By Sally Squires
Tuesday, September 18, 2007
Vitamin D is best known for building strong bones, but it may have another benefit: longevity.
In a recent analysis of more than 18 studies involving nearly 60,000 people, those who took vitamin D supplements had a 7 percent reduction in mortality from all causes compared with those who didn't take the vitamin. The numbers improved slightly for people who took vitamin D for three years or more. They had an 8 percent lower risk of dying.
"The results are remarkable," notes Harvard School of Public Health's Edward Giovannucci in an editorial published in the Archives of Internal Medicine, where the study also appeared this month.
Even better, the study found no "negative surprises" from taking vitamin D, as long as doses were kept between 300 to 2,000 international units (IU) per day.
That's key because recent excitement over the health benefits of vitamins has been tempered by sobering results: Scandinavian studies, for example, found that smokers who took beta carotene -- converted in the body to vitamin A -- had an increased risk of developing lung cancer compared with those who didn't take the supplements.
These new findings add to the growing interest in vitamin D -- often dubbed the "sunshine vitamin" because in its natural form, the vitamin is produced by the skin under the sun's ultraviolet rays.
Beyond its proven bone benefits, vitamin D is critical for immunity, prompting production of antimicrobial substances that seem to act like natural antibiotics and antiviral agents.
Some experts think that the reduced sun exposure during winter could help account for the seasonal ebb and flow of colds and influenza. "It's always been a mystery why influenza disappears in the summertime," notes John J. Cannell, a psychiatrist at Atascadero State Hospital in California who heads the nonprofit Vitamin D Council.
Emerging research also points to a role for vitamin D in cancer prevention, particularly against breast, colon, prostate and lung tumors. Vitamin D could help with cancer treatment. One recent study found that lung cancer patients who either got a lot of sun or had a high intake of vitamin D had three times the survival rate of their counterparts with lower vitamin D levels.
Another possible benefit of vitamin D is prevention of Type 2 diabetes, which affects an estimated 17 million Americans. And in an upcoming paper, Cannell speculates, based on population studies, that vitamin D deficiency during pregnancy may play a role in the worldwide increase of childhood autism.
So how could one vitamin have so many potentially wide-ranging effects? Unlike other vitamins, D acts both as a vitamin and as a hormone that can be activated as needed by the body.
This wider role of vitamin D has led scientists to weigh whether the current recommended daily intake is high enough. In the meantime, a growing number of experts, including Harvard's Giovannucci, advise routine measurement of vitamin D blood levels to detect deficiencies that aren't severe enough to produce clinical signs.
"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.