (10,000 to 76,000 IU/100 g)
·Animal or fish liver
·Dandelion greens
·Kohlrabi
·Palm Oil
·Parsley
·Turnip greens
·Mint
·Spinach
Medium (1,000 to 10,000 IU/100 g)
Apricots
Beet greens
Beef, lamb
or pork kidney
Broccoli
Butter
Cantaloupe
Cheese
Cherries
Chives
|
Cream
Egg Yolk
Endive
Kale
Milk
Mustard
Nectarines
|
Peaches
Squash
Sweet
potatoes
Swiss chard
Tomatoes
Watercress
Whitefish
|
10 of the Top Vitamin A-Rich Foods Most
Consumed:
· Liver, 3.5 oz, braised - 10,602 mg
· Sweet Potato, 4 oz. baked - 2,487 mg
· Carrot, 1 raw - 2,025 mg
· Mango, 1 medium - 805 mg
· Spinach, 1/2 cup, boiled - 737 mg
· Cantaloupe, 1 cup - 515 mg
· Apricots, dried, 10 halves - 253 mg
· Milk, 1 cup - 149 mg
· Egg yolk - 84 mg
· Mozzarella Cheese, part skim, 1 ounce - 50 mg
Summary of
Vitamin E Deficiency Symptoms
· Poor Night Vision
· Corneal Ulcerations
· Skin Conditions Such As:
· Eczema
· Acne
· Psoriasis
· Goose Flesh or Scleroderma (Also known as
Alligator Skin)
· Rough Dry or Scaly Skin
· Degeneration of the Mucous Membranes of the:
· Respiratory System
· Gastrointestinal System
· Urinary Tract
· Xeropthalmia or Xerosis
· Anorexia
· Inhibited Growth
· Faulty Bone and Tooth Development
· Decreased Efficiency of the Immune System
|
