Vitamin D's wild days: Who to test, what to take?

Vitamin D and calcium go hand in hand. You need a lifetime of both to build strong bones. We get D in three ways: sun exposure, dietary supplements or certain foods, particularly D-fortified milk, orange juice and cereals.

Scientists have been interested in vitamin D's possibly broader effects for decades, since noticing that cancer rates between similar groups of people were lower in sunny southern latitudes than in northern ones. In recent years, studies have linked low levels of vitamin D with breast, prostate and colon cancer, heart disease, diabetes and certain other ailments - as well as an overall increased risk of death.

Much of the evidence is circumstantial. There's a chicken-or-egg question: Does correlating how much vitamin D is in someone's blood at a certain time really mean it triggered or worsened a disease - or did the disease, or other risk factors, trigger the low vitamin D? Being a couch potato, for instance, is a key risk for heart disease, and also keeps you out of the sun.

Moreover, not all vitamin D studies find that the nutrient helps. The disappointing ones seldom make headlines.

But the increasing interest in vitamin D parallels increasing concern that people aren't getting enough - and increasing confusion about how much that might be.

Currently, the government and other health authorities recommend consuming anywhere from 200 to 600 international units a day from food or supplements, depending on your age - levels that many vitamin D proponents say are too low.

There's no consensus on how to balance the risk of skin cancer from sun exposure with vitamin D needs. Some specialists recommend 10 to 15 minutes daily without sunscreen. Others say sunscreen doesn't completely block vitamin D production so sunscreen users will get enough. Regardless, time of day, season and geography play a role.

There's not even good agreement on what's a low level - different studies use different definitions, notes a newly published research review sponsored by the National Institutes of Health.

The government has begun discussions with the Institute of Medicine to determine if the daily recommended intake needs changing.

Meanwhile, because megadoses may be toxic, the government considers 2,000 IUs a day the upper limit, although doctors may recommend 10,000 or even 50,000 IUs for a short period if someone needs a rapid boost.

By Lauran Neergaard

Terra/AP

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