Going to the gym really does keep you young, according to new study findings.
Researchers found brief vigorous exercise tends to slow the ageing process, which could help explain why those with healthy lifestyles are likely to live longer.
Just 15 minutes of energetic activity a day can reduce stress and prevent the deterioration of vital cells which lead to us feeling and looking older.
The study shows increasing activity levels not only makes people feel better, it improves the ability of the body's cells to fight disease and premature ageing.
The effect comes from increasing levels of the enzyme telomerase which plays a critical part in the control of cell ageing.
Psychologist Dr Eli Puterman, who led the US research team at the University of California in San Francisco, said the study built on previous work that showed how changes in DNA in the body resulted in ageing.
He said 'We have extended those findings to show that, in fact, there are things we can do about it.
'If we maintain the levels of physical activity recommended by public health bodies we can prevent the unyielding damage that psychological stress may have on our body.'
The researchers studied the effect of exercise on protective strips of DNA called telomeres - tiny 'caps' on the ends of chromosomes which protect against inflammation and other ageing processes.
Telomeres have been called the 'chromosomal clock' because they appear to be central to biological ageing.
They shorten over time and after a certain point are no longer able to prevent the DNA falling apart.
Scientists believe this process is at the heart of many age-related diseases, and may even place a final limit on human lifespan.
The latest study found women who did about 15 minutes of exercise a day were able to stop the strips of DNA from shortening.
Dr Elissa Epel, an associate professor in the UCSF Department of Psychiatry, said 'Telomere length is increasingly considered a biological marker of the accumulated wear and tear of living.
'Even a moderate amount of vigorous exercise appears to provide a critical amount of protection for the telomeres.'
As little as 42 minutes of vigorous exercise over a three-day period was shown to protect telomere length.
The researchers recruited 62 post-menopausal women, many of whom were caring for spouses or parents with dementia.
The women reported at the end of each day how many minutes of vigorous activity they had engaged in and how stressed they felt.
Vigorous activity was defined as 'increased heart rate and/or sweating', says a report in the Public Library of Science's online journal.
The women also gave blood samples so the researchers could measure the telomere length of their immune cells.
The results showed psychological stress promoted immune cell ageing through shortening of telomeres.
But even the highly stressed women were able to stop this ageing process if they had exercised.