Confused about taking vitamins? So is the government.
A staple on U.S. store shelves since the 1930s, vitamins have been the focus of hundreds of studies. Vitamins have also mushroomed into a $7 billion business, with more than half of all Americans either taking a vitamin or a supplement in one form or another.
Lacking a consensus on whether vitamins actually work, a federal panel this spring completed its search for evidence that they help people stay healthy by preventing cancer and other diseases.
The group commissioned a review of every vitamin study conducted during the past 40 years, met with the scientists who conducted that review and held a three-day conference last month to question leading experts.
The bottom line? Vitamins may help. Or not.
"The present evidence is insufficient to recommend either for or against the use" of vitamins, says the 18-page report, released May 17 by a National Institutes of Health panel.
Conclusions are elusive because there have been so few randomized, controlled trials of vitamins' effectiveness. These studies, which compare results from a group that ingests a product with a group that receives a placebo, are the gold standard for medical research. They are also expensive.
"The whole area of nutrition research over the years has been underfunded," said Diane Birt, a member of the NIH panel and an expert on diet and cancer prevention at Iowa State University.
The federal panel recommended more thorough studies and a requirement that supplement makers report toxic reactions to the Food and Drug Administration, as pharmaceutical firms must do. Currently, supplement makers do not have to report these problems to the government.
The panel, appointed by the NIH Office of Medical Applications of Research and the Office of Dietary Supplements, issued specific recommendations about vitamins for post menopausal women, women of childbearing age, smokers and those at risk of early-stage macular degeneration, a disease that causes blindness.
But critics said the panel set the bar too high by limiting its conclusions to findings derived from human clinical trials. "The panel's rules and design were so limited, it seems like the conclusions were preordained," said Brian Sanderoff, a pharmacist with an Owings Mills outlet that specializes in the sale of vitamins and nutritional supplements.
The panel seemed to ignore overwhelming evidence that popular multivitamins can help millions of people who don't eat properly, said Jeffrey Blumberg, nutrition professor at Tufts University.
Only 3 percent of the U.S. population follows the government's 2005 Dietary Guidelines, and the guidelines advisory committee reported last year that many people aren't getting enough vitamin E, calcium, magnesium, potassium and fiber, Blumberg said.
"I'm not saying [multivitamins] are a substitute for a healthy diet, but you're not going to have a shortfall of any vitamin if you take a multivitamin," said Blumberg, who was not on the panel but attended the conference.
Blumberg said the NIH panel should have looked at smaller studies that link vitamin use to reduced cancer risks, improved bone density among women and other health benefits.
"There's lots and lots of smaller studies, observational studies, that suggest there are benefits to multivitamins," Blumberg said.
But panel members say these so-called "observational" studies are less reliable because they are based on recording the health of people who take vitamins in the first place - and tend to take better care of themselves than the average American.
That makes it difficult to know why someone experiences a specific health benefit, such as lower blood pressure or reduced bone loss, Birt said.
"The question becomes, `Is it the exercise, is it the person's diet or is it the multivitamin they're taking?' We just don't know," Birt said.
The NIH panel commissioned researchers at the Johns Hopkins Bloomberg School of Public Health to review every scientific study conducted from 1966 to February of this year, looking for any clinical trials that examined the effects of vitamin use on overall population groups.
There were only two - and neither is particularly useful to researchers interested in U.S. populations, said Han-Yao Huan, the nutrition expert at Hopkins who led the research.
In one, researchers examined the effects of multivitamin use by rural Chinese peasants. The other focused on diets among the French from 1996 to 2004.
The Chinese study was 20 years old and focused on a nutritionally deprived population, Huan said. And, although the French have the same problems with fatty foods and high cholesterol levels as Americans, they have different diets and generally are at lower risk for heart disease. That's a phenomenon researchers often refer to as the "French Paradox," she said.
The lack of clinical trials involving vitamins is not surprising because studies are costly and take a long time, Huan said.