question: Why does fair skin help people in environments with low sun absorb Vitamen D when the color white reflects light while the color black absorbs light? Does skin work differently?
comment: Sunlight isn't beaming vitamin D into the skin, it is catalyzing the generation of the vitamin from precursors within the cells of the skin. Pigment in the skin blocks UV light before it can penetrate deep enough to cause that reaction as well as damage skin cell DNA. The greater absorption of light of dark pigmented skin then doesn't imply an increase in the vitamin D production but rather a decrease.
passages: With the increase of vitamin D synthesis, there is a decreased incidence of conditions that are related to common vitamin D deficiency conditions of people with dark skin pigmentation living in environments of low UV radiation: rickets, osteoporosis, numerous cancer types (including colon and breast cancer), and immune system malfunctioning. Vitamin D promotes the production of cathelicidin, which helps to defend humans' bodies against fungal, bacterial, and viral infections, including flu. When exposed to UVB, the entire exposed area of body’s skin of a relatively light skinned person is able to produce between 10 - 20000 IU of vitamin D.
   Whilst not all of these genes directly affect melanin production, most of them code for proteins that may play a significant role in melanogenesis and control melanin concentration. Some of these genes are found to be more prevalent in certain population than others.
  
  Section::::Health implications.
  
  Skin pigmentation is an evolutionary adaptation to the various UV radiation levels around the world. There are health implications of light-skinned people living in environments of high UV radiation. Various cultural practices increase problems related to health conditions of light skin, for example sunbathing among the light-skinned.
   Dark skin offers great protection against UVR because of its eumelanin content, the UVR-absorbing capabilities of large melanosomes, and because eumelanin can be mobilized faster and brought to the surface of the skin from the depths of the epidermis. For the same body region, light- and dark-skinned individuals have similar numbers of melanocytes (there is considerable variation between different body regions), but pigment-containing organelles, called melanosomes, are larger and more numerous in dark-skinned individuals. Keratocytes from dark skin cocultured with melanocytes give rise to a melanosome distribution pattern characteristic of dark skin. Melanosomes are not in aggregated state in darkly pigmented skin compared to lightly pigmented skin. Due to the heavily melanised melanosomes in darkly pigmented skin, it can absorb more energy from UVR and thus offers better protection against sunburns and by absorption and dispersion UV rays. Darkly pigmented skin protects against direct and indirect DNA damage. Photodegration occurs when melanin absorbs photons. Recent research suggest that the photoprotective effect of dark skin is increased by the fact that melanin can capture free radicals, such as hydrogen peroxide, which are created by the interaction of UVR and layers of the skin. Heavily pigmented melanocytes have greater capacity to divide after UVR irradiation, which suggests that they receive less damage to their DNA. Despite this, UVB damages the immune system even in darker skinned individuals due to its effect on Langerhans cells. The stratum corneum of people with dark or heavily tanned skin is more condensed and contains more cornified cell layers than in lightly pigmented humans. These qualities of dark skin enhance the barrier protection function of the skin.
   Humans with light skin pigmentation have skin with low amounts of eumelanin, and possess fewer melanosomes than humans with dark skin pigmentation. Light skin provides better absorption qualities of ultraviolet radiation. This helps the body to synthesize higher amounts of vitamin D for bodily processes such as calcium development. Light-skinned people who live near the equator with high sunlight are at an increased risk of folate depletion. As consequence of folate depletion, they are at a higher risk of DNA damage, birth defects, and numerous types of cancers, especially skin cancer.
   Endogenous defense mechanisms provide protection of the skin from damages induced by UV.
  
  Section::::Defense Mechanisms.:Epidermal thickness.
  
  UV exposure which would lead to an increase in epidermal thickness could help protect from further UV damage.
  
  Section::::Defense Mechanisms.:Pigment.
  
  It has been reported in many cases that fairer individuals who have lesser melanin pigment show more dermal DNA photodamage, infiltrating neutrophils, keratinocyte activation, IL-10 expression and increased MMPs after UV exposure. Therefore, the distribution of melanin provides protection from sunburn, photoaging, and carcinogenesis by absorbing and scattering UV rays, covering the skin lower layers and protecting them from the radiation. 
   Section::::Health implications.:Disadvantages of light pigmentation in high sunlight environments.
   Section::::Health implications.:Advantages of light skin pigmentation in low sunlight environments.
   For the same body region, individuals, independently of skin colour, have the same amount of melanocytes (however variation between different body parts is substantial), but organelles which contain pigments, called melanosomes, are smaller and less numerous in light-skinned humans.
   Dark-skinned people living in low sunlight environments have been recorded to be very susceptible to vitamin D deficiency due to reduced vitamin D synthesis. A dark-skinned person requires about six times as much UVB than lightly pigmented persons. This is not a problem near the equator; however, it can be a problem at higher latitudes. For humans with dark skin in climates of low UVR, it can take about two hours to produce the same amount of vitamin D as humans with light skin produce in 15 minutes. Dark-skinned people having a high body-mass index and not taking vitamin D supplements were associated with vitamin D deficiency. Vitamin D plays an important role in regulating the human immune system and chronic deficiencies in vitamin D can make humans susceptible to specific types of cancers and many kinds of infectious diseases. Vitamin D deficiency increases the risk of developing tuberculosis fivefold and also contributes to the development of breast, prostate, and colorectal cancer. The most prevalent disease to follow vitamin D deficiency is rickets, the softening of bones in children potentially leading to fractures and deformity. Rickets is caused by reduced vitamin D synthesis that causes an absence of vitamin D, which then causes the dietary calcium to not be properly absorbed. This disease in the past was commonly found among dark-skinned Americans of the southern part of the United States who migrated north into low sunlight environments. The popularity of sugary drinks and decreased time spent outside have contributed to significant rise of developing rickets. Deformities of the female pelvis related to severe rickets impair normal childbirth, which leads to higher mortality of the infant, mother, or both. Vitamin D deficiency is most common in regions with low sunlight, especially in the winter. Chronic deficiencies in vitamin D may also be linked with breast, prostate, colon, ovarian, and possibly other types of cancers. The relationship between cardiovascular disease and vitamin D deficiency also suggest link between health of cardiac and smooth muscle. Low vitamin D levels have also been linked to impaired immune system and brain functions. In addition, recent studies have linked vitamin D deficiency to autoimmune diseases, hypertension, multiple sclerosis, diabetes, and incidence of memory loss. Outside the tropics UVR has to penetrate through a thicker layer of atmosphere, which results in most of the UVB reflected or destroyed en route; because of this there is less potential for vitamin D biosynthesis in regions far from the equator. Higher amount of vitamin D intake for dark-skinned people living in regions with low levels of sunlight are advised by doctors to follow vitamin D rich diet or take vitamin D supplements, although there is recent evidence that dark-skinned individuals are able to process vitamin D more efficiently than lighter-skinned individuals so may have a lower threshold of sufficiency.
   Some populations who had diets rich in vitamin D were less affected by the evolutionary selection for light skin. Vitamin D3 is available in low quantities in fish and liver. Populations who lived in coastal areas or areas with access to abundant sources to seafood could get their proportion of vitamin D from food. Some Arctic populations, such as the Inuit, could retain some of their skin pigmentation in areas of low UV radiation. In the spring they receive high levels of UV radiation as reflection from the snow, and their relatively darker skin protects them from the sunlight.
   BULLET::::- Oxyhemoglobin: It is also found in blood and is not a pigment of the skin. It develops a red color.
  
  There is a correlation between the geographic distribution of UV radiation (UVR) and the distribution of indigenous skin pigmentation around the world. Areas that highlight higher amounts of UVR reflect darker-skinned populations, generally located nearer towards the equator. Areas that are far from the tropics and closer to the poles have lower concentration of UVR, which is reflected in lighter-skinned populations.
   People living far from the equator developed light skin to produce adequate amounts of vitamin D during winter with low levels of UV radiation. Genetic studies suggest that light-skinned humans have been selected for multiple times.
   In the 1960s, biochemist W. Farnsworth Loomis suggested that skin colour is related to the body’s need for vitamin D. The overwhelming positive effect of UV radiation in land-living vertebrates is the ability to synthesize vitamin D3 from it. A certain amount of vitamin D which penetrates the skin helps the body to absorb more calcium which is essential for building and maintaining bones, especially for developing embryos. Vitamin D production depends on exposure to sunlight. Humans living at latitudes far from the equator developed light skin in order to help absorb more vitamin D. People with light (type II) skin can produce previtamin D3 in their skin at rates 5–10 times faster than dark-skinned (type V) people. People living far from the equator were under evolutionary pressure to develop light skin, which allowed more penetration of UV radiation and helped to produce more of the essential vitamin D. The evolution of blond hair in some populations is related to the development of light skin, but in other populations is independent.
   Section::::Health implications.
  
  Skin pigmentation is an evolutionary adaptation to various UVR levels around the world. As a consequence there are many health implications that are the product of population movements of humans of certain skin pigmentation to new environments with different levels of UVR. Modern humans are often ignorant of their evolutionary history at their peril. Cultural practices that increase problems of conditions among dark-skinned populations are traditional clothing and vitamin D-poor diet.
  
  Section::::Health implications.:Advantages of dark skin pigmentation in high sunlight environments.
   Another group of hypotheses contended that dark skin pigmentation developed as antibacterial protection against tropical infectious diseases and parasites. Although it is true that eumelanin has antibacterial properties, its importance is secondary as a physical absorbed to protect against UVR induced damage. This hypothesis is not consistent with the evidence that most of the hominid evolution took place in savanna environment and not in tropical rainforests. Humans living in hot and sunny environments have darker skin than humans who live in wet and cloudy environments. The antimicrobial hypothesis also does not explain why some populations (like the Inuit or Tibetans) who live far from the tropics and are exposed to high UVR have darker skin pigmentation than their surrounding populations.
   Not all the effects of pigmentation are advantageous. Pigmentation increases the heat load in hot climates, and black people absorb 30% more heat from sunlight than do white people, although this factor may be offset by more profuse sweating. In cold climates black skin entails more heat loss by radiation. Pigmentation also hinders synthesis of vitamin D, so that in areas of poor nutrition black children are more liable to rickets than white children. Since pigmentation appears to be not entirely advantageous to life in the tropics, other hypotheses about its biological significance have been advanced, for example a secondary phenomenon induced by adaptation to parasites and tropical diseases.
   An abundance of clinical and epidemiological evidence supports that light skin pigmentation developed due to the importance of maintaining vitamin D3 production in the skin. As a consequence, there must have been a strong selective pressure for the evolution of light skin in areas of low UV radiation. The evidence that dark skin evolved as a protection against the effect of UV radiation is overwhelming, and research shows that eumelanin protects against both folate depletion and direct damage to DNA. This accounts for the development of dark skin pigmentation of people living near the equator but does not account for the increasingly lighter-skinned people living outside the tropics.
   Migration of humans out of Africa to higher latitudes involved less exposure to sunlight. Since UVA and UVB rays are crucial for the synthesis of vitamin D, which regulates calcium absorption and thus is essential for bone health, living at higher latitudes would mean a substantial reduction in vitamin D synthesis. This put a new selective pressure on skin colour trait, favouring lighter skin colour at higher latitudes.
   Section::::Health implications.:Disadvantages of dark skin pigmentation in low sunlight environments.
   The dermis is thinner in whites than in other races; the exposed skin is vulnerable to sunburn because of the lower amount of melanin in the skin than in other races. These traits cause problems in warm climates, but the nearly transparent skin allows more sunlight to reach the inner layers of the epidermis, thereby increasing Vitamin D production far above the level found in other racial groups. A study of skin cultured from the hip region of Europeans and Africans living in Nigeria showed that European skins allow penetration of between 3 and 4 times as much UV radiation incident upon the skin.
   Data collected from studies on "MC1R" gene has shown that there is a lack of diversity in dark-skinned African samples in the allele of the gene compared to non-African populations. This is remarkable given that the number of polymorphisms for almost all genes in the human gene pool is greater in African samples than in any other geographic region. So, while the "MC1R"f gene does not significantly contribute to variation in skin colour around the world, the allele found in high levels in African populations probably protects against UV radiation and was probably important in the evolution of dark skin.
   Section::::Evolution.:Earlier hypotheses.
  
  There were two other main hypotheses that have been put forward to explain the development of light skin pigmentation: resistance to cold injury, and occurrence of a mutation effect; now both of them are considered unlikely to be the main mechanism behind the evolution of light skin.
   Section::::Biochemistry.
   A 1978 study examined the effect of sunlight on folate – a vitamin B complex – levels. The study found that even short periods of intense sunlight are able to halve folate levels if someone has light skin. Low folate levels are correlated with neural tube defects, such as anencephaly and spina bifida. UV rays can strip away folate, which is important to the development of healthy foetuses. In these abnormalities children are born with incomplete brain or spinal cord. Nina Jablonski, a professor of anthropology and expert on evolution of human skin coloration, found several cases in which mother's visits to tanning studios were connected to neural tube defects in early pregnancy. She also found that folate was crucial to sperm development; some male contraception drugs are based on folate inhibition. It has been found that folate may have been the driving force behind the evolution of dark skin.
   Dark-skinned people who live in high latitudes with mild sunlight are at an increased risk – especially in the winter – of vitamin D deficiency. As a consequence of vitamin D deficiency, they are at a higher risk of developing rickets, and numerous types of cancers, and possibly cardiovascular disease and low immune system activity. However, some recent studies have questioned if the thresholds indicating Vitamin D deficiency in light-skinned individuals are relevant for dark-skinned individuals, as they found that, on average, dark-skinned individuals have higher bone density and lower risk of fractures than lighter-skinned individuals with the same levels of Vitamin D. This is attributed as, possibly, due to lower presence of Vitamin D binding agents (and thus higher bioavailability) in dark-skinned individuals.
  
presuppositions: Fair skin should lower the vitamen D that you get.  Vitamin D is absorbed into the skin through sunlight.
corrections: Fair skin makes it easier to absorb vitamen D than a darker skined person.  The generation of Vitamin D is catalyzed by sunlight.
raw_presuppositions: Fair skin should lower the vitamen D that you get.  Vitamin D is absorbed into the skin through sunlight.
raw_corrections: Fair skin makes it easier to absorb vitamen D than a darker skined person.  The generation of Vitamin D is catalyzed by sunlight.