Experts differ in their estimates of how many Americans might be candidates for a triglyceride-lowering drug. If the eligible group included all adults with triglyceride levels of 200 or more — the normal level is 150 or less — that would mean about 20 percent of adult Americans. If it were just those with the highest levels, above 500, then 2 percent to 3 percent of adults would qualify.

The discovery announced on Wednesday was hinted at in 2008 in a much smaller study of the Amish conducted by researchers from the University of Maryland’s medical school. One in 20 Amish people has a mutation that destroys a gene, involved in triglyceride metabolism, compared with one in 150 Americans generally. The scientists were intrigued but did not have enough data to nail down the gene’s role in heart attacks.

Sam, a 55-year-old Amish farmer who declined to have his last name published, saying he was uncomfortable about being conspicuous, has such a beneficial mutation. He recalls little heart disease in his family. On a cold day last fall, as an icy rain fell outside, he sat at a small wooden table in his daughter’s house and laid out a sheet of paper that showed he had a triglyceride level of 45. The average in the United States is 147.

“It’s nice that something came out that is positive,” he said.

Triglycerides have long puzzled researchers, although they are routinely measured along with cholesterol in blood tests and are often high in people with heart disease. Many experts were unconvinced they caused heart attacks. Clinical trials of drugs that lowered triglycerides by a small amount added to doubts about their role: The drugs had no effect on heart attack rates.

As for triglycerides themselves, “do they just keep bad company or are they independently doing something to risk?” asked Dr. Robert Hegele, a heart disease expert at Western University in London, Ontario, who was not involved in the new studies.