Cells lining our digestive system are more likely to commit suicide the older we get, according to researchers at the University of Manchester and the Paterson Institute.
"These stem cells live on a knife-edge. If damaged, they kill themselves," Tom Kirkwood told the Health Span Conference at the University of Manchester this week. They appear to change with age - older cells kill themselves in response to attacks that are withstood by younger ones.
Stem cells renew the lining of the intestines. When they go wrong it affects digestion, nutrition and water balance.
Professor Kirkwood is working on a computer model to identify how stem cells age. There are three main mechanisms: cells can accumulate aberrant proteins, oxidative damage or their internal powerhouses - mitochondria - accumulate mutations. "These mechanisms interact," Professor Kirkwood said. By entering data, such as the concentration of key molecules in the cells, he hopes to unravel the interaction. For example, one process might initiate cell death but another might finish it.
There is also a genetic basis to longevity. "We all know of someone who lived to a ripe old age and did everything wrong," said Professor Kirkwood. Other researchers are trying to discover whether certain genes protect people, such as smokers, from developing diseases like lung cancer.
One possibility is that genes that predispose people to developing a disease in youth actually protect against it in older age. A French study found that many centenarians have a gene that would normally be a risk factor for heart disease. Cholesterol also becomes protective in old age.
n What needs to be done about an ageing population, Anthea Tinker of King's College London asked the conference.
The number of pensioners has risen from 6 per cent of the population in 1901 to 18 per cent today. By 2025, almost a quarter of the population will be pensioners.
The proportion of very old people is also expanding. In 1995, 4 per cent of the population was over 80; by 2025, this will have climbed to more than 5 per cent.
"Many older people are still in good health and work," said Professor Tinker. As part of a study to be published next year, she interviewed 40 people who were older than 85. "Some of them said: 'Please don't come until after 7pm because I am at work until then'."
But she also found that the cost of healthcare for the over-85s was more than twice that for younger pensioners. It costs Pounds 2,530 a year to look after an 85-year-old, compared with Pounds 1,050 for people aged between 65 and 74 years.
She is worried about a shortage of nurses and women doctors. As the number of elderly people from ethnic minorities grows, authorities must realise that females prefer to be treated by female doctors.