Melanin has essentially a protective function, as it protects the genome from the harmful action of ultraviolet rays. Near the nucleus of keratinocytes, it forms a sort of protective screen that acts as a filter, absorbing and repelling part of the solar radiation.
To defend itself against solar radiations, the body also initiates an auxiliary process that stimulates the proliferation of the stratum corneum, favoring the thickening of the epidermis.
The quantity and distribution of melanin in the skin also varies depending on many parameters, related to race, age (children's skin is less protected from solar radiation) and the body region (the most exposed areas are more pigmented but the they are also covered air, like the mammary areola or the anogenital areas).
Habits and living conditions also affect the distribution of melanin. In this regard, latitude is undoubtedly one of the most interesting and influential aspects.
The individuals that populate the equatorial regions generally have darker skin than the northern populations which, on the contrary, possess a particularly pale complexion. These differences are not random, but derive from the natural process of evolutionary selection, which selects and maintains only those characteristics necessary for the preservation and improvement of the species. Millennium after millennium, natural selection has benefited individuals with dark skins in tropical regions and individuals with light skins in the northern regions.
In the tropics the high presence of melanin is essential to protect the skin from damage from solar radiation (melanoma). Beyond this commonly known aspect, it seems that the most important region underlying the selection and differentiation process is linked to the ability of dark skin to prevent folate destruction, caused by strong exposure to ultraviolet rays. Folate, or vitamin B9, is extremely important during pregnancy, given that their deficiency is accompanied by a high risk of neural tube changes (an embryonic formation designed to form the canal that encloses the spinal cord). If the feeding of the pregnant woman is low in folates, the neural tube of the embryo can develop abnormally, giving rise to extremely serious pathologies such as spina bifida. Folate is also essential for male fertility, since it participates in the process of spermatogenesis, which slows down in the absence of these vitamins, causing sterility.
To sum up: UVAs, which have a high penetration power, destroy the folate in the blood, compromising the reproduction of the species. Dark skin prevents the destruction of folate caused by UV and is therefore an advantageous feature for equatorial populations.
In the Nordic countries, where solar radiations are weak, the light complexion of the skin is fundamental to guarantee an adequate synthesis of vitamin D. This vitamin is fundamental for the intestinal absorption of calcium, therefore for the health of the skeletal system.
In the Nordic countries solar radiation is less intense and, to guarantee an adequate synthesis of vitamin D, it is important that the skin is able to capture as many as possible.
QUESTION: why do the inhabitants of the Arctic areas, like the Eskimos, have rather dark skin? Probably because their diet is naturally rich in calcium (fish) and the dark color of the skin (<vitamin D) is not a limiting factor for bone health.
QUESTION: Why do women of all populations generally have lighter skin (3-4%) than men? The calcium requirement of women is higher than in men, especially during pregnancy and lactation. Recall that to facilitate the absorption of this mineral it is necessary that the skin is clear, since this feature guarantees a greater synthesis of vitamin D, with consequent increase in the absorption and fixation of calcium in the bones.
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