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healthy immune system. Vitamin D maintains the balance of calcium and phosphorus necessary for the constant process of bone formation and remodelling (Hobday, 1999). 3.1.2 Vitamin D Production Process The skin of man has a high concentration of sterol cholesterol which is converted by enzymes in the skin to the sterol 7-dehydrocholesterol. Exposure of skin to sunlight for regular intervals results in the photochemical conversion of 7-dehydrocholesterol into pre-vitamin D3 (Rhoades & Tanner, 2003). This is sometimes referred to as ‘soltriol’, which means ‘hormone of sunlight’ (Hobday, 1999). This pre-vitamin D3 is then transported to the liver and is converted to 25- hydroxycholecalciferol and then to the kidney where it becomes the biologically active 1.25- hydroxycholecalciferol or Calcitriol. Dietary sources of vitamin D2 are derived from ergosterol (plant source), whereas vitamin D3 is derived from cholesterol (animal source). This form of vitamin D3 and D2 are then activated in the kidneys to become Calcitriol (Rhoades & Tanner, 2003). Calcitriol is a hormone that, together with parathyroid hormone, regulates blood calcium levels and, in turn, bone density. In this role, calcitriol targets the intestine, where it promotes calcium absorption; and bone, where it catalyzes calcium release to help restore depleted blood calcium levels (Johnson, 2005). Only about 15 percent of the available 7-dehyrocholesterol in the skin actually becomes vitamin D3, as prolonged exposure to the sun also converts pre-vitamin D3 into substances called lumisterol and tachysterol, which are both biologically inert. Therefore prolonged exposure to the sun will not create more vitamin D than shorter exposure time. Also the production of melanin in the skin in response to the UV radiation within sunlight reduces the production of vitamin D. Therefore people who spend a lot of their time in the sun will synthesise less vitamin D due to the increased darkness of the skin (Hobday, 1999). Skin care products that block out UVB radiation reduce the skin’s ability to produce vitamin D by at least 95 percent. However exposure to sunlight is not always guaranteed to produce enough Vitamin D for optimum health. During certain times of the day and year the UVB content in the sunlight fluctuates. The optimal times for vitamin D absorption are in the mid- morning and mid-afternoon periods of the day, and during the early summer months of the year. This exposure to sunlight is needed to store up enough vitamin D reserves for the winter months when UV radiation is at its lowest (Hobday, 1999). 23PDF Image | THE USE OF LIGHT AND COLOUR AS A HEALING MODALITY
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