Encyclopedia of Health and Medicine

vitamins and health 207
vitamin C supplementation for people recovering
from major surgery, serious burns, and significant
dental procedures.
Vitamin D (Calciferol)
Without vitamin D, the body cannot use calcium.
Vitamin D is unique among vitamins in that the
body can manufacture it as a process of photosynthesis
(exposure to sunlight) that converts a form
of cholesterol stored in the cells of the skin into
vitamin D. Only a small portion of vitamin D
enters the body from dietary sources (namely, fortified
dairy products) in the form of vitamin D 2
(ergocalciferol) or vitamin D 3 (cholecalciferol).
The circulating, active form of vitamin D is calcitriol,
which functions as a HORMONE. Calcitriol, in
tandem with PARATHYROID HORMONE, regulates the
amount of calcium in the blood. This regulation
determines the availability of calcium to the
bones. Vitamin D also influences IMMUNE SYSTEM
functions important for fighting tumors.
Vitamin D deficiency affects bone structure,
preventing bone tissue from accepting new calcium
and allowing calcium to leave the bones to
enter the blood circulation. Vitamin D deficiency
can cause rickets in children and OSTEOMALACIA in
adults. Both are conditions of demineralization
that are reversible with vitamin D supplementation,
though severe rickets may result in residual
deformity particularly of the pelvis. Sustained
vitamin D deficiency in adults leads to OSTEOPORO-
SIS, an irreversible loss of bone tissue.
Vitamin D toxicity may develop with excessive
consumption from vitamin supplements, which
can be supplementation within normal limits in
healthy people who get adequate vitamin D from
dietary sources and is a particular risk among people
who take megavitamins. The toxic level is fairly
low. Vitamin D toxicity is also a risk in people who
are receiving treatment for HYPOPARATHYROIDISM.
Excessive levels of vitamin D affect calcium reabsorption
in the kidney (HYPERCALCEMIA) and often
cause kidney stones (NEPHROLITHIASIS) that can
result in permanent damage to the KIDNEYS.
Vitamin E (Tocopherol)
A fat-soluble vitamin, vitamin E’s most important
function is as an antioxidant. It blocks the reaction
of free radicals to produce more free radicals and
some metabolism of fatty acids. Vitamin E also
maintains the integrity of erythrocytes (red blood
cells), which are vulnerable to damage, in the
blood circulation. Though vitamin E has a reputation
for a wide range of actions in the body to prevent
diseases such as cancer and CARDIOVASCULAR
DISEASE (CVD); to treat conditions such as FIBROCYS-
TIC BREAST DISEASE; and to enhance physical
ENDURANCE, LIBIDO, and reproduction, research has
thus far failed to support these claims. Some
research suggests that excessive amounts of vitamin
E may in fact contribute to the development
of certain cancers. Much research remains under
way to better understand the roles of vitamin E in
health and in disease.
Vitamin E deficiency may occur in disorders of
fat absorption or metabolism though is quite rare.
When present vitamin E deficiency may result in
hemolytic anemia. Vitamin E toxicity is also
uncommon and nearly always occurs in people
who take excessive amounts of vitamin E supplements.
Vitamin E toxicity can have deleterious
effects on the mechanisms of COAGULATION, leading
to hemorrhage.
Vitamin K (Quinone)
BACTERIA in the SMALL INTESTINE synthesize 80 percent
or more of the VITAMIN K the body needs and
uses. The other 20 percent comes from plant-based
foods, notably spinach, broccoli, and other dark
green vegetables. The bacterial form of vitamin K is
menaquinone; the plant form of vitamin K is phylloquinone.
Vitamin K is essential for the activation
of several CLOTTING FACTORS (VII, IC, X) and prothrombin,
which regulate the blood’s ability to clot.
Vitamin K deficiency may occur in disorders
that interfere with the absorption of fats into the
body, such as GALLBLADDER DISEASE and gastrointestinal
malabsorption disorders. Long-term
antibiotic therapy can significantly reduce the bacteria
count in the small intestine, restricting the
body’s ability to synthesize vitamin K. Anticoagulant
medications such as warfarin work by blocking
the action of vitamin K. Untreated vitamin K
deficiency can result in life-threatening hemorrhage.
Vitamin K toxicity is rare and occurs nearly
always when taking vitamin K supplements. It can
cause JAUNDICE and, when severe, permanent
BRAIN damage. Some multivitamin supplements