A Too-Good-to-Be-True Nutrient?

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

Food -- especially fatty fish such as salmon and mackerel -- is a prime source of Vitamin D. Just 3.5 ounces of either fish provides 90 percent of the daily value for most adults. Other foods naturally rich in Vitamin D include sardines, tuna, eggs and liver. And there are plenty of foods, including milk, margarine and some breakfast cereals, that are fortified with Vitamin D.

Another option for increasing the nutrient's levels in the blood is to take a dietary supplement with Vitamin D, a practice that DeLuca began a couple of years ago. "There is enough reason to believe that if you aren't crazy and get too much Vitamin D, you can improve your risk," he says.

DeLuca takes 1,000 international units (IU) of Vitamin D daily. He also follows a diet that includes foods fortified with the vitamin. He estimates that his total intake hovers around 2,000 IU daily, the level that the National Academy of Sciences sets as the tolerable upper limit for adults. (Adequate daily intakes are 200 IU for those 19 to 50 years old; 400 IU for adults 51 to 70; and 600 IU for those 71 or older.)

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

Aside from taking a dietary supplement, spending time in the sun remains the most significant source of Vitamin D. Just 20 minutes spent in the sun without sunscreen enables the skin to produce about 20,000 IU of Vitamin D -- equivalent to drinking about 400 glasses of milk. Contrary to taking mega doses of dietary supplements, sun exposure does not appear to cause Vitamin D toxicity.

But there's another downside to sun exposure: increased risk of skin cancer -- the reason that DeLuca, dermatologists and many scientists say that it's best to get Vitamin D "from fortified foods and capsules."