The stress of caring for a sick child can add 10 or more years to the biological age of a woman's cells, researchers have found.
Telomeres shorten each time a cell divides
A University of California team suggest stress speeds up cell ageing.
It does this by affecting key pieces of DNA called telomeres which are involved in regulating cell division, they say.
The team say the Proceedings of the National Academy of Sciences study shows how stress could be linked to the early onset of age-related diseases.
Telomeres are strips of DNA at the end of chromosomes which appear to protect and stabilise the chromosome ends.
However, they shorten each time a cell divides, until there is nothing left, making cell division less reliable and increasing the risk of age-related disorders.
Previous research had suggested that premature ageing was partly caused by stress, but how the mechanism was unclear.
In this study, the researchers examined 58 pre-menopausal women. Nineteen had healthy children, the rest had children with chronic illnesses.
All the women completed questionnaires asking them to evaluate the level of stress they felt they had been under during the previous month.
Blood samples were also taken so scientists could carry out DNA analysis of telomeres.
Levels of an enzyme called telomerase, which helps build and maintain telomeres, in immune cells were also measured.
The researchers found that women who had reported higher levels of psychological stress - those who were caring for sick children - had shorter telomeres.
They said that, on average, the difference was equivalent to over a decade of additional ageing compared with women who classed themselves as having low levels of stress.
The higher-stress group also had lower levels of telomerase in immune cells.
The researchers, led by Dr Elissa Epel, said this implied the immune cells could function less well and could die sooner.
It was also found that the high-stress women also had higher oxidative stress levels - cumulative damage caused by molecules called "free radicals" - which has been shown to speed up the shortening of telomeres in lab studies.
Writing in Proceedings, the researchers said it was not clear exactly how stress affected telomeres, but they suggest changes in stress hormone levels could have an effect.
They add that their findings showed how cellular aging could be a way in which psychological stress was linked to the earlier onset of age-related diseases.
Professor Thomas von Zglinicki of the clinical medical sciences department at the University of Newcastle, said: "The paper confirms the general perception that stress 'wears you out' and makes you ageing faster by measuring telomere length, which is one possible bio-marker of ageing and age-related disease."
He said the study confirmed what his team had suggested in 2000.
"We said then that telomere length in human blood might be markers for oxidative stress and the capacity of the individual to cope with it."
But Professor von Zglinicki added: "The study is small, and there are a number of unpublished studies that could not confirm telomere length as a strong biomarker.
"So this paper is very interesting and might be very important, but we still need some caution."