It is a bit of a misnomer to call Vitamin D a vitamin at all. “Vitamin” implies that adequate amounts of it can be obtained naturally through the diet. Vitamin D is not actually a vitamin, but is a pre-prohormone . As a species, we evolved a photosynthetic mechanism which produces large amounts of vitamin D3 in the skin, thus making it the “sunshine vitamin.” By and large, vitamin D does not occur naturally in our food supply, but the production of Vitamin D [cholecalciferol] in the skin is astonishingly rapid: a light-skinned person sunbathing for 30 minutes midday (enough to produce slight redness) in the summer releases as much Vitamin D as individuals ingesting 10,000 to 25,000 IU of vitamin D2 - 17-41 times the RDA .
If we evolved to produce this hormone via sun exposure how do we obtain adequate amounts in the absence of sun exposure? At latitudes above 40o [e.g. New York City] there is insufficient sunlight intensity to produce adequate vitamin in the winter months, no matter how much of the body is exposed . Anything that blocks UVB light will block vitamin D production, including darker skin pigmentation; sunscreens with a sun protection of 15 reduce the skin’s ability to manufacture Vitamin D by 99% .
Furthermore, body fat absorbs vitamin D putting some populations at especially high risk, such as obese African-American individuals . In fact, Yanoff LB  found that nearly 60% of obese African Americans were deficient. Africans living near the equator, however, do not exhibit a high prevalence of vitamin D deficiency , underscoring the role of sun exposure.
Most cells in the human body have Vitamin D receptors and hundreds, if not thousands, of genes may be affected by vitamin D. These genes enhance immune function, insulin function and glucose handling, and maximize bone mineralization, among many other functions . The emerging data regarding vitamin D’s role in health prompted a recent reevaluation of vitamin D guidelines by the Institute of Medicine [IOM]. They tripled the amount of recommended dietary vitamin D from 200 to 600 IU daily, with an upper tolerable limit of 4000 IU daily . It is generally accepted that the toxicity threshold is between 10,000 – 40,000 IU daily, or serum values consistently >500 nmol/L 
Many experts agree that − in the absence of adequate sun exposure – most children and adults require 800- 1000 IU/day  from dietary sources. We know that a minimum blood level of 34 ng/ml is needed to optimize calcium absorption, thus affecting bone health  but a level of 52 ng/ml is required to reduce breast cancer risk . People working outside in the summer, as we evolved to do, have blood levels of about 50 ng/ml . Based on these and similar findings many experts recommend serum Vitamin D levels to be at the high end of the natural range, 40-70 ng/ml .
It has been suggested that the de-pigmentation of skin in far northern latitudes was a necessary evolutionary adaptation which allowed adequate Vitamin D synthesis . While much attention has been focused on the reduction of skin cancers by reducing sun exposure, little consideration has been given to the positive effects of UV radiation . Since 90% of our Vitamin D requirements are typically met with sun exposure, it is unsurprising that sun avoidance results in Vitamin D deficiency . This data has led dermatologists to recommend “sensible” sun exposure, in addition to Vitamin D supplementation . Excessive UV exposure should be avoided, particularly sun burns in childhood, and avoidance of excessive sun exposure during the summer months . It should be noted that even in summer months, the sun’s rays are oblique during the early morning and late afternoon, and are therefore almost entirely absorbed by the ozone layer. Thus, skin exposure during those hours results in very little or no Vitamin D production .
How much sun exposure is needed to maintain adequate serum Vitamin D levels depends upon a person’s skin pigmentation, the latitude, weather and time of year. Holick states that a Caucasian at a latitude similar to Boston, MA , in June, will produce adequate amounts of Vitamin D by exposing their skin to mid-day sun for 5 – 15 minutes, a few times per week. After that amount of time, application of sunscreen with a protection factor of 15 or greater is recommended to uncovered skin .
Athletes, even those who train outside, are also vulnerable to vitamin D insufficiency. In a study of 41 athletes training at high altitude - which ought to increase their exposure to UVB - 25% had sub-optimal Vitamin D status in the summer [<40 ng/ml] but 60% had insufficient vitamin D levels in the winter . American professional football players were also found to have widespread Vitamin D insufficiency in the spring. Eighty percent had Vitamin D levels below 32 ng/ml . Unsurprisingly, Vitamin D status of athletes involved in indoor sports such as dancers is especially poor. Even in sun-drenched Israel, Constantini et al. reported that 73% of young athletes were found to have vitamin D insufficiency [serum levels < 30 ng/ml]: 94% of dancers, and 67% of martial artists were Vitamin D insufficient, vs. 48% of those engaged in outdoor sports .
Low Vitamin D levels are associated with many disease states, such as osteoporosis, cancers, cardio-vascular disease , type 2 diabetes , auto-immune diseases such as multiple sclerosis, and infections [14,21, 24]. Vitamin D supplementation of 1200 IUM daily has been shown to reduce the risk of influenza by 42%, and adults with Vitamin D levels > 38 ng/ml had two-fold less risk of acute respiratory infections, and less sick days taken, compared to those with insufficient [<38 ng/ml] vitamin D levels .
Several epidemiologic reports suggest that heavily training athletes are at increased risk of illness [5,29] : athletes in heavy training are especially prone to upper respiratory infections . Impaired immune function has been noted for a few days following a bout of heavy exertion . Vitamin D monitoring and supplementation with Vitamin D3, therefore, should be considered by all athletes and especially by those such as marathoners, who are often in a state of over-training and thus at risk of infection.
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