Taking megadoses of vitamins and minerals, using amounts that people could never consume through food alone, could be even more problematic.
“There’s something appealing about taking a natural product, even if you’re taking it in a way that is totally unnatural,” Ms. Price said.
Early studies, for example, suggested that beta carotene, a substance found in carrots, might help prevent cancer.
In the tiny amounts provided by fruits and vegetables, beta carotene and similar substances appear to protect the body from a process called oxidation, which damages healthy cells, said Dr. Edgar Miller, a professor of medicine at Johns Hopkins School of Medicine.
Experts were shocked when two large, well-designed studies in the 1990s found that beta carotene pills actually increased lung cancer rates. Likewise, a clinical trial published in 2011 found that vitamin E, also an antioxidant, increased the risk of prostate cancer in men by 17 percent. Such studies reminded researchers that oxidation isn’t all bad; it helps kill bacteria and malignant cells, wiping them out before they can grow into tumors, Dr. Miller said.
“Vitamins are not inert,” said Dr. Eric Klein, a prostate cancer expert at the Cleveland Clinic who led the vitamin E study. “They are biologically active agents.
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