Vitamin D is important for bone and skeletal health. Studies show that vitamin D may also play a role in the prevention of breast, colon and prostate cancers, and autoimmune diseases like multiple sclerosis and diabetes.

Vitamin D is manufactured in the skin during exposure to the sun. It is also found in fortified foods and supplements.

"What is surprising is that large numbers of people are vitamin D-deficient, or insufficient, almost by any measures," says Anthony Young of Kings College, London. He says while exposure to sunlight is a major source of vitamin D, experts do not agree on how much is necessary for good health. That, he says is because "for many years western health agencies have been stating that one needs to avoid the sun to reduce the risk of skin cancer."

Mr. Young says that message may have gone too far. It is difficult, he says, to get adequate vitamin D solely from fortified food products and supplements. He recommends NOT turning away from the sun. "I think one can easily achieve vitamin D exposure without the need to get sunburned. In other words, one can do this with relatively low levels of exposure," he says.

Mr. Young calls for a consensus among researchers on ideal levels of Vitamin D. Signs of vitamin D deficiency are rickets in children and bone thinning in adults.