In recent years, many Americans have embraced vitamin D and fish oil pills, their enthusiasm fueled by a steady trickle of suggestive research studies linking higher levels of vitamin D with lower rates of cancer and other ills, and fish consumption with reduced heart disease.

Now a large and rigorous government-funded random ized 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.”