Vitamin Studies Spell Confusion for Patients

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That's not to say supplements shouldn't be used at all, Katz said. He recommends omega-3 fatty acids and vitamin D for most of his patients, plus calcium for women. Prenatal vitamins and folic acid supplementation are also on that list.

There's evidence behind those supplements, he said. The GISSI trial found cardiovascular benefits for omega-3s. Study after study has shown that the majority of Americans are deficient in vitamin D, and the supplement study flood included more positive findings for folic acid supplementation around the time of conception.

As for the rest of the supplement lot -- give them only in the face of deficiencies, Katz said.

It's long established, for instance, that B12 deficiency plays a role in dementia and other neurological disorders, and supplementation can stave that off.

As well, certain vitamins are established treatments for a host of diseases, from vitamin C in scurvy to B12 in pernicious anemia.

Who Is Deficient?

But how can clinicians be sure that their patients actually need specific nutrients, especially since there are usually no obvious symptoms?

Katz said, in most cases, doctors should ask about their patients' diets and about what vitamins they currently take, though he makes the exception for children who are meeting growth milestones or for adults with good muscle tone.

The majority of clinicians reporting in a MedPage Today poll -- 70% -- said they supported annual screening of specific vitamin levels to treat deficiencies.

They may simply be following recommendations -- in June, the Endocrine Society recommended screening for vitamin D deficiency in at-risk patients, setting various levels of treatment for different risk populations, to keep at bay rickets and bone complications.

Rise and Fall

Just as vitamin E was recently found to be unable to prevent prostate cancer -- it was actually associated with an increased risk of developing the cancer, albeit a "marginally significant" one -- several other nutrients once touted for their preventive benefits flopped in trials.

"We fell in love with vitamin C in middle of the century, but trials didn't confirm the benefits of preventing colds or cancer," Katz said.

Similarly, B-complex vitamins promised to lower heart disease risk, he said. While they did lower homocysteine, trials turned up no clinical cardiovascular benefits.

The carotenoid antioxidants beta-carotene and lycopene also proved to be a bust, with both having no benefit in prostate cancer and the former no use in eye disease nor heart disease.

"We have a long litany of barking up the wrong tree," Katz said.

Why Don't They Work?

It's not clear why these vitamins don't do what they're expected to do, and researchers are actively looking into why.

The latest theory is that vitamin isolates don't work quite as well on their own. The vitamin E isolate used in the prostate cancer trial was alpha-tocopherol, which is only one in the family of E-complex vitamins, Katz said.

Rather, it may take the full blend of antioxidants and phytochemicals found within the context of a whole food in order to deliver any potential benefits.

That appears to concur with clinician consensus on the majority of supplements, particularly in healthy patients: they're not needed. Eat a healthy diet instead.

"[Patients] should stop trying to look for health in a pill," Lee Green, MD, of the University of Michigan, said in an email. "Health is not found in pills. It's found in good food and regular exercise. There's something in our psyche that makes us want to believe in magic, and that desire to believe has focused on vitamins."

This article was developed in collaboration with ABC News.

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