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Fat soluble vitamins are
found in meats, liver, dairy, egg yolks, vegetable seed oils, and leafy
green vegetables. Some foods such as milk and margarine are artificially
fortified with vitamins A and D. These vitamins are metabolized along
with fat in the body and require fat
for absorption in the
gut. The fat-soluble vitamins may be stored in large amounts, and this
gives them the potential to cause toxicity if consumed in high amounts.
Deficiencies are rare in adults but may be seen in children. Megadosing
of fat soluable vitamins, except where indicated by a medical
professional, is potentially dangerous and should be avoided. Two of the
fat-soluble vitamins, A and D, have hormone
like actions, causing specific cells to increase or decrease the
expression of certain genes.
Vitamin A (retinol) plays a vital role in the
functioning of the retina,
growth and maturation of the cells lining the inner and outer surfaces
of the body (the epithelial
cells), growth of bone, reproduction and embryonic development.
Several compounds have vitamin A activity and they are referred to as
retinoids. They function with certain carotenoids to protect against the
development of certain cancers and to enhance immune function.
Carotenoids are substances that are consumed in the diet,
some of which are converted to vitamin A. They may also have antioxidant
activity.
Deficiency
Vitamin A deficiency causes night
blindness, a condition in which vision is impaired in dim light.
Dryness and ulceration of the eyes, skin eruptions and dryness, abnormal
cells of the mucous membranes, urinary stones, and impaired taste and
smell also characterize the deficiency. Many children in developing
countries have irreversible blindness resulting from vitamin A
deficiency. In the US, it occurs more commonly in chronic diseases that
affect fat absorption such as pancreatic insufficiency and portal
cirrhosis, or following removal of a portion of the stomach.
Vitamin A is stored in several sites in the body, so when a deficiency
occurs, supplements must be given long enough for these stores to be
replenished.
Toxicity
Toxicity from excessive doses of vitamin A can also occur. Chronic
ingestion of toxic doses results in symptoms progressing from mild dermatitis
to hemorrhage, increased intracranial pressure, and liver damage.
Pregnant women that ingest quantities above that which is recommended
can cause the development of fetal malformations. Women that have been
treated with synthetic retinoids that accumulate in fat may require
several drug free years or longer before the increased risk to the fetus
subsides.
Dietary Sources & RDA
The best sources of vitamn A are highly pigmented vegetables and
fortified margarine. A normal diet
contains both carotenoids and vitamin A, so calculations of dietary
vitamin A combines both sources and is often measured in retinol
equivalents (REs). The RDA for males 11 years and older is 1000 RE (3300
units); for females 11 years and older is 800 RE (2700 units); for
pregnant females, 800 RE (2700 units); for lactating females during the
first 6 months, 1300 RE (4300 units) and during the second 6 months,
1200 RE (4000 units). Topical forms of vitamin A are used in the
treatment of several conditions including acne, psoriasis and to reverse the
damage resulting from sun exposure.
Vitamin E (tocopherol) has many actions in the
body. It mainly acts as an antioxidant
of lipids, protecting cell
membranes and preventing damage to membrane associated enzymes. The
manifestation of a deficiency is rare in the US because it is present in
many foods and there are large total body stores. Vitamin E is present
as alpha, beta and gamma tocopherols. Supplements may contain the alpha
tocopherol that is either in the "d" form or a combination of
the "d" and "l" forms. The "d" form is
more active than the "l" form but when comparing supplements,
an equivalent number of international units (IU) indicate equivalent
activity. Less information is available about the action of the beta and
gamma tocopherols, but they appear to have different antioxidant
effects. A supplement that contains all forms may provide the greatest
benefits. There are several conditions in which vitamin E
supplementation may have a beneficial effect but these remain to be
proven by long-term studies. It may slow the progression of
Alzheimer’s disease, prevent heart
disease, improve immune function in the elderly, reduce the risk of cataracts
and decrease the pain associated with arthritis.
Deficiency
There is no known syndrome associated with vitamin E deficiency in
adults but in premature infants, anemia may be seen.Vitamin E
supplements are sometimes given to people at a risk for developing a
deficiency such as children with cystic fibrosis, cholestatic liver
disease or other disorders of gastrointestinal
absorption.
Toxicity
A high dietary intake may decrease the risk of some cancers,
but oral supplements do not appear to have the same effect. High doses
of supplements interfere with blood clotting and may increase the risk
of bleeding in people with bleeding tendencies or those on
anticoagulants (blood thinners). Megadose intake may induce blurred
vision or headaches.
Dietary Sources & RDA
The major sources for this vitamin are vegatable seed oils. The RDA for
d-alpha tocopherol for males 11 years and older is 15 IU; for females 11
years and older, 12 IU; for pregnant females, 15 IU; for lactating women
during the first 6 months, 18 IU and during the second 6 months, 16.5 IU.
Vitamin D (calciferol) acts as a hormone
that promotes formation of bone by increasing the blood levels of calcium
and phosphorus. It is synthesized by the skin
upon exposure to sunlight. Receptors for vitamin D are found throughout
the body and it is thought that they mediate a variety of activities,
some of which are unrelated to calcium metabolism.
The precursor for vitamin D3 (cholecalciferol) is a product of
cholesterol metabolism. The precursor for vitamin D2 (ergocalciferol) is
found in plants. In humans, vitamins D2 and D3 appear to have the same
activity and they are referred to collectively as vitamin D. Vitamin D
is activated by the liver and kidney
(although several other organs can participate in its activation) to
calcitriol. The ability and necessity of maintaining tight control of
calcium levels in the blood is served by interaction of many components
including vitamin D and parathyroid hormone (PTH). When calcium levels
decrease, vitamin D stimulates increased intestinal absorption of
calcium and phosphorus, increased release of calcium from the bone, and
decreased excretion of calcium in the urine.
Deficiency
Without vitamin D, there is insufficient absorption of calcium and
phosphorus, and the blood levels of calcium are maintained at the
expense of bone break down. In children, impaired bone and cartilage
formation leads to rickets, characterized by skeletal deformities. In
adults, osteomalacia can occur in which impaired bone mineralization can
lead to bone pain and deformity in advanced cases.
Vitamin D supplements are used in the treatment of nutritional rickets.
Supplements may also be used in the treatment of conditions associated
with poor absorption such as diarrhea and biliary obstruction, and in
metabolic disorders involving abnormalities of vitamin D metabolism.
Other uses are in patients with hyperparathyroidism and osteoporosis.
Calcium supplements frequently contain vitamin D. Other nontraditional
uses of vitamin D are being investigated.
Toxicity
Excessive consumption of vitamin D, known as hypervitaminosis D, can
cause calcium levels in the blood to reach toxic, life-threatening
levels. Toxicity can also occur in the fetus.
Dietary Sources & RDA
Vitamin D has been added to milk purchased in the U.S. and it is
naturally formed via the action of ultraviolet
light on precursors in the skin.
Vitamin K is essential for the formation of
several blood clotting factors, therefore the main symptom of a
deficiency is an increased tendency to bleed. It antagonizes the action
of oral anticoagulants
(blood thinners) such as warfarin. Vitamin K1 (phytonadione) is found in
plants, vitamin K2 (the menaquinones) is made by bacteria in the gut,
and vitamin K3 (menadione) is the precursor to menaquinone-4.
Deficiency
Defective blood clotting is present in vitamin K deficiency. Symptoms
can be produces by coumarin anticoagulants and by antibiotic therapy.
Inadequate vitamin K may be absorbed with obstruction to bile flow since
the vitamin is poorly absorbed in the absence of bile. Other forms of
malabsorption from the gastrointestinal tract and certain types of liver
disease may also lead to a vitamin K deficiency. Vitamin K may be given
to infants with bleeding tendencies. Doses of vitamin K are
individualized and should only be given with medical supervision.
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