When cells divide, they use carbon 14 to build DNA, a phenomenon that can serve as a birthmark for new cells. By looking at DNA from people born before 1955, when the first nuclear bombs were tested, researchers could see whether heart cells were born after the people in the study were born.

About 1 percent of the heart muscle cells are replaced every year at age 25, and that rate gradually falls to less than half a percent per year by age 75, concluded a team of researchers led by Dr. Jonas Frisen of the Karolinska Institute in Stockholm.

To cardiologists, this is a blockbuster discovery, since the heart has been pegged as a disadvantaged organ in terms of injury, healing, and repair. Susceptible to coronary blockages that can cut off blood and destroy major hunks of heart muscle at one time in a heart attack, the heart can only heal itself slowly, often leaving behind thinned and baggy scar tissue devoid of healthy, beating muscle. And the distortion and remodeling of the heart that comes with this muscle loss sets the patient up for cardiac failure, blood clots, and nasty heart rhythms.