Child suffering from rickets. Rickets is a disease occuring in growing children in which the bones do not harden and are malformed due to a deficiency of vitamin D. Vitamin D is necessary for the uptake of calcium from food; without the vitamin not enough calcium salts are deposited in the bones to make them rigid. Vitamin D is synthesised in the skin when it is exposed to ultraviolet light. The deficiency can therefore be caused by lack of sunlight as well as insufficient dietary intake. In adults, vitamin D deficiency causes osteomalacia, a progressive softening and decalcification of bony tissues.