To millions of Americans, Bob Harper was the picture of health, a celebrity fitness trainer who whipped people into shape each week on the hit TV show “The Biggest Loser.”

But last February, Mr. Harper, 52, suffered a massive heart attack at a New York City gym and went into cardiac arrest. He was saved by a bystander who administered CPR and a team of paramedics who rushed him to a hospital, where he spent two days in a coma.

When he awoke, Mr. Harper was baffled, as were his doctors. His annual medical checkups had indicated he was in excellent health. How could this have happened to someone seemingly so healthy?

The culprit, in turned out, was a fatty particle in the blood called lipoprotein(a). While doctors routinely test for other lipoproteins like HDL and LDL cholesterol, few test for lipoprotein(a), also known as lp(a), high levels of which triple the risk of having a heart attack or stroke at an early age.

For most people, lp(a) is nothing to worry about. Levels are strongly determined by genetics and the majority of people produce very little of it.

But up to one in five Americans, including Mr. Harper, have perilously high levels of it in their blood. Studies show that diet and exercise have almost no impact on lp(a), and cholesterol-lowering drugs only modestly lower it.

“People don’t know about it, physicians don’t know about it, and we have to get an education program out there, but that’s expensive,” said Dr. Henry N. Ginsberg, the Irving Professor of Medicine at Columbia University and a leading expert on lp(a). “I would say that somewhere between 15 to 20 percent of the population would clearly benefit from knowing that this is their problem.”

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