<P> Adequate amounts of vitamin D can be produced with moderate sun exposure to the face, arms and legs, averaging 5--30 minutes twice per week, or approximately 25% of the time for minimal sunburn . The darker the skin, and the weaker the sunlight, the more minutes of exposure are needed . Vitamin D overdose is impossible from UV exposure; the skin reaches an equilibrium where the vitamin degrades as fast as it is created . </P> <P> Sunscreen absorbs or reflects ultraviolet light and prevents much of it from reaching the skin . Sunscreen with a sun protection factor (SPF) of 8 based on the UVB spectrum decreases vitamin D synthetic capacity by 95%, and SPF 15 decreases it by 98% . </P> <P> The skin consists of two primary layers: the inner layer called the dermis, composed largely of connective tissue, and the outer, thinner epidermis . Thick epidermis in the soles and palms consists of five strata; from outer to inner, they are: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale . Vitamin D is produced in the keratinocytes of two innermost strata, the stratum basale and stratum spinosum . </P> <P> Vitamin D can be synthesized only by a photochemical process . Phytoplankton in the ocean (such as coccolithophore and Emiliania huxleyi) have been photosynthesizing vitamin D for more than 500 million years . Primitive vertebrates in the ocean could absorb calcium from the ocean into their skeletons and eat plankton rich in vitamin D . </P>

Where is vitamin d synthesized in the skin
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