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Conventionally, however, it is referred to as vitamin, which we will adopt in the remainder of this text. Unlike other essential vitamins, that have to be absorbed from food, vitamin D is also produced by the body. Using ultraviolet radiation - the short-wave portion  (UV, mainly 270-290nm wavelengths) of the visible sunlight - it transforms provitamin D (or 7-dehydrocholesterol, which the body makes from cholesterol) in your skin into vitamin D; more exactly, vitamin D3, or cholecalciferol.

Efficiency of this process vary individually, mainly depending on how much of the ultraviolet radiation is absorbed by your skin pigment (in general, the darker, more pigmented skin, the more of the radiation is absorbed, and less available for vitamin D "photosynthesis").

It is sometimes stated that daily exposure of relatively small skin area (face and arms) to sunshine, direct or indirect, for an hour or so, is sufficient to satisfy your vitamin D needs. This, however, may vary individually due to the complexity of body's vitamin D utilization: vitamin D3 produced in your skin is converted by the liver into its five times more active form, 25(OH)D3, which is then converted by the kidneys into twice as active D3 form, 1.25(OH)₂D3, calcitriol.

Depending on how effective - or ineffective - are your conversion enzymes, you might need up to several times longer sun exposure to produce sufficient amounts of vitamin D.

Other than its natural form, cholecalciferol, or vitamin D3, formed in your skin exposed to sunshine (and also found in fish oil), there is also synthetic, plant-derived form of vitamin D, called ergocalciferol (vitamin D2). For decades, the two were considered near equally potent; however, recent studies (as well as some ignored older studies) showed that vitamin D3 is at least four times, and possible as much as ten times more potent. The reason may be inefficient conversion of the synthetic vitamin into more potent forms.

The main function of vitamin D is enabling your body to regulate absorption and use of calcium and phosphorus. Hence, it is indirectly involved in all important body processes dependant on these two major minerals. While vitamin D is stable and suffers only minor losses in food processing and preparation, number of foods that contain its natural form is quite limited. This, combined with insufficient sunlight exposure - common with the elderly - may put you at the risk of vitamin D deficiency, more so if you are dark-skinned. Possible consequences include impaired calcium absorption, muscle weakness and spasms, bone pain and, longer-term, rickets, osteoporosis, multiple sclerosis and even cancer. 

On the other hand, with more processed foods coming vitamin D fortified, and vitamin D supplementation widely available, excessive vitamin D intake is also possible. The body stores vitamin D, and it can become toxic if excessively accumulated. It can lead to soft tissue calcification (from increased intestinal absorption of calcium) resulting, among other, in possible joint pains, kidney damage and arteriosclerosis (hardening of the arteries), abdominal pain, nausea and high blood pressure.

Vitamin D DRI (Dietary Reference Intakes, the most recent set of dietary recommendations set by the government) for an average healthy adult is 0.005mg (5mcg) a day for both, females and males. It is given as "adequate intake" which, in general, means that there is no sufficient scientific basis to determine its "recommended intake". Since 1mcg=40I.U. of vitamin D, the adequate intake can also be given as 200 I.U. a day. It is doubled for those older than 50y of age, and tripled for those above 70y.

It is important to note that the DRI is for cholecalciferol (vitamin D3), significantly more potent form of vitamin D than ergocalciferol (vitamin D2), the more commonly used form. While at the time when the standards were set the two were considered as being of similar potency, today this would imply at least four - and up to 10 times - higher nominal DRI for vitamin D2 (i.e. 800 I.U. to 2000 I.U.).

As with most others DRI levels, the one set for vitamin D is considered to be too low; current educated opinion is that optimum intake level should be above 2000 I.U. of vitamin D3 daily, and definitely not less than 1000 I.U. More so knowing that vitamin D deficiency is rather common.

Cholecalciferol (D3) is preferred vitamin D form, since D2 is more likely to be toxic in larger quantities. What is the toxic level? Since a total body sun exposure for as little as 15-20 minutes produces approximately 10,000 I.U. of vitamin D3, it should be safe up to this intake level of D3, unless your sun exposure is unusually high. Taking up to 40,000 I.U.-100,000 I.U. of vitamin D2 equivalent, on the other hand, doesn't seem to be a good idea, due to possible - and rather probable - toxic effects.