As Vitamins Go, D, You Are My Sunshine

"Given the high probability of benefit, and the low likelihood of harm, it seems prudent that physicians measure" levels in their patients, he writes.

Doing that will be costly: Vitamin D testing runs at least $100 per test. There are other drawbacks, too. No optimal blood levels have been set for the vitamin. (Unlike the international units used to gauge intake of vitamin D, blood levels are measured in nanograms per milliliter.) Giovannucci proposes 30 to 40 nanograms per milliliter as a "reasonable target."

Others aren't so sure. "At the moment, it's really difficult to be able to pinpoint whether a particular level can be used in all circumstances to say this person has the best level of vitamin D that they can have," says Ann Prentice, director of the Medical Research Council's Human Nutrition Research Laboratory in Cambridge, England. "Most of the work has been in the disease of old age and largely in Caucasians. We really don't know that those levels of vitamin D would provide the same benefits in children and pregnant women, or in different ethnic groups. There are lots of reasons to think that they probably wouldn't be."

So for now, Prentice lets her skin produce vitamin D by having brief bouts of sun exposure both in the U.K. and in Western Africa, where she does research.

But she advises her mother to take a vitamin D supplement. That's because her mother doesn't get much sun exposure, and with age, the skin's ability to produce vitamin D drops significantly. Adults 65 or older make only 25 percent of the vitamin D produced by those ages 20 to 30.

Here's how you can make sure you get enough vitamin D:

¿ Eat salmon or mackerel. Just 3.5 ounces of either provides 90 percent of the daily value for vitamin D. Other foods naturally rich in vitamin D include sardines, tuna, eggs and liver. Foods fortified with vitamin D include milk, margarine and some breakfast cereals.

¿ Grab a few rays. Emphasis here is on "few," since prolonged sun exposure increases risk of skin cancer significantly. Just 20 minutes of sun exposure without sunscreen enables the skin to produce about 20,000 IU of vitamin D. "You'd have to drink about 400 glasses of milk to get that same amount," Cannell notes. And contrary to taking mega doses of dietary supplements, it appears that sun exposure does not cause toxic levels of vitamin D.

¿ Take a vitamin D supplement. The National Academy of Sciences sets 200 IU per day as the adequate intake for those 19 to 50 years old; 400 IU for adults 51 to 70; and 600 IU for those 71 or older. In the latest study that showed the 7 percent reduction in mortality, the average intake was about 500 IU per day.

Too much vitamin D can be toxic -- the reason the National Academy of Sciences sets 2,000 IU per day as the tolerable upper limit for adults.

Multivitamins provide vitamin D, but the amount varies widely, so read the labels: Men's One-A-Day contains 400 IU; Centrum Silver has 500 IU; Women's One-A-Day contains 800 IU; and Nature Made Multivitamins provides 1,000 IU.

Many calcium supplements also contain vitamin D, providing between 200 and 400 IU. And single vitamin D supplements are another option. Just make sure that the combination of dietary supplements and food sources of vitamin D don't exceed the upper limit. Signs of toxicity, which include bone loss and kidney problems, begin at about 10,000 IU daily.