Who is Vitamin D Deficient?
Vitamin D is produced in the skin as a result of Ultra-violet radiation from the sun.

When the sun is low on the horizon, little UV light reaches us. Pollution and cloud cover serve to reduce even further our UV exposure. During winter months in northern areas, the sun is never high enough in the sky to provide sufficient UV to produce Vitamin D.

These periods are referred to as “Vitamin D winters". In northern Europe, Canada and North America this “Vitamin D winter” can extend for six months when absolutely no UV reaches us. During the autumn and early spring UV radiation may only be strong enough to produce Vitamin D during a few hours around mid day. (1)

Consequently, <strong>Vitamin D3</strong> deficiency is more common the farther north of the equator one lives. While many people are significantly deficient, the vast majority do not have optimal levels.

The half life vitamin D in the body is only about 30 days, so even if we build up levels during the summer we are soon in a state of deficiency. In addition, our lifestyle and the belief that exposure to the sun is damaging to our skin, results in many people being deficiency even in the summer.

If you do not get enough vitamin D from the sun, it is impossible to receive enough through diet alone.
Some foods are now fortified with Vitamin D, but the amounts are designed to avoid absolute deficiency as opposed to providing enough to achieve optimal levels. An 8oz. glass of milk contains only 100IU. To achieve the necessary dose to reach optimal levels would require drinking more than 20 glasses a day. (1)

Fifteen minutes each day, in the mid-day sun during summer with the majority of the body exposed would be required, to keep levels optimal. However, we have been told to avoid sun light due to the fear of melanoma and the suns aging effects. Covering up and the use of sun blocks contributes significantly to our state of deficiency. (1)
African-Americans and other dark skinned races are particularly at risk of deficiency. This is in keeping with data showing that African-Americans have higher cancer rates, and when cancer occurs, it is often very aggressive.(2) African Americans die prematurely and have been shown to have half the vitamin D levels of Caucasians.(3)

Other at risk populations are the elderly and obese. The elderly are less able to produce vitamin D and produce far less than a 20 year old when exposed to the same amount of sunlight. (4)

Obesity is a risk factor because vitamin D binds to fat. Consequently obese people, even when producing enough Vitamin D may be deficient as their adipose tissue acts as a reservoir resulting in the rest of the body suffering from deficiency.
Studies have shown that Vitamin D deficiency is widespread. Vitamin D levels below optimal levels are almost universal in Northern regions. Just because levels are sufficient to prevent symptoms of deficiency does not mean our levels are optimal to achieve everything that Vitamin D has to offer.
It is now known that while the classic symptoms of deficiency i.e. Rickets, occurs at levels below 15ng/ml much higher levels are believed to be needed to ensure adequate cardiac function and the prevention of cancer. (See "How much do you need")
If we look at Vitamin D levels achieved in humans who constantly work or live in the sun, we find they achieve levels between 50-70ng/ml (5). It is these higher levels that are believed to prevent cancer.
A study at Massachusetts General Hospital found that 57% of inpatients had levels <15ng/ml and almost no one had optimal levels. (5)
In a study conducted in Italy where one might expect the population to be exposed to more

Signs of Vitamin D Deficiency
It is important to distinguish between the levels of vitamin D that differentiate between absolute deficiency and suboptimal levels. The two are very different. A state of deficiency is considered to occur when levels are below 15ng/ml. The signs and symptoms described below will only occur when vitamin D levels are at or below this level.

Optimal levels needed for disease prevention are much higher (30-70ng/ml). Those who fail to maintain optimal levels are symptom free in the short term, but as we are now learning, they may not be protected from the development of chronic disease and in particular, cancer and heart disease.
   Severe deficiency presents in children as Rickets, a curvature of the long bones of the legs. Due to vitamin D deficiency, the bones fail to harden and literally buckle under the weight of the growing child.

In adolescence and adulthood unexplained fractures may occur. Indeed, many have been accused of childhood abuse because of finding fractures in children when, in fact, they may have been simply suffering from vitamin D deficiency rather than actual abuse. (1) 
In adult life, symptoms of deficiency are often quite non-specific and may involve any of the following:

Weakness of the arms and legs Heaviness in the legs Chronic musculoskeletal pain Fatigue Hypersensitivity of the skin
These symptoms can be quite severe and the cause is often missed. Many elderly persons who are wheel chair bound maybe so due to nothing more than vitamin D deficiency. A study of wheel chair bound elderly folk showed that many of them regained their mobility after correcting their deficiency. 

Who should not take vitamin D?
As with all medications, the absolute contradiction to taking <strong>Vitamin D3 supplements</strong> is signs of toxicity or allergy. There are no reports in the worldwide literature of cases of allergic reactions to Vitamin D and the possibility of toxicity is extremely unlikely (1)
If you have high calcium levels (hypercalcemia) or any disease of your parathyroid glands, you should not take Vitamin D supplements without consulting with your physician.
There are a few rare skin conditions where exposure to sunlight and, hence, possibly Vitamin D supplementation may be detrimental. However, there is no evidence that the toxic manifestations of over exposure to sunlight in these conditions are due to Vitamin D overdose.
The conditions in question are:
Xeroderma pigmentosa
There are certain drugs that result in abnormal sensitivity to sunlight. Again there is no evidence that the toxic effects are due to vitamin D overproduction.  
Those drugs that may be problematical are:
If you have had any form of skin cancer, excessive exposure to sunlight may be dangerous. However, a recent study in melanoma patients demonstrated that those patients with higher exposure to sun light had a reduced mortality from their condition. (20)
Although the liver is involved in the hydroxylation of vitamin D, even patients with advanced liver disease appear to be able to meet these needs. (21)
Hypersensitivity to Vitamin D may occur in sarcoidosis, tuberculosis and rarely in some cancer patients, most commonly non-Hodgkin’s lymphoma and oat cell carcinoma of the lung. Sometimes these patients appear to be able to complete the final renal hydroxylation in other tissues resulting in abnormally high levels of vitamin D activity. (22) Again these situations are extremely rare.

The way in which Vitamin D is metabolized in the body (Cytochrome P450 Enzymes) is shared with many commonly used drugs. If you are on drugs for a cardiac condition, depression or other mental illnesses you should consult with your physician and use only the lower doses of supplementation as the data is incomplete on possible interactions although that have been done to date suggest that Vitamin D levels are lowered by many drugs. (21).

Can you take too much Vitamin D?
Vitamin D toxicity is extremely rare and it is impossible to achieve toxic levels due to excessive exposure to sunlight. When skin production of Vitamin D reaches a maximum, further exposure to UV radiation causes the breakdown of Vitamin D. The body appears to have a built in system for ensuring that toxicity does not occur. This maximum level of production is in the region of 20000IU per day(1) and the recommended dose is Vitamin D3 5000 IU.
One of the primary functions of vitamin D is to facilitate absorption of calcium. Over consumption of vitamin D can lead to high levels of calcium (hypercalcemia). The symptoms caused by high levels of calcium that commonly identifies vitamin D toxicity.
Symptoms of Hypercalcemia
Loss of appetite Weight loss Nausea Vomiting Thirst Fatigue Muscle weakness Restlessness Confusion
However, high calcium levels only occur when Vitamin D levels exceed 150ng/ml. (23, 24). As we know, it is difficult enough to maintain cancer preventative levels of above 38ng/ml so the likelihood of toxicity is exceedingly unlikely. 
Adults would need to take in excess of 10,000IU per day for many months or even years.

Should toxicity occur it can be reversed simply by stopping supplementation. Many patients consume 10000IU on a chronic basis and to date no reports of toxic effects at that dose have appeared in the world literature. (1)

Toxic effects have only been seen when doses of 40,000IU per day have been used over prolonged periods (25)

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   Vitamin D Deficiency 

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