impulses, and help blood to clot. Vitamin D helps the
body get the calcium it needs by increasing the amount
of calcium absorbed in the small intestine. Vitamin D
is an active part of the feedback loop that maintains a
normal level of calcium in the blood.
To maintain health, the amount of calcium in the
blood must stay within a very narrow range. When the
amount of calcium in the blood falls below normal, the
drop is sensed by the parathyroid glands. The para-
thyroid glands are four separate clusters of specialized
cells in the neck. Low blood calcium levels stimulate
the parathyroid glands to secrete parathyroid hor-
mone (PTH). PTH travels through the bloodstream
and stimulates the kidney to increase the conversion ofvitamin D2 and D3 into its active form. Active vitamin
D is released into blood and stimulates the cells lining
the small intestine to increase the amount of calcium
that they absorbed from digesting food. Vitamin D
also causes the kidney to conserve calcium so that less
is lost in urine. If these actions do not return the level
of calcium in the blood to normal, vitamin D activates
cells called osteoclasts that break down bone and
return calcium from the bone to the bloodstream.
People who do not have enough vitamin D absorb
less calcium from the food they eat. To make up for
this, calcium is taken from their bones and the bones
weaken and break more easily.
CANCER PREVENTION AND TREATMENT.Vitamin D
also helps regulate cell differentiation. During devel-
opment, cells divide over and over again. At some
point, they are triggered to specialize (differentiate)
into different types of cells, for example, skin, muscle,
blood, or nerve cells. Vitamin D joins with other com-
pounds to turn on and off more than 50 different genes
that stop cell growth and start cell differentiation.
One characteristic of cancer cells is that they grow
wildly, dividing many times more than normal cells
without differentiating. Since vitamin D can stimulate
cells to stop dividing and begin differentiating,
researchers are investigating whether vitamin D can
protect people from getting certain cancers, especially
colon,prostate, skin, and breast cancer. The research
has produced mixed results. Some studies found that
vitamin D protected against colon cancer, while other
found it offered no protection. The official position of
the American Cancer Society described in their 2006
Nutrition and Physical Activity Guidelines states,
‘‘There is a growing body of evidence from population
studies (not yet tested in clinical trials) that vitamin D
may have helpful effects on some types of cancer,
including cancers of the colon, prostate, and breast.’’
However, the American Cancer Society makes no rec-
ommendations on the amount of vitamin D needed to
have a beneficial effect. Clinical trials are underway to
determine safety and effectiveness of vitamin D in a
variety of situations. Individuals interested in partic-
ipating in a clinical trial at no charge can find a list of
open trials at<http://www.clinicaltrials.gov>.
OTHER DISORDERS.Vitamin D has been proved to
successfully to treat a few other disorders. Psoriasis, a
skin disorder, often responds to ointments that con-
tain synthetic vitamin D3 when other treatment
options have failed. When the parathyroid glands fail
to function or are removed during surgery, vitamin D
supplements help make up for the lack of PTH. Sup-
plements are also used to treat rare inherited familial
hypophosphatemia and Fanconi syndrome-relatedKEY TERMS
Cell differentiation—The process by which stem
cells develop into different types of specialized
cells such as skin, heart, muscle, and blood cells.
Fat-soluble vitamin—A vitamin that dissolves in
and can be stored in body fat or the liver.
Hormone—A chemical messenger that is produced
by one type of cell and travels through the blood-
stream to change the metabolism of a different type
of cell.
Mineral—An inorganic substance found in the
earth that is necessary in small quantities for the
body to maintain a health. Examples: zinc, copper,
iron.
Osteoporosis—A condition found in older individ-
uals in which bones decrease in density and
become fragile and more likely to break. It can be
caused by lack of vitamin D and/or calcium in the
diet.
Placebo—A pill or liquid given during the study of a
drug or dietary supplement that contains no medi-
cation or active ingredient. Usually study partici-
pants do not know if they are receiving a pill
containing the drug or an identical-appearing
placebo.
Steroid—A family of compounds that share a sim-
ilar chemical structure. This family includes the
estrogen and testosterone, vitamin D, cholesterol,
and the drugs cortisone and prendisone.
Vitamin—A nutrient that the body needs in small
amounts to remain healthy but that the body cannot
manufacture for itself and must acquire through
diet.Vitamin D