lack of sunlight

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Invalids, shift workers and people whose exposure to sunlight may be minimal can suffer from insufficient amounts of vitamin D, which is required for calcium metabolism, without which rickets and osteoporosis(bone thinning) has been observed. Ultraviolet light is the stimulus to vitamin D formation in skin. It is blocked by cloud, fog, smoke, smog, ordinary window glass, curtains and clothing. The maximum recommended daily supplement intake of vitamin D is 400 i.u.