In January, an editorial in The Journal of the National Cancer Institute noted that most trials had shown no cancer benefits from vitamins  with a few exceptions, like a finding that calcium appeared to lower the recurrence of precancerous colon polyps by 15 percent.

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But some vitamin studies have also shown unexpected harm, like higher lung cancer rates in two studies of beta carotene use. Another study suggested a higher risk of precancerous polyps among users of folic acid compared with those in a placebo group.

In 2007, The Journal of the American Medical Association reviewed mortality rates in randomized trials of antioxidant supplements. In 47 trials of 181,000 participants, the rate was 5 percent higher among the antioxidant users. The main culprits were vitamin A, beta carotene and vitamin E; vitamin C and selenium seemed to have no meaningful effect.

“We call them essential nutrients because they are,” said Marian L. Neuhouser, an associate member in cancer prevention at the Fred Hutchinson Cancer Research Center in Seattle. “But there has been a leap into thinking that vitamins and minerals can prevent anything from fatigue to cancer to Alzheimer’s. That’s where the science didn’t pan out.”

Everyone is struggling to make sense of the conflicting data, said Andrew Shao, vice president for scientific and regulatory affairs at the Council for Responsible Nutrition, a vitamin industry trade group. Consumers and researchers need to “redefine our expectations for these nutrients,” he said. “They aren’t magic bullets.”

Part of the problem, he said, may stem from an inherent flaw in the way vitamins are studied. With drugs, the gold standard for research is a randomized clinical trial in which some patients take a drug and others a placebo. But vitamins are essential nutrients that people ingest in their daily diets; there is no way to withhold them altogether from research subjects.