A simple urine test could measure how much our bodies have aged and even how long left we have to live, new research suggests.
Scientists discovered a certain substance, known as 8-oxoGsn, which indicates cell damage, increases in people's urine as they get older.
By determining people's 'internal age', it may be possible to assess their risk of suffering age-related illnesses and even premature death, according to the researchers.
The efficient urine-analysis technique used by the scientists, which assessed the samples of up to 10 individuals in an hour, may be useful in future studies investigating the link between 8-oxoGsn and biological ageing, they add.
Critics argue, however, ageing alone does not accurately indicate the onset of diseases, adding the study did not investigate a link between 8-oxoGsn and conditions associated with the elderly, such as Parkinson's.
Study author Dr Jian-Ping Cai, from the National Centre of Gerontology in China, said: 'Urinary 8-oxoGsn may reflect the real condition of our bodies better than our chronological age, and may help us to predict the risk of age-related diseases.'
'Urinary 8-oxoGsn is promising as a marker of ageing'
Previous studies suggest 8-oxoGsn levels increase in the urine of animals as they age.
The researchers therefore analysed urine samples from 1,228 Chinese residents aged between two and 90 years old.
Dr Cai said: "We found an age-dependent increase in urinary 8-oxoGsn in participants [aged] 21 years old and older."
"Therefore, urinary 8-oxoGsn is promising as a new marker of ageing."
Results suggest levels of 8-oxoGsn were roughly the same between the men and women participating in the study, except in post-menopausal females, who demonstrated higher levels.
Speaking of the findings, Ilaria Bellantuono, professor of musculoskeletal ageing at the University of Sheffield, who was not involved in the study, said: 'This paper shows that it is possible to detect 8-oxoGsn in human urine and its increase correlates with increasing age.
"However, ageing is not a disease but a risk factor for age-related diseases, in the same way smoking is for lung cancer.
"Therefore it is not possible to say whether this could be used as marker to predict the occurrence of disease.
"For example, this paper does not show whether this marker is present at higher levels in people affected by diseases such as osteoporosis, diabetes, Parkinson's or whether it is present at higher level in people who then go on to develop those diseases in longitudinal studies."
The findings were published in the journal Frontiers in Ageing Neuroscience.
Internal ageing may begin post-menopause in women
Changes in the level and function of oxygen is thought to reflect people's internal age.
Dr Cai said: "Oxygen by-products produced during normal metabolism can cause oxidative damage to biomolecules in cells, such as DNA and RNA."
"As we age, we suffer increasing oxidative damage and so the levels of oxidative markers increase in our body."
This may occur after the menopause in women due to oestrogen having a known antioxidant effect.
Having children ages women's DNA by 11 years
This comes after research released earlier this month suggested having children ages women's DNA by 11 years.
Giving birth shortens women's telomeres, which 'cap' the end of DNA strands and are associated with health, by around 4.2 per cent, a study found.
Longer telomere lengths are linked to slower aging, longer lifespans and improved overall health.
Such an extent of telomere shortening is greater than the effects of smoking or obesity demonstrated in previous studies.
Study author Dr Anna Pollack from George Mason University, Virginia, told the New Scientist: 'We were surprised to find such a striking result. It is equivalent to around 11 years of accelerated cellular ageing.'
The researchers believe this may be due to the stress of raising children, particularly in countries without mandatory maternity leave, such as the US.
They stress, however, more research into the link between motherhood and genetic ageing is required, with Dr Pollack adding: "We're not saying 'don't have children".