Imagine a nutrient that could help prevent cancer, heart disease and tuberculosis, preserve bones and thwart autoimmune diseases such as multiple sclerosis, rheumatoid arthritis and juvenile diabetes.

Too good to be true?

But that's the potential promise for vitamin D, a nutrient whose usefulness was once thought to be limited to prevention of rickets and severe bone loss. Known as the "sunshine" vitamin because it is naturally produced when the skin is exposed to ultraviolet light, vitamin D has been garnering increasing attention recently.

The American Journal of Clinical Nutrition recently published a special supplement highlighting widespread vitamin D deficiencies "in various populations throughout the world, including 'healthy' people in developed countries where it was thought that [the] deficiency was obsolete."

The National Cancer Institute and the federal Office of Dietary Supplements have both convened scientists to review vitamin D data this past year. And last fall, the federal Agency for Healthcare Research and Quality issued a review of vitamin D that found it essential for bone health at all ages.

Among the more intriguing findings is a recent review of 18 studies involving nearly 60,000 people that showed that those who took vitamin D supplements had a 7 percent reduction in mortality compared with those who didn't take the vitamin.

Experts believe that 25 to 40 nanograms per milliliter is a reasonable target for vitamin D blood levels. But population studies suggest "that if you can get to 60 to 70 nanograms per milliliter that maybe you can address some other health problems," says Hector DeLuca, a University of Wisconsin biochemist and a pioneer in vitamin D research.

Rates of colon cancer, for example, are about 50 percent lower in sunny parts of the world and in regions where there's a high consumption of fatty fish, which are rich in vitamin D.

Based on the recent vitamin D research, Dr. DeLuca began taking 1,000 international units (IU) of vitamin D daily through supplements. He also consumes a diet that includes vitamin D-fortified foods. He estimates his total daily intake at around 2,000 IU daily – the level that the National Academy of Sciences sets as the tolerable upper-limit intake for adults.

Too much vitamin D can be toxic. "I am only confident to go to about 2,000 or 3,000 IU per day," Dr. DeLuca says. "We don't have adequate safety data to go much above that."