the increased diameter, number, and blood flow of the capillaries. people with moderately pigmented skin (types iii-iv) are able to produce melanin in their skin in response to uvr. normal tanning is usually delayed as it takes time for the melanins to move up in the epidermis. heavy tanning does not approach the photoprotective effect against uvr-induced dna damage compared to naturally occurring dark skin, however it offers great protection against seasonal variations in uvr. gradually developed tan in the spring prevents sunburns in the summer. this mechanism is almost certainly the evolutionary reason behind the development of tanning behaviour. 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. advantages in low sunlight humans with light skin pigmentation living in low sunlight environments experience increased vitamin d synthesis compared to humans with dark skin pigmentation due to the ability to absorb more sunlight. almost every part of the human body, including the skeleton, the immune system, and brain requires vitamin d. vitamin d production in the skin begins when uv radiation penetrates the skin and interacts with a cholesterol-like molecule produce pre-vitamin d3. this reaction only occurs in the presence of medium length uvr, uvb. most of the uvb and uvc rays are destroyed or reflected by ozone, oxygen, and dust in the atmosphere. uvb reaches the earth's surface in the highest amounts when its path is straight and goes through a little layer of atmosphere. the farther a place is from the equator, the less uvb is received, and the potential to produce of vitamin d is diminished. some regions far from the equator do not receive uvb radiation at all between autumn and spring. vitamin d deficiency does not kill its victims quickly, and generally does not kill at all. rather it weakens the immune system, the bones, and compromises the body's ability to fight uncontrolled cell division which results in cancer. a form of vitamin d is a potent cell growth inhibitor; thus chronic deficiencies of vitamin d seem to be associated with higher risk of certain cancers. this is an active topic of cancer research and is still debated. the vitamin d deficiency associated with dark skin leads to higher levels of schizophrenia in such populations residing in northerly latitudes. 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 and 20000 iu of vitamin d. disadvantages in high sunlight fatal neural tube defect light-skinned people living in high sunlight environments are more susceptible to the harmful uv rays of sunlight because of the lack of melanin produced in the skin. the most common risk that comes with high exposure to sunlight is the increased risk of sunburns. this increased risk has come along with the cultural practice of sunbathing, which is popular among light-skinned populations. this cultural practice to gain tanned skin if not regulated properly can lead to sunburn, especially among very lightly-skinned humans. the overexposure to sunlight also can lead to basal cell carcinoma, which is a common form of skin cancer. another health implication is the depletion of folate within the body, where the overexposure to uv light can lead to megaloblastic anemia. folate deficiency in pregnant women can be detrimental to the health of their newborn babies in the form of neural tube defects, miscarriages, and spina bifida, a birth defect in which the backbone and spinal canal do not close before birth. the peak of neural tube defect occurrences is the highest in the may–june period in the northern hemisphere. folate is needed for dna replication in dividing cells and deficiency can lead to failures of normal embryogenesis and spermatogenesis. individuals with lightly pigmented skin who are repeatedly exposed to strong uv radiation, experience faster aging of the skin, which shows in increased wrinkling and anomalies of pigmentation. oxidative damage causes the degradation of protective tissue in the dermis, which confers the strength of the skin. it has been postulated that white women may develop wrinkles faster after menopause than black women because they are more susceptible to the lifetime damage of the sun throughout life. dr. hugh s. taylor, of yale school of medicine, concluded that the study could not prove the findings but they suspect the underlying cause. light-coloured skin has been suspected to be one of the contributing factors that promote wrinkling.