For the sake of heart disease research, 809 members of the Old Order Amish community agreed to go to a clinic in Lancaster, Pa., near their homes, and drink a rich milkshake that was made mostly of heavy cream. Over the next six hours, a group of investigators took samples of their blood, determining how much fat was churning through their bloodstreams.

Most of the study participants responded as expected  their levels of triglycerides, a common form of fat in the blood, rose steadily for three to four hours and then declined. But about 5 percent had an extraordinary reaction: their triglyceride levels started out low and hardly budged.

It turns out, the researchers report in the Friday issue of the journal Science, that those individuals who barely responded have a mutation that disables one of their two copies of a gene called apoC-III. The gene codes for a protein, APOC3, that normally slows the breakdown of triglycerides.

With the mutated gene, people break down triglycerides unusually quickly. And, the investigators find, they also have low levels of LDL cholesterol, which at high levels increases heart disease risk. They have high levels of HDL cholesterol, which is associated with a decreased risk of heart disease. And they appear to have arteries relatively clear of plaque.