New research is suggesting that black skin may enhance the body’s natural immune system and provide better protection against disease than white skin. The melanin in dark skin is believed to help enhance the body’s natural ability to combat pathogens.
By Bernie Douglas (January 8, 2008), Revised February 17, 2009
Melanin Protects against Bacterial Infection and Disease:New research is suggesting that black skin may enhance the body’s natural immune system and provide better protection against disease than white skin (Mackintosh, 2001). The melanin in dark skin is believed to help enhance the body’s natural ability to combat pathogens (Nosanchuk and Casadevall, 2006; Mackintosh, 2001; Manning et al, 2003). A major function of melanocytes, melanosomes and melanin in skin is to inhibit the proliferation of bacterial, fungal and other parasitic infections of the dermis and epidermis (Mackintosh, 2001), so that the melanization of skin and other tissues form an important component of the innate immune defense system (Mackintosh, 2001; Nosanchuk and Casadevall, 2006). Protection from bacterial infection is also believed to be the reason that melanocyte and melanization patterns among different parts of the body do not reflect exposure to sunlight (Mackintosh, 2001). For example, many parts of the body which are hardly ever exposed to sunlight, such as genitalia, throats and nasal passages are often packed with melanin cells. In addition, several types of cells that are unrelated to skin also accumulate melanin (e.g. cells in the brain, and muscles) and in these instances immuno-enhancing or antimicrobial activity defend cells from oxidative stress (Mackintosh, 2001; Riley, 1992).
Pigmentation has also been implicated in the maintenance of calcium homeostasis (mechanism by which the body maintains adequate calcium levels) and is known to be involved with calcium ion regulation in the inner ear, which may improve hearing (Furuta et al, 1998; Norris et al, 2005; Nosanchuk and Casadevall 2003, 2006; William, 2007). Other research dealing with melanin as it relates to hearing has also found that skin pigmentation reduces the effects of noise-induced temporary hearing loss. For example, empirical studies have shown that people with more skin pigmentation suffer significantly less temporary hearing loss than those with pale skin (Barrenäs and Lindgren, 1990; Maurer et al, 1999). In addition to this, melanin offers resistance to heat or cold, as well as resistance to the activity of inorganic antimicrobial compounds (Garcia-Rivera and Casadevall, 2001; Rosas and Casadevall, 1997) thus improving natural overall health.
Pigmentation has widely been viewed as photoprotective largely because darkly pigmented skin is at substantially lower risk of developing skin cancer than is fair skin (Agar and Young, 2005). In the U.S., for example, some forms of skin cancer are “50 times” higher in Caucasians than in African Americans, and African Americans have a 13-fold lower incidence of melanoma than do Caucasians (Tadokoro et al, 2003). However, research is now showing that the relationship between skin pigmentation and photoprotection is much more complex than was once previously assumed (Agar and Young, 2005). The lower incidences of skin cancer observed in darker skinned individuals are now argued to result from the DNA repair facilitated by melanin (Izagirre et al, 2006; Agar and Young, 2005; Mackintosh, 2001). Melanin plays important roles in the quenching of free radicals produced by ultraviolet radiation (Césarini and I.N.S.E.R.M., 1999); this is also believed to be the reason why individuals with pale skin tan after sun exposure (Agar and Young, 2005).
The Genetics of Skin Color: Is Pale Skin Related to Albinism?Genetic research has shown that four genes involved in skin pigmentation have shown clear evidence of selection in Europeans (OCA2, MYO5A, DTNBP1, and TYRP1). All four genes are associated with Mendelian disorders that cause lighter pigmentation or albinism, and all are in different genomic locations, indicating the action of separate selective events (Voight et al, 2006). One of these genes, “OCA2”, is associated with the third longest haplotype on a high frequency SNP anywhere in the genome for Europeans (Voight et al, 2006). The ‘OCA2’gene is associated with “Type II oculocutaneous albinism,” which is an autosomal recessive disorder in which the biosynthesis of melanin pigment is reduced in the skin, hair, and eyes. ‘OCA2’ is also the most common form of albinism found in African and African-American patients (Spritz et al, 2004; Stevens et al, 2004).
A fifth gene, “SLC24A5”, has recently been shown to impact skin pigmentation and to have a derived selected allele near fixation in Europeans (Lamason et al., 2005; Voight et al, 2006; Soejima and Koda, 2006). Genetic studies show that the high frequencies of this variant allele in European populations may be associated with a substantial reduction in regional heterozygosity (Lamason et al., 2005; Soejima et al, 2006). This tends to tie in with other molecular genetic research that has shown Europeans to have among the world’s lowest levels of genetic variation which results in lower heterozygosity levels (Weber et al, 2002; Watkins et al 2003). Heterozygosity is thought to enhance resistance of hosts to infectious diseases, offers a greater degree of fitness for one or the other allele, and improves overall health (Gangestad & Buss 1993, Thornhill & Gangestad 1993). Reduced levels of genetic variation and heterozygosity have also been linked to lower reproductive and evolutionary fitness (Lacy 1987; Reed and Frankham, 2003).
Melanin and its other Protective Qualities:Melanin also acts as a free radical scavenger and has been shown to protect against Cryptococcus neoformans; this is a parasite of the central nervous system (Curie & Casadevall, 1994; Mitchell & Perfect, 1995). Interestingly, it is known that Caucasians suffer from heightened rates of many diseases that affect the central nervous system. For example, Huntington’s disease, optic neuritis, Parkinson’s disease, Multiple Sclerosis, PKU (Phenylketonuria), Anencephaly and Spinal bifida (Bates et al, 2002; Okano et al, 1990; Tanner et al, 1996; Nelson et al, 1997; Modi et at, 2001; Kurtzke, 1979; Alan, 2003). While there may not be any ‘direct’ relationship between melanin levels and these particular neurodegenerative conditions, it should be noted that (eu)melanin is believed to protect folate and folic acid (Jablonski and Chaplin, 2000; Branda and Eaton, 1978). These are essential vitamins that are particularly important for neural tube development in the unborn (Vorobey et al, 2006; Jablonski and Chaplin, 2000). Depleted levels of these vitamins are caused by UV light exposure (Vorobey et al, 2006; Lapunzina, 2006; Branda and Eaton, 1978), which Caucasians are particularly susceptible (Jablonski and Chaplin, 2000). Depleted levels of these vitamins have been linked to a variety of different birth defects including ‘spinal bifida’ and ‘anencephaly’ (brain malformation). Other research shows a low prevalence of folate and folic acid deficiencies in black Africans and African Americans (Wiswell et al, 1990; Lawrence, 1983); which is generally attributed to their levels of eumelanin.
Research has also shown that melanin is actually a poor sunscreen, which does not protect particularly well against UVB radiation (Mackintosh, 2001; Hill, 1992; Scheibner et al., 1986). Thus the notion that black skin is simply and adaptation to hotter climates remains controversial. Studies indicate that black skin can produce adequate levels of vitamin D as far north as the Arctic Circle, even if only 10.5% of the body’s surface is exposed (Beatle, 1977; Robins, 1991). Indeed, it only requires brief sun exposure to generate all the vitamin D one needs, and summer sunshine even in the North Temperate Zone is more than adequate even for heavily pigmented skin to provide a supply of Vitamin D that can be stored up in fat and muscle tissue in sufficient quantities to last the rest of the year (Robins, 1991), suggesting that pigmentation plays important roles outside of simple photoprotection and/or UV filtration.
In addition, it is known that people of African decent generally possess superior bone mineral density and suffer significantly lower rates of osteoporosis than do Caucasians, even when living in the same or ‘higher’ latitudinal environments (Finkelstein et al, 2002; Luckey et al, 1996; Barrett-Connor, 2005). UVB light is assumed to be a precursor for vitamin D, and low vitamin D levels are associated with lower bone mineral density and higher risk of bone fractures. Because a major role of vitamin D is to promote the development of strong and healthy bones, one would not expect to find significantly lower bone mineral density and higher rates of osteoporosis among people of lighter complexion (e.g. Whites and Asians) living at the same or similar latitudes as people of darker complexion, if dark skin were simply an adaptation for filtering UVB light. In fact, the very opposite would be expected! Vitamin D deficiency has also been linked to ‘multiple sclerosis’ (Vieth, 1999), which is a condition that is also known to affect Caucasians overwhelmingly (Modi et al, 2001), and is rare among people of African decent (Modi et at, 2001; Kurtzke, 1979).
Photoprotection, Ethnicity and more:Tadokoro et al. (2003) reported for the first time the effects of melanin on UV responses in different ethnic groups. This team found that DNA damage in all subjects was greatest immediately after UV exposure and was gradually repaired thereafter. Rates and efficiencies of removal of DNA lesions differed dramatically between subjects in all groupings, but DNA damage was most severe in the light skin. The overall findings were that melanin affords significant protection against DNA damage in underlying skin cells, a concept previously supported by tissue culture and organ model systems. It was also found that Melanin can absorb UV efficiently at most wavelengths.
Differences in pigmentation, aside from being largely genetic, also result from the production of different amounts of melanins in skin tissues and on their distribution by neighboring keratinocytes (the primary cell types found in skin). Lighter skin has less melanin and what is produced is typically found arranged in clusters of melanosomes in keratinocytes, while darker skin has more melanin and the melanosomes are distributed individually in keratinocytes, thus absorbing light more efficiently (Miyamura, 2006). Recent studies report that the density of melanocytes in various types of racial/ethnic skin is virtually identical (Alaluf et al., 2003; Tadokoro et al., 2003), however, the amount of melanins detected in those melanocytes varies greatly (Tadokoro et al., 2003; Miyamura, 2006).
Caucasian people, while producing overall lower levels of melanin (Tadokoro et al., 2003; Miyamura, 2006), have also been shown to possess higher concentrations of a more deleterious form of melanin known as “Pheomelanin.” The kind of melanin that helps to cause blonde and red hair (blue and green eyes) also increases the potential for cell death! Melanin filters out UV radiation, but it also increases UV harmful effects and causes cell death, particularly when the melanin is the kind found in light hair or skin (Brash et al., 2004; Maresca, 2006). The resulting sunlight sensitivity of individuals with any particular melanin type would reflect the balance between protection and damage, with pheomelanin contributing 3-fold greater damage than eumelanin and perhaps less efficient protection (Brash et al., 2004). In this scenario, when a pheomelanin-containing individual visits the beach, his melanosomes act as microscopic x-ray sources to generate superoxide and OH_ radicals. Pheomelanin also produces almost five times as much superoxide as eumelanin after UV exposure (Persad et al, 1983; Brash el al, 2004).
Melanin and the EyesMelanin plays important roles in vision. Norris et al (2005) show that melanin acts like a neutralizing sponge inside cells in the retina to soak up and destroy reactive oxygen species. Reactive oxygen species, or free radicals, which are energized by light, are thought to play a major role in “macular degeneration”, the leading cause of blindness in people over the age of 60. The disorder is more prevalent among whites than among African-Americans (Evans, 2003; Norris et al, 2005; Klein et al, 1999). The disorder is also found in higher frequencies among people with light eyes than people with dark eyes (Nicolas et al, 2003). Macular degeneration causes gradual loss of central vision by damaging the RPE (retinal pigment epithelial) cells that lie underneath the macula, the small region of the retina responsible for fine detail at the center of the field of vision (See, Norris et al, 2005). Oxidative damage occurs throughout one’s life, which suggests that macular degeneration may be the result of accumulated retinal damage. The retina is a typical example in which oxidative damage manifests in what is termed as “retinal aging” (Winkler, 1999). Dark eyes, “in general”, are known to absorb UV better than blue or green. Melanin provides protection to the lens of the eye from UV damage, protecting against Uveal melanoma, Choroidal Hemangioma, iris naevi, and choroidal naevi. Melanin also provides near optimum protection to the retina, glare is reduced in dark eyes; and, thus, vision is enhanced.
Although African Americans (6.6%) were found in a longitudinal study to have slightly higher rates of myopia (near sightedness), not significantly different from white Americans (4.4%), Asians were shown to have the highest prevalence (18.5%), followed by Hispanics (13.2%) (Kleinstein et al, 2005). For hyperopia, whites have the highest prevalences (19.3%), followed by Hispanics (12.7%). Asians were shown to have the lowest prevalence of hyperopia (6.3%) and were not significantly different from African Americans (6.4%). For astigmatism, Asians and Hispanics had the highest prevalences (33.6% and 36.9%, respectively) and did not differ from each other. African Americans had the lowest prevalence of astigmatism (20.0%), followed by whites (26.4%) (Kleinstein et al, 2005). This suggests that with respect to general visual acuity blacks may have a strong advantage relative to other groups. However, judging from the mean group differences, this is unlikely to be the result of an advantage offered by higher melanin levels, as Hispanics are seen to have considerably worse vision than whites.
Researchers continue also to examine the distinctiveness of motor performance by dark- versus light-eyed individuals. For example, it has been shown that Dark-eyed individuals generally perform better at reactive type tasks, while light-eyed individuals perform better at “some” self-paced tasks (Miller et al, 1992; Beer and Fleming, 1988; Rowe and Evans, 1994; Beer and Beer, 1989). Strange as this research may seem, Beer and Fleming (1988) using multiple regression analysis, found that dark-eyed students hit a target with a frisbee more times than did light-eyed students. While Rowe and Evans (1994) had College students (61 men, 64 women) perform a forehand rally with different colored racquetballs. Eye color, sex, and total hits were recorded for each subject. Men scored significantly better with balls of each color than did women. Dark-eyed men performed better than other subjects and performance was better with blue balls than yellow or green balls. Other research shows that children with blues eyes tend also to be more timid and socially un savvy in preschool when compared with children with darker eye color (Moehler et al, 2006; Coplan et al, 1998), and blue eyed children are more susceptible to alcohol abuse in later years (Bassett and Dabbs, 2001).
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