Fish oil may be the true elixir of youth, according to new evidence of its effect on biological ageing. Omega-3 fatty acids from fish oil preserve 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 rates after a heart attack, reduce mental decline in old age and help to prevent age-related changes in the eye that can lead to blindness. Research has also shown that rodents live one-third longer when given a diet enriched with fish-derived omega-3.

Although omega-3 fatty acids have powerful anti-inflammatory properties and lower levels of some blood fats, the mechanisms behind these effects are poorly understood. The new research suggests that omega-3 has a direct effect on biological ageing by slowing down the rate at which protective caps on the ends of chromosomes shorten.

The caps, called telomeres, are made from copied strands of DNA and have a similar function to bookends or the plastic ends of shoelaces. They prevent the ends of chromosomes – the "packages" of DNA in the cell nucleus – becoming damaged and keep the DNA organised and contained.

Each time a cell divides, its telomeres get shorter until a critical point is reached. DNA then becomes damaged and the cell stops dividing, and may die. In this way, the telomere acts like a biological fuse.

The rate at which the fuse "burns" can vary both between individual people and individual cells. This is believed to have an impact on age-related diseases.

US scientists conducting the research looked at the effect of omega-3 fatty acids on telomere shortening in 608 hospital out-patients with heart disease. At the start of the study, measurements were taken of the length of chromosomal telomeres in the patients' white blood cells. Blood levels of the two fish-derived omega-3 fatty acids docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) were also measured.

The tests were carried out again after five years, and showed a clear correlation with omega-3 intake. Patients consuming the least omega-3 had the fastest rate of telomere shortening, while those in the top 25% of consumption levels had the slowest rate.

The scientists, led by Dr Ramin Farzaneh-Far of the University of California at San Francisco, wrote in the Journal of the American Medical Association: "The present findings identify deceleration of telomere attrition as a potentially novel pathway for the anti-ageing effects of marine omega-3 fatty acids.

"In summary, among patients with stable coronary artery disease, there was an inverse relationship between baseline blood levels of marine omega-3 fatty acids and the rate of telomere shortening over five years ... These findings raise the possibility that omega-3 fatty acids may protect against cellular ageing in patients with coronary heart disease."