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Krishna: I wish I could answer this Q in a simple ‘yes’ or ‘no’. But Vitamin D production by the skin in the presence of sunlight is a complicated process and depends on several factors.

During exposure to sunlight, the UV B photons enter the skin and photolyze 7-dehydrocholesterol to previtamin D3 which in turn is isomerized ( change from one isomer to another) by the body's temperature to vitamin D3. Most humans have depended on sun for their vitamin D requirement. Skin pigment (melanin), sunscreen use, aging, time of day, season and latitude dramatically affect previtamin 13 synthesis (1).

The synthesis of vitamin D in skin is a two-stage process that begins with the production of previtamin D after irradiation of 7-dehydrocholesterol by ultraviolet (UV) radiation. A number of personal and environmental factors control the probability of a suitable UV photon reaching a molecule of 7-dehydrocholesterol in the skin. These are astronomical factors that govern the solar zenith angle (SZA), and the local state of the atmosphere, determining the available solar UV radiation; skin pigmentation and age, determining competing absorbers of UV radiation and available 7-dehydrocholesterol; individual behaviour in the local surroundings, determining exposure of unprotected skin to available UV radiation. The only one of these influences that can be determined unequivocally for any situation is the SZA. The other influences must be considered either as individual case studies, or be represented by "typical" and "idealised" situations for the weather, skin and behaviour. At large SZAs there is insufficient solar UV radiation to initiate significant vitamin D synthesis. At smaller SZAs assessment of solar exposure necessary for vitamin D synthesis can only be indicative and application of any such assessment necessarily requires awareness of both self- and the local environment (2).

Image source: Boston University

Exposure to sunlight continues to play a major role in providing adequate vitamin D nutrition for most of the population of the world, including those who live in countries that practice fortification of dairy, margarine, and cereal products with vitamin D. During exposure to sunlight, the high-energy UV photons (290-315 nm) penetrate the epidermis and photolyze 7-dehydrocholesterol (provitamin D3) to previtamin D3. Once formed, previtamin D3 undergoes a thermally induced isomerization to vitamin D3 that takes 2-3 days to reach completion. Melanin effectively competes with provitamin D3 for the UV radiation that enters the epidermis and limits its photolysis to previtamin D3. However, this is not the major factor that prevents excess production of vitamin D in the skin of people who are constantly exposed to sunlight. During the initial exposure to sunlight, provitamin D3 is efficiently converted to previtamin D3. However, because previtamin D3 is photolabile, continued exposure to sunlight causes the isomerization of previtamin D3, principally to lumisterol. Thus, no more than 10-20% of the initial provitamin D3 concentrations ultimately end up as previtamin D3. These things are affected by aging, sunscreens, seasonal changes, time of day, ozone pollution, and latitude (3).

In the Northern hemisphere, exposure to sunlight during the months of November through February will not produce any significant amounts of cholecalciferol in the skin.

Because windowpane glass absorbs ultraviolet B radiation, exposure of sunlight through glass windows will not result in any production of cholecalciferol. It is now recognized that vitamin D insufficiency and vitamin D deficiency are common in elderly people, especially in those who are infirm and not exposed to sunlight or who live at latitudes that do not provide them with sunlight-mediated cholecalciferol during the winter months (4).

A study was conducted to determine the levels of UV radiation in relation to Vitamin D3 induction with six commonly encountered shade environments for the larger solar zenith angles observed during autumn and winter. Spectral UV irradiance measurements were made under relatively clear sky conditions at a sub-tropical Southern Hemisphere site for six specific shade environments and solar zenith angle between 35 degrees and 60 degrees to investigate the biologically effective UV irradiances for pre-Vitamin D3 production. Data from this research indicates that pre-Vitamin D3 effective UV wavelengths in the shade were most significant for tree shade and a shade umbrella. Compared to that in full sun, pre-Vitamin D3 effective UV wavelengths were at levels of approximately 52 and 55%, respectively, beneath the shade umbrella and in tree shade. UVB irradiance levels in the shade of a northern facing covered veranda and in a car with windows closed were significantly less than those beneath the shade umbrella, with levels of approximately 11 and 0%, respectively, of those in full sun (5).

So the right answer to you question is ‘it depends’. On several factors.

Sunlight doesn’t actually “provide” you with vitamin D. Rather, your body produces vitamin D when skin is exposed to the sun’s ultraviolet rays, which trigger vitamin D synthesis. The liver and kidneys convert this biologically inert form of vitamin D into biologically active forms the body can use to promote calcium absorption and bone health.

But sunlight consists of both ultraviolet A, or UVA, which penetrates deep within the skin layers and can cause premature aging; and ultraviolet B, or UVB, which causes the redness of sunburn. It’s the UVB rays that trigger the synthesis of vitamin D.

Exposure to sunlight through glass results in almost nil production of Vitamin D. Virtually all commercial and automobile glass blocks UVB rays. As a result, you will not be able to increase your vitamin D levels by sitting in front of a sunny window, though much of the UVA radiation will penetrate the glass and may be harmful.

In cloudy, hazy conditions sunlight will be inadequate for the skin to produce Vitamin D

Diffused sunlight through other media gives mixed results.

If your skin contains high melanin pigment, if you are old, again the results will be disappointing.

So those concerned about low vitamin D levels can get more of the vitamin through foods. The best dietary source for vitamin D is cod liver oil. Other dietary sources include swordfish and salmon and, to a lesser extent, fortified milk, orange juice and yogurt, as well as sardines canned in oil, egg yolks and fortified cereals.

Footnotes:

  1. Sunlight, ultraviolet radiation, vitamin D and skin cancer: how muc...
  2. Who, what, where and when-influences on cutaneous vitamin D synthes...
  3. Photosynthesis of vitamin D in the skin: effect of environmental an...
  4. Environmental factors that influence the cutaneous production of vi...
  5. http://DOI: 10.1016/j.jsbmb.2005.04.039

Q: How'd we get vitamin D during cloudy days? What should Invalides do for D /D3 vitamin apart chemical supplements?

Krishna: 

In cloudy, hazy conditions sunlight will be inadequate for the skin to produce Vitamin D.

Yes, you can use UV light to treat Vitamin D apart from using supplements (1).

Conclusion of the research studies : “A UV lamp that emits ultraviolet radiation similar to sunlight and thus produces vitamin D3 in the skin is an excellent alternative for CF, and SBS patients who suffer from vitamin D deficiency due to fat malabsorption, especially during the winter months when natural sunlight is unable to produce vitamin D3 in the skin. This UV lamp is widely available for commercial home use and could potentially be prescribed to patients with CF or SBS”.

Footnotes:

  1. Treatment of vitamin D deficiency with UV light in patients with ma...

Q: Can we use uv c bulbs for indoor source of vit D for people with little or no mobility?

Krishna: 

 The answer is ‘yes, you can use UV light to treat Vitamin D apart from using supplements ‘(1).

Conclusion of the research studies : “A UV lamp that emits ultraviolet radiation similar to sunlight and thus produces vitamin D3 in the skin is an excellent alternative for CF, and SBS patients who suffer from vitamin D deficiency due to fat malabsorption, especially during the winter months when natural sunlight is unable to produce vitamin D3 in the skin. This UV lamp is widely available for commercial home use and could potentially be prescribed to patients with CF or SBS”.

Footnotes:

  1. Treatment of vitamin D deficiency with UV light in patients with ma...

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