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The compound we call vitamin
D can no longer properly be considered a vitamin. For most mammals, it
is not in any sense even a nutrient. Nevertheless, vitamin D resembles
true vitamins inasmuch as humans -- who are cut off from the critical
solar ultraviolet wavelengths by reason of latitude, clothing, or shelter
-- depend on an external source of the substance, just as they do for
the true essential nutrients.
What is Vitamin D?
Vitamin D, calciferol, is a
fat-soluble vitamin. It is found in food, but also can be made in your
body after exposure to ultraviolet rays from the sun. Vitamin D exists
in several forms, each with a different activity. Some forms are relatively
inactive in the body, and have limited ability to function as a vitamin.
The liver and kidney help convert vitamin D to its active hormone form.
The major biologic function
of vitamin D is to maintain normal blood levels of calcium and phosphorus.
Vitamin D aids in the absorption of calcium, helping to form and maintain
strong bones. It promotes bone mineralization in concert with a number
of other vitamins, minerals, and hormones.
Without vitamin D, bones can
become thin, brittle, soft, or misshapen. Vitamin D prevents rickets in
children and osteomalacia in adults, which are skeletal diseases that
result in defects that weaken bones.
What are the sources of
vitamin D?
Food sources
Fortified foods are the major
dietary sources of vitamin D. Prior to the fortification of milk products
in the 1930s, rickets (a bone disease seen in children) was a major public
health problem in the United States. Milk in the United States is fortified
with 10 micrograms (400
IU) of vitamin D per quart, and rickets is now uncommon in
the US.
Exposure to sunlight
Exposure to sunlight is an
important source of vitamin D. Ultraviolet (UV) rays from sunlight trigger
vitamin D synthesis in the skin.
Season, latitude, time of day,
cloud cover, smog, and suncreens affect UV ray exposure. For example,
in Boston the average amount of sunlight is insufficient to produce significant
vitamin D synthesis in the skin from November through February.
Sunscreens with a sun protection
factor of 8 or greater will block UV rays that produce vitamin D.
Vitamin D supplements are often
recommended for exclusively breast-fed infants because human milk may
not contain adequate vitamin D.
Vitamin D and Bone Health
It is estimated that over 25
million adults in the United States have, or are at risk of developing
osteoporosis. Osteoporosis is a disease characterized by fragile bones.
It results in increased risk of bone fractures.
Rickets and osteomalacia were
recognized as being caused by vitamin D deficiency 75 years ago; their
prevention and cure with fish liver oil constituted one of the early triumphs
of nutritional science. The requirement for vitamin D has been pegged
to these disorders ever since.
Having normal storage levels
of vitamin D in your body helps keep your bones strong and may help prevent
osteoporosis in elderly, non-ambulatory individuals, in post-menopausal
women, and in individuals on chronic steroid therapy.
Researchers know that normal
bone is constantly being remodeled (broken down and rebuilt). During menopause,
the balance between these two systems is upset, resulting in more bone
being broken down (resorbed) than rebuilt.
Vitamin D deficiency has been
associated with greater incidence of hip fractures. A greater vitamin
D intake from diet and supplements has been associated with less bone
loss in older women. Since bone loss increases the risk of fractures,
vitamin D supplementation may help prevent fractures resulting from osteoporosis.
The use of vitamin D is well
accepted, but the mere absence of clinical rickets can hardly be considered
an adequate definition either of health or of vitamin D sufficiency.
The fact that it takes 30 or
more years to manifest itself makes it no less a deficiency condition
than a disorder that develops in 30 days. It is easy to understand how
long-period deficiency diseases could never have been recognized in the
early days of nutritional science, but with modern methods and a better
grasp of the relevant physiology, failing to recognize a slowly developing
condition as a true deficiency state, can no longer be justified.
Vitamin D nutrition probably
affects major aspects of human health, as listed below, other than its
classical role in mineral metabolism. The rest of the article addresses
some of the newly recognized uses of vitamin D.
Cancer
Today, it is well established
that besides playing a crucial role in the establishment and maintenance
of the calcium in the body, the active form of vitamin D also acts an
effective regulator of cell growth and differentiation in a number of
different cell types, including cancer cells.
Laboratory, animal, and epidemiologic
evidence suggest that vitamin D may be protective against some cancers.
Clinical studies now show vitamin D deficiency to be associated with four
of the most common cancers:
- Breast
(23)
- Prostate 24-27
- Colon 28-31
- Skin 32,33
Diabetes
Vitamin D deficiency has been
associated with insulin deficiency and insulin resistance. (1-3) In fact,
last year it was shown that vitamin D deficiency is likely to be a major
factor for the development of type one diabetes in children. (4)
Heart Disease
Insulin resistance is also
one of the major factors not only leading to the cancers mentioned above,
but also to the number one killer in the US, heart disease. Northern countries
have higher levels of heart disease and more heart attacks occur in the
winter months. (5,6)
Arthritis
Progression of degenerative
arthritis of the knee and hip is faster in people with lower vitamin D
concentrations (33-34)
Infertility and PMS
Infertility is associated with
low vitamin D(7), and PMS has been completely reversed by addition of
calcium, magnesium and vitamin D.(8)
Fatigue, Depression and
Seasonal Affective Disorder
Activated vitamin D in the
adrenal gland regulates tyrosine hydroxylase, the rate limiting enzyme
necessary for the production of dopamine, epinephrine and norepinephrine.
Low vitamin D may contribute
to chronic fatigue and depression. (9-10) Seasonal Affective Disorder
has been treated successfully with vitamin D. In a recent study covering
30 days of treatment comparing Vitamin D and 2 hour daily use of 'light
boxes', depression completely resolved in the D group, but not in the
light box group.(11)
Autoimmune Disorders
Multiple Sclerosis, (12) Sjogren's
Syndrome, rheumatoid arthritis, thyroiditis and Crohn's disease have all
been linked with low vitamin D levels.
Single, infrequent, intense,
skin exposure to UV-B light suppresses the immune system and causes harm.
However chronic low-level exposure
normalizes immune function and enhances immune cell production. This reduces
abnormal inflammatory responses such as found in autoimmune disorders,
and reducing occurrences of infectious disease. (14-18)
Obesity
Vitamin D deficiency has been
linked with obesity. (18, 19) Vitamin D has recently been shown to lower
leptin secretion. (20) Leptin is a hormone produced by fat cells and is
involved in weight regulation. It is thought that the hormone signals
the brain when fat cells are "full," but exactly how the hormone
controls weight is not entirely clear.
Additionally, obesity by itself
probably further worsens vitamin D deficiency due to the decreased bioavailability
of vitamin D(3) from skin and dietary sources, because of its being deposited
in body fat. (36)
Syndrome X
Vitamin D deficiency has been
clearly linked with Syndrome X. (21) Syndrome X refers specifically to
a group of health problems that can include insulin resistance (the inability
to properly deal with dietary carbohydrates and sugars), abnormal blood
fats (such as elevated cholesterol and triglycerides), overweight, and
high blood pressure.
Vitamin D and Steroids
Steroids, like prednisone,
are often prescribed to reduce inflammation from a variety of medical
problems. These medicines may be essential for a person's medical treatment,
but they have potential side effects, including decreased calcium absorption.
There is some evidence that
steroids may also impair vitamin D metabolism, further contributing to
the loss of bone and development of osteoporosis associated with steroid
medications. For these reasons, individuals on chronic steroid therapy
should consult with their physician or registered dietitian about the
need to increase vitamin D intake through diet and/or dietary supplements.
The above document was edited from:
National
Institutes of Health Document on Vitamin D
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