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updated 10/24/2012 5:26:29 PM ET 2012-10-24T21:26:29

Whether you need more vitamin D is not simply a matter of how much is pulsating through your body but rather which doctor you talk to.

Confusion and doubt still linger in the medical community nearly two years since the esteemed U.S. Institute of Medicine revised the recommendations for daily vitamin D intake. It was in November 2010 when the IOM lowered the vitamin D blood level deemed sufficient, from 30 nanograms/milliliter to 20 ng/ml.

But what this change has meant for the general population, most of whom don't even know their vitamin D blood levels, has been a mystery … until now.

Researchers at Loyola University Chicago Stritch School of Medicine have done some number-crunching and have calculated that 78.7 million adults once considered to have insufficient vitamin D levels now have sufficient levels under the new guidelines.

Yet many doctors and medical organizations, such as the Endocrine Society, still abide by the old guidelines. This means a sizeable chunk of the U.S. adult population — over one-third — is in "recommendation limbo."

A team led by Holly Kramer of Loyola published its calculation today (Oct. 24) in the journal PLOS ONE.

D's source and purpose

Humans get most of their vitamin D from the sun. The sun's ultraviolet radiation interacts with a type of steroid deep in the skin to make cholecalciferol, colloquially known as vitamin D3. There are only a few food sources rich in vitamin D, mostly fatty fish such as catfish and salmon. This type of vitamin D is called ergocalciferol, or D2. [ Infographic: The Power of Vitamin D ]

Vitamin D is needed primarily for healthy bones. Low levels can cause bone softening, a disease called rickets in children and osteomalacia in adults.

Some doctors think vitamin D improves the immune system and lowers the risk of cardiovascular disease, cancer, multiple sclerosis and other ailments. But the data supporting these claims are lacking, a fact reflected in the IOM's lowering of the recommended vitamin D blood-level target.

Because most Americans do not eat much fatty fish, the sun remains the primary source of vitamin D. However, many scientists say that, outside of the warmer months, the sun's rays are too weak to generate enough vitamin D anywhere north of 37 degrees latitude, an imaginary line stretching roughly from Washington, D.C., and across St. Louis to San Francisco.

Those with darker skin will have a harder time making vitamin D. Air pollution and sunscreens also reduce the amount of UV light hitting the skin. Hence, dietary supplements are recommended.

Talk to your doctor?

The Loyola-led study primarily examined the effect of the new vitamin D recommendations on people with chronic kidney disease. Chronic kidney disease in adults is linked to low vitamin D levels.

The study found that death rates from chronic kidney disease were essentially the same for people with vitamin D levels anywhere in the range of 20 to 40 ng/ml and that only those patients with very low levels of vitamin D had the highest risk of death.

So the IOM's lowering of the guidelines to 20 ng/ml hasn't worsened the death rate from chronic kidney disease. But in calculating this, using data from a large national health survey, the Loyola team could infer the number of Americans with no chronic kidney disease and with a vitamin D level smack in between 20 and 30 ng/ml. That's the 78.7 million. [ 9 Good Sources of Vitamin D ]

Kramer would not comment on what levels are best, stating only that those people confused about the appropriate level should "consult with their doctors."

What your doctor might say is anyone's guess.

"The [Endocrine] Society's figure of 30 is still quite reasonable; indeed, even it may not be high enough," said Robert Heaney, a professor at Creighton University School of Medicine in Omaha, Neb., who was an author of the Endocrine Society's clinical practice guideline on vitamin D. (Heaney added that this is his opinion and that he's not authorized to speak on the behalf of the society.)

Walter Willett, a respected nutritionist at Harvard School of Public Health in Boston, also has said the IOM guidelines for vitamin D blood levels are too low. In a comment co-written by Heike Bischoff-Ferrari of the University of Zurich and published in Harvard's Nutrition Source newsletter about the IOM recommendations, Willett cites two large studies from 2009 demonstrating that the 20 ng/ml level is too low to prevent bone fractures from falls.

Willett and Bischoff-Ferrari added that, beyond bone health, "the evidence for benefit is quite strong for some [disease prevention], especially colorectal cancer," albeit not conclusive.

The IOM, part of the United States National Academies, is considered a "who's who" of medical research, whose distinguished new members are elected by current members. Willett, in fact, is a member. The IOM has not ruled out revising its guidelines, particularly as stronger studies about vitamin D's role beyond bone health come to light.

Christopher Wanjek is the author of a new novel, " Hey, Einstein! ", a comical nature-versus-nurture tale about raising clones of Albert Einstein in less-than-ideal settings. His column, Bad Medicine, appears regularly on LiveScience.

© 2012 LiveScience.com. All rights reserved.

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