Vitamin D helps the body absorb calcium and phosphorous, which is essential for building bones and making them strong and dense, according to the Merck Manual Home Edition. Vitamin D also improves muscle strength. Vitamin D helps the nerves relay messages between the brain and muscles, states the National Institutes of Health. The vitamin also assists the immune system in warding off foreign bodies.
In some cases, vitamin D restricts the growth of cancer cells, according to the Harvard School of Public Health. Vitamin D also may reduce the risk of heart disease. Vitamin D is a hormone and a nutrient. When exposed to sunlight, the body makes vitamin D from cholesterol. Vitamin D also can be obtained through the diet. Dairy products, fortified cereals, mushrooms and fatty fish, such as salmon and sardines, are good sources of the vitamin.
Vitamin D deficiency is a common condition, the Merck Manual states. In these cases, the skin may not receive enough sunlight. In addition, many natural foods don't contain much vitamin D. Children who don't receive enough vitamin D may develop rickets, a rare disease characterized by soft bones. Adults may develop a similar condition called osteomalacia. A deficiency of vitamin D also can worsen osteoporosis.