The compound we call vitaminD can no longer properly be considered a vitamin. For most mammals, itis not in any sense even a nutrient. Nevertheless, vitamin D resemblestrue vitamins inasmuch as humans -- who are cut off from the criticalsolar ultraviolet wavelengths by reason of latitude, clothing, or shelter-- depend on an external source of the substance, just as they do forthe true essential nutrients.

What is Vitamin D?

Vitamin D, calciferol, is afat-soluble vitamin. It is found in food, but also can be made in yourbody after exposure to ultraviolet rays from the sun. Vitamin D existsin several forms, each with a different activity. Some forms are relativelyinactive 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 functionof vitamin D is to maintain normal blood levels of calcium and phosphorus.Vitamin D aids in the absorption of calcium, helping to form and maintainstrong bones. It promotes bone mineralization in concert with a numberof other vitamins, minerals, and hormones.

Without vitamin D, bones canbecome thin, brittle, soft, or misshapen. Vitamin D prevents rickets inchildren and osteomalacia in adults, which are skeletal diseases thatresult in defects that weaken bones.

What are the sources ofvitamin D?

Food sources

Fortified foods are the majordietary sources of vitamin D. Prior to the fortification of milk productsin the 1930s, rickets (a bone disease seen in children) was a major publichealth problem in the United States. Milk in the United States is fortifiedwith 10 micrograms (400IU) of vitamin D per quart, and rickets is now uncommon inthe US.

Exposure to sunlight

Exposure to sunlight is animportant source of vitamin D. Ultraviolet (UV) rays from sunlight triggervitamin 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 significantvitamin D synthesis in the skin from November through February.

Sunscreens with a sun protectionfactor of 8 or greater will block UV rays that produce vitamin D.

Vitamin D supplements are oftenrecommended for exclusively breast-fed infants because human milk maynot contain adequate vitamin D.

Vitamin D and Bone Health

It is estimated that over 25million adults in the United States have, or are at risk of developingosteoporosis. Osteoporosis is a disease characterized by fragile bones.It results in increased risk of bone fractures.

Rickets and osteomalacia wererecognized as being caused by vitamin D deficiency 75 years ago; theirprevention and cure with fish liver oil constituted one of the early triumphsof nutritional science. The requirement for vitamin D has been peggedto these disorders ever since.

Having normal storage levelsof vitamin D in your body helps keep your bones strong and may help preventosteoporosis in elderly, non-ambulatory individuals, in post-menopausalwomen, and in individuals on chronic steroid therapy.

Researchers know that normalbone is constantly being remodeled (broken down and rebuilt). During menopause,the balance between these two systems is upset, resulting in more bonebeing broken down (resorbed) than rebuilt.

Vitamin D deficiency has beenassociated with greater incidence of hip fractures. A greater vitaminD intake from diet and supplements has been associated with less boneloss 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 wellaccepted, but the mere absence of clinical rickets can hardly be consideredan adequate definition either of health or of vitamin D sufficiency.

The fact that it takes 30 ormore years to manifest itself makes it no less a deficiency conditionthan a disorder that develops in 30 days. It is easy to understand howlong-period deficiency diseases could never have been recognized in theearly days of nutritional science, but with modern methods and a bettergrasp of the relevant physiology, failing to recognize a slowly developingcondition as a true deficiency state, can no longer be justified.

Vitamin D nutrition probablyaffects major aspects of human health, as listed below, other than itsclassical role in mineral metabolism. The rest of the article addressessome of the newly recognized uses of vitamin D.

Cancer

Today, it is well establishedthat besides playing a crucial role in the establishment and maintenanceof the calcium in the body, the active form of vitamin D also acts aneffective regulator of cell growth and differentiation in a number ofdifferent cell types, including cancer cells.

Laboratory, animal, and epidemiologicevidence suggest that vitamin D may be protective against some cancers.Clinical studies now show vitamin D deficiency to be associated with fourof the most common cancers:

• Breast (23)
  
• Prostate 24-27
  
• Colon 28-31
  
• Skin 32,33

Diabetes

Vitamin D deficiency has beenassociated with insulin deficiency and insulin resistance. (1-3) In fact,last year it was shown that vitamin D deficiency is likely to be a majorfactor for the development of type one diabetes in children. (4)

Heart Disease

Insulin resistance is alsoone 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 countrieshave higher levels of heart disease and more heart attacks occur in thewinter months. (5,6)

Arthritis

Progression of degenerativearthritis of the knee and hip is faster in people with lower vitamin Dconcentrations (33-34)

Infertility and PMS

Infertility is associated withlow vitamin D(7), and PMS has been completely reversed by addition ofcalcium, magnesium and vitamin D.(8)

Fatigue, Depression andSeasonal Affective Disorder

Activated vitamin D in theadrenal gland regulates tyrosine hydroxylase, the rate limiting enzymenecessary for the production of dopamine, epinephrine and norepinephrine.

Low vitamin D may contributeto chronic fatigue and depression. (9-10) Seasonal Affective Disorderhas been treated successfully with vitamin D. In a recent study covering30 days of treatment comparing Vitamin D and 2 hour daily use of 'lightboxes', depression completely resolved in the D group, but not in thelight box group.(11)

Autoimmune Disorders

Multiple Sclerosis, (12) Sjogren'sSyndrome, rheumatoid arthritis, thyroiditis and Crohn's disease have allbeen 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 exposurenormalizes immune function and enhances immune cell production. This reducesabnormal inflammatory responses such as found in autoimmune disorders,and reducing occurrences of infectious disease. (14-18)

Obesity

Vitamin D deficiency has beenlinked with obesity. (18, 19) Vitamin D has recently been shown to lowerleptin secretion. (20) Leptin is a hormone produced by fat cells and isinvolved in weight regulation. It is thought that the hormone signalsthe brain when fat cells are "full," but exactly how the hormonecontrols weight is not entirely clear.

Additionally, obesity by itselfprobably further worsens vitamin D deficiency due to the decreased bioavailabilityof vitamin D(3) from skin and dietary sources, because of its being depositedin body fat. (36)

Syndrome X

Vitamin D deficiency has beenclearly linked with Syndrome X. (21) Syndrome X refers specifically toa group of health problems that can include insulin resistance (the inabilityto properly deal with dietary carbohydrates and sugars), abnormal bloodfats (such as elevated cholesterol and triglycerides), overweight, andhigh blood pressure.

Vitamin D and Steroids

Steroids, like prednisone,are often prescribed to reduce inflammation from a variety of medicalproblems. 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 thatsteroids may also impair vitamin D metabolism, further contributing tothe loss of bone and development of osteoporosis associated with steroidmedications. For these reasons, individuals on chronic steroid therapyshould consult with their physician or registered dietitian about theneed to increase vitamin D intake through diet and/or dietary supplements.

The above document was edited from:
NationalInstitutes of Health Document on Vitamin D