Deficiency in vitamin D may arise from a lack of exposure to sunlight (sunlight stimulates the body to produce vitamin D), causing bone loss, pain and weakness. Bones soften and bend and often results in bowlegs.
Although misnamed a "vitamin" long ago, it is actually a steroid hormone that is made by the skin under the action of sunlight. It is a common additive to dairy products for years. Along with calcium, phosphorus, iron, vitamin B12 and others, it is essential for strong healthy bones and blood producing marrow.