Nature
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.
Background
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.