When we say that vitamin D is not like any other vitamins, it's because, unlike other vitamins, this functions like a hormone. It can be produced inside the body. Each cell in the body has receptors for producing vitamin D. These receptors get activated when skin is exposed to sunlight. This vitamin helps in maintaining the bones and teeth health, supports the immune system, brain and central nervous system. It is also responsible for regulating insulin levels in the body. People often wonder that, If the body produces vitamin D, then how can there be a deficiency of it? Deficiency of Vitamin D is related to the time a person spends in sunlight. Covered homes, covered cars and covered offices often make it difficult to get enough sunlight. National Institute of Health, (NIH), USA, has recommended at least 400-800 IU of vitamin D should be taken daily. Apart from exposure to sunlight, you can also increase your vitamin D consumption by eating foods such as fatty fish and fortified dairy products. Here are some unknown signs of vitamin D deficiency that your body is sending out.