Omega-3 fatty acids from fish oil extend the genetic "fuse" that determines the lifespan of cells, say scientists.
The discovery, made in heart disease patients, may explain many of the claimed health benefits of omega-3.
Taking fish oil supplements is said to protect against heart disease, improve survival after a heart attack, reduce mental decline, and prevent age-related changes in the eye that can lead to blindness.
Dr Ramin Farzaneh-Far from the University of California at San Francisco, wrote in the Journal of the American Medical Association:
"These findings raise the possibility that omega-3 may protect against cellular ageing in patients with coronary heart disease."