When we eat a fatty meal, the fats are packaged into triglycerides by the intestine and put into the blood.
An enzyme on blood vessels then cleaves the triglycerides so the fats can be used for energy in the heart and skeletal muscles. Fats not used as energy are deposited in adipose tissue.
Excess blood triglycerides can be deposited in artery walls, raising the risk of heart disease, said Dan Rader, chairman of the department of genetics at the University of Pennsylvania's Perelman School of Medicine.
A gene called APOC3 contains the recipe for a protein that inhibits, or slows down, the activity of this fat-cleaving enzyme and raises the level of blood triglycerides and risk of heart disease.
Scientists have previously shown that people who carry one copy of this mutation are at lower risk of getting heart disease.
Recently, researchers have identified human APOC3 "knockouts" — people in Pakistan who inherited two defective copies of this gene, one from each parent. On average, they have even lower levels of triglycerides than the Amish people with one copy.