Now a large and rigorous government-funded randomized trial — the only such study of omega-3 fish oils ever carried out in healthy adults, and the largest trial ever done of high-dose vitamin D — has found the supplements do not lower cancer rates in healthy adults. Nor do they reduce the rate of major cardiovascular events, a composite of heart attacks, strokes and deaths from cardiovascular disease. The trial is of the kind considered the gold standard in medicine.

“It’s disappointing, but there have always been such high expectations that vitamin D can do all these different things,” said Dr. Clifford J. Rosen of the Maine Medical Center Research Institute in Scarborough, who was a co-author of an editorial on the studies in The New England Journal of Medicine. He said doctors had engaged in “magical thinking about vitamin D,” often testing their patients’ blood levels and advising them to take supplements.

“In terms of preventing cancer, I think the door is closed. I don’t think there is anything there,” Dr. Rosen said. The study also showed that omega-3 fatty acid supplements do not reduce major cardiovascular events or cancer, “period,” he said. “That’s what people should take home with them.”

The results hinted at some potential benefits, which would have to be explored in separate trials. Secondary analysis of the data found, for example, a reduction in cancer deaths for people who took vitamin D for at least two years, and fewer heart attacks in people who took fish oil. And people do need vitamin D in order to absorb calcium and for other bodily functions.

Regarding fish oil, Dr. Rosen asked, “Are there subgroups where it may be effective, or reduce heart attacks? It’s possible.

“But that’s got to be proven in another trial. If this were a drug being tested for approval by the F.D.A., it wouldn’t make it.”

Dr. JoAnn E. Manson, an investigator at Brigham and Women’s Hospital who led the trial, said that since fish oil did not reduce strokes, it did not have an impact on the overall risk of cardiovascular disease, a measure that combined heart attacks, strokes and cardiovascular deaths. But she said another analysis looking separately at heart attack found a 28 percent reduction among those taking fish oil, with a 40 percent reduction in people who eat little fish and a 77 percent reduction in African-Americans. She said the 28 percent reduction was “pretty amazing. That’s what you see with statins.”

“The data have to be very strong before you go out and recommend to everyone in the world to take supplements, and we’re certainly not doing that,” Dr. Manson said. “We’re saying, ‘Talk to your doctor, especially if you have low fish intake or are African-American.’”