The human skin: between sun protection and vitamin D synthesis

“The evolution of skin pigmentation in humans is related to the evolution of hairless-ness and enhanced sweating abilities in equatorial Africa, under conditions of high daytime temperatures and high ultraviolet radiation (UVR). The exposed skin on the face and on the dorsal surfaces of the hands and feet had active melanocytes capab-le of producing melanin in response to sun exposure but unexposed skin remained unpigmented. Loss of functional body hair occurred early in the evolution of the genus Homo and was associated with the evolution of an efficient whole-body cooling system based on eccrine sweating. Naked skin is more vulnerable to damage from solar radiation, including UVR, and compensation for this came from evolution of increased thickness of the epidermis and of permanent protective eumelanin (the dominant form of melanin) pigmentation to prevent the most energetic and damaging wavelengths of UVR from penetrating the body. Ultraviolet radiation damages DNA and the constituents of cell membranes, causing a toxic cascade of events that produces reactive oxygen species (ROS) and disrupts normal chemical reactions in cells. These processes are greatly attenuated by eumelanin, especially when it is present close to the surface of the skin.

Many hypotheses have been advanced to account for the evolution of dark pigmentation under high UVR regimes: 1) lowered mortality due to protection from sunburn and skin cancer; 2) enhancing survival through camouflage in poorly lit forested environments; 3) the antimicrobial properties of eumelanin in pathogen-rich environments; and 4) protection of folate metabolism against deficiencies caused by high UVR. Our research has focused on the last of these hypotheses. The mostly deleterious effects of UVR on biological systems are well known. Darkly pigmented, eumelanin-rich skin protects against much of the damage to DNA caused by UVR, and is associated with much lower rates of skin cancer than lightly pigmented skin. Increases in the incidence of melanoma in the last 50 years are the result of lightly pigmented people being exposed to more intense or longer periods of sunlight and UVR and experiencing more painful sunburns because of migration to sunny places or recreational sun-tanning, the so-called ‘vacation effect’. These conditions were not typical of our species prior to the twentieth century, when most people moved very little during their lifetimes because they lacked the means of transportation to do so. Thus, reduction of sunburn and melanoma risk probably contributed only to a minor extent in the evolution of darkly pigmented skin.

The deleterious effects of UVR are not limited to DNA, and it is probably the effects of UVR on folate metabolism that have been of greatest importance in the evolution of dark pigmentation in humans. The importance of folate in human development and health has been highlighted by studies demonstrating that interdiction of cell proliferation because of folate deficiency interferes with normal development, and causes birth defects. The role of folate deficiency in causing neural tube defects is now firmly established. Folate sufficiency is also necessary for maintenance of active spermatogenesis and in the formation of myelin and the production of many neurotransmitters including serotonin. Folate can only be obtained from foods or from supplements as folic acid. Healthy levels of folate are difficult to maintain in the body because natural food folates are unstable, suffer from low bioavailability, and tend to break down when foods are boiled or stored. Folate deficiencies can be caused by insufficient intake of folate, improper absorption of the vitamin from the gut, or the breakdown of folate or its main serum form, 5-methylhydrofolate (5-MTHF) by alcohol or UVR, in particular ultraviolet A (UVA). Ultraviolet radiation and the ROS generated by UVA lower levels of folate 5-MTHF in the body because competition for folate is intense when high UVR simultaneously stimulates multiple folate-requiring processes, including cell division, DNA repair, and melanogenesis. Reduction of levels of folate and 5-MTHF is particularly serious if the body’s demand for folate is high – as during pregnancy – or if folate levels are already low because of low intake. The essential connections between folate metabolism and the evolution of dark skin pigmentation are, firstly, the relationship between UVR exposure and folate deficiency and, secondly, the relationship between UVR-induced folate deficiency and reduced fitness due to failures of normal embryogenesis and spermatogenesis. Supporting evidence also comes from epidemiological studies, which indicate trends associating dark pigmentation with lower rates of neural tube defects.

“The evidence that permanent dark skin pigmentation evolved as protection against the deleterious effects of UVR is overwhelming, but it does not explain the clinal distribution of increasingly lightly pigmented skin outside of the tropics. The strength of UVR, and of UVB in particular, declines greatly north of the Tropic of Cancer and south of the Tropic of Capricorn. Low levels of UVR are not beneficial despite the fact that most UVR radiation is harmful. This is because UVR has one overwhelmingly positive action, photosynthesis of vitamin D3 (cholecalciferol) in the skin by UVB. This reaction, which becomes less efficient with advancing adult age, only occurs in the presence of wavelengths of 290–315 nm in the UVB range and is dependent upon season and latitude, time of day, and on the amount of pigment and thickness of the skin. Vitamin D is also available in low quantities in some foods, but vitamin D3 is found in physiologically significant quantities only in oily fish and liver or supplements. The most obvious function of vitamin D in humans is in the building and maintenance of the bony skeleton. The presence of vitamin D-receptors in tissues of the brain, heart, stomach, pancreas, skin, gonads, in the activated T and B lymphocytes of the immune system, and many sites elsewhere in the body has heightened awareness of the varied and important roles vitamin D plays in the body. Chronic deficiencies in vitamin D have been associated recently with breast, prostate, colon, ovarian and possibly other cancers, and research in this area is extremely active. Low vitamin D status is also linked to impaired immune system activity and to abnormal development and function of the brain.

The theory that light skin pigmentation evolved in order to permit cutaneous vitamin D production under conditions of reduced sunlight at high latitudes is an old one that has considerable new, positive support from clinical, nutritional, epidemiological, genetic and other observational studies. The extent of vitamin D’s importance to health is evidenced by the seriousness and diversity of problems related to hypovitamosis D, which afflicts people throughout the life cycle. Thus, skin pigmentation in modern humans is the result of the action of two reciprocal clines working to promote the UVB-induced photosynthesis of pre­vitamin D3 in the skin on the one hand and prevent the damage caused by UVB and UVA on the other. Today, the global disease burden linked to the vitamin D deficiencies caused by low UVR exposure exceeds the burden connected with high UVR exposure. In the evolution of humanity, it is a relatively new phenomenon that we cover ourselves with so much clothing and reside mainly in doors. Before, people did not suffer as much vitamin D deficiency.”

Based on: Jablonski N. G. The evolution of human skin colouration and its relevance to health in the modern world. J R Coll Physicians Edinb. 2012; 42:58–63.