Vitamin D is crucial for bone and muscle health and effective for preventing and treating acute respiratory infections like colds and flu. 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. It also causes muscle wasting in malnourished children and adults.
Although misnamed a "vitamin" long ago, Vitamin D 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.
In the USA, rickets was once a major health problem but the addition of vitamin D to milk in the 1930s had virtually eliminated the disease; the government stopped keeping statistics on it. However, Vitamin D deficiency is on the rise again. Research reported in 2009 revealed that 77 percent of adults are deficient and that deficiency levels had doubled between the two study periods 1988 to 1994 and 2001 to 2004. Indications are: (1) 57% of 290 patient admitted to Massachusetts General Hospital in 2001 were low in vitamin D. Not only were persons either housebound or over age 65 experiencing the deficiency, but 42% of the 77 healthy, nonelderly people with an average age of 44 were likewise affected; (2) in the late 1990s, around one in 200,000 children in the state of Georgia was hospitalized with rickets. The government attributes the comeback of childhood rickets to the popularity of milk substitutes like soy that lack certain nutrients; the failure to supplement breast milk with vitamin D; and a lack of childhood exposure to sunlight as parents are keeping children indoors more, leaving them at day-care centers or trying to protect them from skin cancer by using sunblocks and screens for UV light. The resurgence has been seen particularly among children breast-fed by a black mother (dark-skinned people absorb less sunlight).
Housebound elderly people who get rickets may not be exposed to enough sunlight. Sitting by the window does not help, because windows filter out the part of the ultraviolet light that produces vitamin D in the skin.