Vitamin d review Научные исследования Октябрь, 2014 Солнечный свет и витамин D: глобальная перспектива для здоровья

Life forms began to evolve in the oceans over 1 billion years ago. They took advantage of sunlight and used it as an energy source to generate carbohydrates. Curiously some of the earliest phytoplankton including Emiliania huxleyi (which is a coccolithophore, i.e., has a calcium carbonate exoskeleton) which has existed unchanged in the Sargasso Sea (Atlantic Ocean) for more than 500 million years when exposed to sunlight not only photosynthesized glucose but also produced vitamin D₂ (Fig. 1).¹ This phytoplankton produces a large amount of ergosterol that when exposed to sunlight absorbs ultraviolet B (UVB) radiation and undergoes a photolysis reaction to form previtamin D₂. Once formed this thermodynamically unstable isomer is transformed into vitamin D₂. Similarly yeast and fungi also contain high amounts of ergosterol and when exposed to sunlight produce vitamin D₂.^1-4

Although the functions of ergosterol and vitamin D₂ are unknown in these primitive unicellular photosynthesizing factories there are at least three possible functions that have been proposed. Ergosterol can efficiently absorb UVB radiation, which would make it an ideal natural sunscreen to protect UVB sensitive macromolecules in the organism including its proteins, RNA and DNA (Fig. 2).^1,2

After absorbing UVB radiation previtamin D₂ is produced. Its absorption spectrum with a wavelength maximum at 260 nm overlaps the UV absorption spectrum for both DNA and RNA and thus would be able to protect DNA and RNA from photodamage (Fig. 2).² When previtamin D₂ is exposed to UVB radiation it is converted to tachysterol₂ which has a UV absorption spectrum with a wavelength maximum at 282 nm which overlaps the UV absorption spectrum for amino acids in proteins that have conjugated double bonds including tryptophan and tyrosine (Fig. 1 and ​and22).^1-4 Thus early in evolution as organisms began to utilize solar energy for photosynthesis they needed a sun protection factor to absorb solar UVB radiation to minimize damage to UVB sensitive molecules. Ergosterol, previtamin D₂, and its photoproducts could have acted as an ideal UVB sunscreen since they could absorb UVB radiation and dissipate its energy by the rearrangement of the double bonds.²

The amount of previtamin D₂ and photoproducts produced during sun exposure could also have been a photochemical signal (actinometer) to tell the organism that it has been exposed to enough solar UVB radiation and to signal it to leave the surface into deeper water where it would no longer be exposed to UVB radiation due to the ocean’s ability to absorb this solar energy.²

It has also been speculated that if ergosterol was principally present in the plasma membrane and contained within the lipid bilayer that this rigid planar structure after exposure to solar UVB radiation would be transformed into a more flexible vitamin D₂ molecule that would likely be released into the extracellular space. This process could alter membrane permeability and possibly open up a pore to permit the entrance and exit of ions including calcium. This could be the connection for why vertebrates including humans have depended on sun exposure for the maintenance of their calcium metabolism.^2,5-7

As the industrial revolution swept across Northern Europe in the 1600s resulting in buildings built in close proximity and coal burning causing a pall of air pollution (Fig. 3) so too appeared a bone deforming disease rickets in children that had devastating health consequences (Fig. 4).^8,9

The first insight into the possible relationship between the industrialization of Northern Europe and rickets was made by Śniadecki¹⁰ (Fig. 5A) in 1822 when he concluded that children who lived in the inner city of Warsaw had a high incidence of rickets because of their lack of sun exposure. This was based on his clinical observations that children living in rural areas outside of Warsaw did not suffer from rickets while children born and raised in Warsaw were plagued with the disease. More than 70 y later Palm¹¹ came to the same conclusion based on reports from his colleagues in third world countries including India and China that rickets was uncommon compared with the high prevalence of the disease in children living in London. Another 30 y would pass before Huldschinsky^12,13 (Fig. 5B) would report that rachitic children exposed to a mercury arc lamp had dramatic radiologic improvement of their rickets several months later. He cleverly also realized that exposing one arm of a child with rickets had the same dramatic radiologic improvement in the forearm of the arm not exposed to the mercury arc lamp. He therefore correctly concluded that it was likely that something was being made in the skin and entered into the circulation to improve the global bone health of the child (Fig. 6).^12,13 Finally in 1921 Hess and Unger^8,14 (Fig. 5C) exposed rachitic children to sunlight on the roof of their hospital in New York City and demonstrated significant radiologic improvement in the children’s rickets. These physicians also realized that children of color were at much higher risk for rickets and concluded that they needed longer exposure to sunlight to both treat and prevent rickets.

By turn of the 20th century it was estimated that upwards of 80–90% of children living in Northern Europe and in Northeastern United States had evidence of rickets.⁸ Steenbock and Black¹⁵ (Fig. 5D) and Hess and Weinstock¹⁶ exposed various foods including cotton seed oil, corn oil, and milk to UVB radiation and demonstrated that this process imparted antirachitic activity for rodents. This led to the addition of ergosterol to milk followed by UVB irradiation or the addition of ergosterol that had been previously exposed to UVB radiation or to the addition of vitamin D₂ to the milk (Fig. 7).⁸

This process was quickly embraced by the dairy processors and in the early 1930s essentially all milk in the United States and in most industrialized countries including Great Britain and other European countries had vitamin D fortified milk. The United States government also established an agency in 1931 whose goal was to promote sensible sun exposure of young children to prevent rickets and improve their bone health (Fig. 8).^9,14 Within a few years these interventions essentially eradicated rickets.^8,17-19

Vitamin D became so popular that in the 1930s and 1940s a wide variety of foods and beverages as well as personal care products were fortified with vitamin D.¹⁹ They included not only milk and other dairy products but also soda pop, beer, hot dogs, custard and even soap and shaving cream (Figs. 9A-D).¹⁹

However in the early 1950s an outbreak of hypercalcemia in infants who had elfin faces, heart problems, and mental retardation led to an investigation by the Royal College of Physicians. The experts concluded that this was most likely due to vitamin D intoxication since a similar presentation had been observed in neonatal rodents born of mothers who were fed high doses of vitamin D.^20-22 Legislation quickly followed banning the fortification of any food or personal use products with vitamin D in Great Britain.^8,17,22 This ban quickly spread across Europe and for the most part remains in effect today with the exception of a few foods including margarine and some cereals being fortified with vitamin D.^8,19-22 It is however likely that these children had Williams syndrome which is associated with elfin faces, mental retardation, heart problems, and hypercalcemia due to a hypersensitivity to vitamin D.²³ Sweden and Finland now permit milk to be fortified with vitamin D.^22,24 It is worth noting that milk has been fortified with 100 IU vitamin D/8 oz for the past 80+ years without any reports of toxicity in infants. In the past 10 y juice products including orange juice have been fortified in the United States with 100 IU vitamin D/8 oz without any reports of toxicity.²²

Photochemistry of Provitamin D₃

During exposure to sunlight solar radiation with wavelengths of 290–315 nm penetrate into the skin and are absorbed by proteins, DNA and RNA as well as 7-dehydrocholesterol.^1,2 Most of this UVB radiation is absorbed in the epidermis and as a result when exposed to sunlight most of the vitamin D₃ that is produced in the skin is made in the living cells in the epidermis. This is the reason why after exposure to sunlight vitamin D₃ remains in the skin even when the skin is washed with soap and water immediately after the exposure to sunlight.

When epidermal 7-dehydrocholesterol absorbs solar UVB radiation with energies of 290–315 nm (Fig. 10), it causes an activation of the double bonds causing them to rearrange and open up the B ring to form the seco-steroid (split steroid) previtamin D₃ (Fig. 11).²⁵

Previtamin D₃ when made in a test tube exists in two confomeric forms, a 5,6-sec-cis-s-cis (czc) and a 5,6-sec-trans-s-cis (czt) form. The czt conformer is the most thermodynamically stable form and thus most of the previtamin D₃ when produced in a test tube exists as this conformer. This conformer however being stable cannot isomerize to vitamin D₃. However the less thermodynamically stable conformer czc does undergo a 1–7 antarafacial sigma shift of a hydrogen from C-19 to C-9 causing rearrangement of the double bonds to form vitamin D₃. As a result it takes several days in a test tube at room temperature and even at body temperature for the czt conformer to isomerize to the czc conformer which then is converted to vitamin D₃ (Fig. 12).^6,7,26,27

From a physiologic perspective it made little sense for it to take several days for previtamin D₃ to convert to vitamin D₃ in the skin as it does in a test tube. Furthermore this was a significant problem for poikilothermic (cold blooded) vertebrates since a lower outside temperature would make it take a much longer time for previtamin D₃ that was produced in the skin to convert to vitamin D₃. Studies of lizard skin exposed to simulated sunlight revealed that the conversion of previtamin D₃ to vitamin D₃ was 10 times faster when compared with previtamin D₃ in a isotropic organic solution (Fig. 12).⁶ A similar observation was made in human skin (Fig. 13).⁷

This suggested that the skin had some property that accelerated the conversion of previtamin D₃ to vitamin D₃. One possible explanation was that an enzyme existed that catalytically converted previtamin D₃ to vitamin D₃. Skin homogenates incubated with previtamin D₃ did not result in an enhancement in its conversion to vitamin D₃. Thus another explanation needed to be found. It was hypothesized and finally proven with several studies that 7-dehydrocholesterol in the skin cell principally existed in the plasma membrane and was incorporated within the fatty acid hydrocarbon side chain and polar head group of the triglycerides (Figs. 11 and ​and14).14). The rigid planar structure of 7-dehydrocholesterol sandwiched in between the triglyceride fatty acid hydrocarbon tails could only be transformed into the planar czc conformer of previtamin D₃ upon exposure to solar UVB radiation (Fig. 11).^6,26-28

Once formed, this unstable conformer rapidly converted to vitamin D₃.^6,7,26-28 To confirm this hypothesis studies were done in liposomes that mimicked the plasma membrane and it was demonstrated that adding double bonds to the triglyceride’s side chain or shortening or lengthening of the side chain resulted in a decrease in the kinetics for the conversion of previtamin D₃ to vitamin D₃ (Fig. 15).²⁷

Because vitamin D₃ is thermodynamically more stable and also more flexible it is ejected out of the plasma membrane into the extracellular space and diffuses into the capillary bed in the dermis where it is bound to the vitamin D binding protein (DBP) for transport to the liver.^{2,5,6,22,29,30}

There has been a lot of discussion as to whether ingesting vitamin D from the diet or from a supplement is the same as producing vitamin D₃ in the skin. Because it takes approximately ~8 h for previtamin D₃ in the skin to fully convert to vitamin D₃^26,27 and it takes additional time for the vitamin D₃ to enter the dermal capillary bed this is at least 2 of the explanations for why it was observed that vitamin D₃ produced in the skin last 2–3 times longer in the circulation when compared with ingesting it orally (Fig. 16).²⁹ Furthermore when vitamin D₃ is produced in the skin 100% of it is potentially bound to the vitamin D binding protein (Fig. 17). When vitamin D₃ is ingested from the diet or supplement it gets incorporated into chylomicrons which are transported into the lymphatic system and then into the venous system were approximately 60% of the vitamin D₃ is bound to the vitamin D binding protein and 40% is rapidly cleared in the lipoprotein bound fraction.²⁹

Sun Controlled Cutaneous Production of Vitamin D₃

During exposure to sunlight after previtamin D₃ is produced it will absorb solar UVB radiation and isomerize into two major photoproducts, lumisterol₃ and tachysterol₃ (Fig. 17).^31,32

Neither of these two photoproducts has any effect on calcium metabolism.³² Thus when the skin is exposed to sunlight it can only convert approximately 15% of 7-dehydrocholesterol to previtamin D₃ (Fig. 18).³² Any further exposure will result in a photoequilibrium whereby previtamin D₃ is converted into lumisterol₃ and tachysterol₃ as well as revert back to 7-dehydrocholesterol (Fig. 17). In addition when vitamin D₃ is made from previtamin D₃ in the skin if it is exposed to solar UVB radiation it will absorb UVB radiation and be converted into several suprasterols and 5,6-trans-vitamin D₃ (Figs. 17 and ​and19).19). In addition previtamin D₃ can also be converted to several toxisterols (Fig. 20).^33-36 Therefore no matter how much sun a human is exposed to vitamin D intoxication will not occur because any excess previtamin D₃ and vitamin D₃ is photodegraded into products that have no calcemic activity.^31,32

This however does not mean that these myriad of photoproducts don't have other biologic effects such as regulating epidermal cell growth and reducing risk of skin cancer. One product, lumisterol_3, if converted to 1,25-dihydroxylumisterol₃ may have anti-tumor effects in the skin.³⁷ Some of the suprasterols also have antiproliferative activity in cultured human keratinocytes (Fig. 21). Therefore sensible sun exposure to produce previtamin D₃, vitamin D₃ and its photoproducts may have some additional benefits above and beyond simply taking a vitamin D₃ supplement or ingesting vitamin D₃ from dietary sources.

Only about one percent of solar UVB radiation ever reaches the earth’s surface even in the summer at noon time.³⁸ The reason is that all of the UV C (200–280 nm) and all of the UVB radiation up to approximately 290 nm is efficiently absorbed by the stratospheric ozone layer.^38,39 In addition the ozone layer absorbs approximately 99% of the UVB radiation with wavelengths 291–320 nm. Therefore increasing the path length by which solar UVB has to travel through the ozone layer will result in a decrease in the number of UVB photons that reach the earth's surface (Fig. 22).⁴⁰ This is the explanation for why during the winter when living above and below approximately 33° latitude very little if any vitamin D₃ can be produced in the skin from sun exposure. People who live farther North and South often cannot make any vitamin D₃ in their skin for up to 6 mo of the year.⁴¹ For example in Boston at 42° North essentially no vitamin D₃ can be produced in the skin from November through February. Inhabitants living in Edmonton Canada at 52° North, Bergen Norway at 60° North, or Ushuaia Argentina at 55° South are unable to produce any significant vitamin D₃ for about 6 mo of the year (Figs. 23 and ​and2424).^2,39,41

People who live in the far Northern and Southern hemispheres had apparently appreciated this fact and were able to satisfy their vitamin D requirement by eating vitamin D rich foods including oily fish, seal blubber, polar bear liver and whale blubber and liver all of which contain large amounts of vitamin D₃.^42,43

In the early morning and late afternoon the zenith angle of the sun is also more oblique similar to winter sunlight and as a result very little if any vitamin D₃ can be produced in the skin before 10 a.m. and after 3 p.m. even in the summer time (Figs. 23 and ​and2525).⁴⁴

Air pollution including nitrous oxide and ozone is common in many large cities including Los Angeles and San Diego (Fig. 26) and will absorb solar UVB radiation and therefore reduce the effectiveness of sun exposure in producing vitamin D₃ in the skin (Fig. 27).^40,45 The amount of UVB radiation available for cutaneous vitamin D₃ production is markedly reduced by the increase of sulfur dioxide in San Diego and Los Angeles, offsetting the fact that both cities are at lower latitudes.⁴⁵

Altitude can also have a dramatic influence on the amount of solar UVB that reaches the earth’s surface because the higher the altitude the shorter the path length that UVB has to travel through and thus the skin can produce more vitamin D₃. This was dramatically demonstrated in Agra (169 M altitude), Katmandu (1400 M), and Mount Everest base camp (5300 M), India (27° North). An analysis of sun-induced vitamin D₃ synthesis in vitro was conducted at higher altitudes at the same latitude during the same month. In November in Agra very little previtamin D₃ was produced during exposure to the sun. It was observed that there was a direct correlation with increased previtamin D₃ production with increased altitude. At Mt Everest base camp (5300 min) there was almost a 5-fold increase in previtamin D₃ production compared with what was observed in Agra (Fig. 28).⁴⁶ Since glass absorbs all UVB radiation, exposure of the skin to sunlight that passes through glass, plexiglass, and plastic will not result in any production of vitamin D₃ in the skin (Fig. 29).³¹

Sunscreens were designed to absorb solar UVB radiation.⁴⁷ A sunscreen with a sun protection factor (SPF) of 30 absorbs approximately 95–98% of solar UVB radiation. Therefore the topical application of a sunscreen with an SPF of 30 reduces the capacity of the skin to produce vitamin D₃ by the same amount i.e., 95–98%.²² This was confirmed with the report that the application of sunscreen with a SPF of only 8 dramatically reduced the blood level of vitamin D₃ after exposure to simulated sunlight in a tanning bed (Fig. 30).^47,48 Farmers in the Midwest who had a history of non-melanoma skin cancer and who wore a sunscreen all the time before going outdoors for more than a year demonstrated that at the end of the summer their blood levels were significantly lower (most were vitamin D deficient) than the levels of the control group (Fig. 31).⁴⁸

Skin Pigment

Humans evolved at the equator. They were constantly exposed to sunlight and developed an efficient natural sunscreen melanin,⁴⁹ which has an absorption spectrum of 290–700 nm and thus can effectively absorb solar UVB radiation (Fig. 32).⁴⁵ However even though Africans have extremely dark heavily pigmented skin a small amount of UVB radiation is able to penetrate into the epidermis to produce vitamin D₃. This was demonstrated when adult whites (skin type 2) and blacks (skin type 5) were exposed to the same amount of UVB radiation in a tanning bed. Whereas the white adults raised their blood levels of vitamin D₃ more than 30 fold the black adults demonstrated no significant increase in their blood levels of vitamin D₃. However when the black adults were exposed to 5 times more UVB radiation, they increased their blood level by about 15-fold (Fig. 33).⁵⁰ This was confirmed when surgically obtained white and black skin was exposed to sunlight in Boston in summer. After 30 min approximately 3% of cutaneous 7-dehydrocholesterol was converted to previtamin D₃ in the white skin sample whereas only about 0.3% of 7-dehydrocholesterol was converted to previtamin D₃ in the black skin (Fig. 34).⁵¹ These findings could explain the positive association between skin lightness and 25-hydroxyvitamin D [25(OH)D] levels as found by Armas et al.⁵² (Fig. 35). The associations between skin lightness, UVB dose and 25(OH)D are documented in Figure 36.

Skin pigmentation or the lack thereof was important in the evolution of humans as they migrated North and South of the equator. Africans such as the Maasai (Fig. 37) living outdoors exposed to sunlight daily throughout the year have robust circulating concentrations of the major circulating form of vitamin D, 25(OH)D, in the range of 46 ng/mL.⁵³

Although there have been several explanations for why skin pigment devolved as humans migrated North and South of the equator one of the most likely explanations is as humans migrated farther North and South of the equator the zenith angle of the sun increased resulting in a decrease in the amount of solar UVB radiation reaching the earth thereby reducing vitamin D₃ synthesis. A decrease in the amount of skin pigment resulted in a decrease in the sun screening protection permitting more of the UVB radiation to reach the epidermal cells. This provided an evolutionary advantage by being more efficient in producing vitamin D₃.⁴⁹ It had long been speculated that our Neanderthal ancestors were heavily pigmented hairy creatures. This however did not make a lot of sense since heavy pigmentation and excessive hair would markedly reduce cutaneous production of vitamin D₃ which was essential for maximizing skeletal health throughout life thereby reducing risk of life-threatening fractures. However, more important is the fact that vitamin D deficiency in utero and during the first few years of life would have caused infantile rickets resulting in a flat deformed pelvis with a small pelvic outlet. Furthermore vitamin D is important for muscle function which is also crucial for birthing.^22,54 These conditions caused by vitamin D deficiency would have made it difficult for females to give birth. Therefore in order to survive and procreate skin pigmentation had to markedly decrease in order to permit more UVB photons to enter the skin to produce sufficient amounts of previtamin D_3.^54,55 Recent evidence has suggested that Neanderthals had a mutation of their melanocyte stimulating hormone receptor resulting in them being redheaded and having Celtic-like fair skin.^56,57 This is the likely explanation for why people in Northern Europe have skin types 1 and 2.