Tons of Americans are taking Vitamin D pills, but studies suggest they're useless

Tens of thousands of people all over the United States get tested for Vitamin D deficiency. But the evidence suggesting that it's a serious problem ― at least as defined in many commercial labs ― is weak. And all those Vitamin D pills getting pushed on patients may not be helping much.

That's the upshot of an article by reporter Gina Kolata published Monday in the New York Times looking at the state of research around Vitamin D.

First, the basic facts:

• The vitamin industry in general is booming. Exact sales figures are hard to come by, especially for individual varieties like Vitamin D. But more than half of US adults take at least some supplements.
• Vitamin D refers to a group of nutrients that help you absorb calcium, as well as other minerals necessary for a healthy body.
• Your body produces Vitamin D naturally. (Just add sun.) A bit of sunlight on your skin gets your chemical factories churning out the nutrient. You can also get it from some foods, like fish and fortified milk.
• Lacking Vitamin D is bad for your joints and bones, and there's a body of research that connects Vitamin D deficiency to everything from breast cancer to fatigue to depression.

However, there's reason to think too many people are being diagnosed with Vitamin D deficiency.

Starting about ten years ago, researchers began to worry that many people, especially in colder climates where winter forces humans indoors, may be lacking Vitamin D. Doctors and medical journals in the US started pushing to screen for deficiencies and increase the Vitamin D intake of the general population. Medicare beneficiaries received 83 times more blood tests for Vitamin D in 2010 than in 2000.

But "deficiency" is a fuzzy term. Many labs will apply it to someone with 20 to 30 nanograms per milliliter in their bloodstream, which is likely a perfectly healthy level. As a result, thousands of patients may be going home with Vitamin D supplements they don't need. And recent studies have shown little in the way of medical benefits (though there is debate on that subject).

There are real consequences to over-consuming Vitamin D, though. Vitamin D toxicity can cause calcium to build up in your bloodstream, leading to fatigue, nausea, and vomiting.

For now, some researchers are urging doctors to cool it on testing every patient for Vitamin D, and only prescribe supplements in cases where they're really necessary.