Life expectancy in elderly people is linked to the length of special structures in their DNA, according to a study published today in the British medical journal Lancet. The shorter these structures -- called telomeres -- the earlier a person died, the report found.
Dr. Richard Cawthon of the University of Utah and co-workers analyzed blood DNA samples from 143 men and women aged 60 years or older. The blood had been drawn for another purpose between 1982 to 1986. At the time of the study, 101 of the people had died.
For each sample, the scientists measured the lengths of the telomeres, which are special, protective "caps" that sit at the end of all chromosomes.
The scientists found that overall, older people had shorter telomeres than younger people. This was expected. Previous studies have shown that human telomeres get steadily shorter through life, and some scientists think this could be a key contributor to aging through the malfunctioning of cells.
However, Cawthon and his colleagues also compared telomere length between people of similar ages to identify those with shorter- or longer-than-average telomeres for their age.
The scientists found that people whose telomere lengths were in the shortest quarter of the group had a mortality rate from infectious disease eight times higher than those with the longest telomeres. People with telomere lengths in the shorter half had a threefold higher mortality rate from heart disease. Deaths from both these diseases increase with age.
Overall, women with shorter telomeres died an average of 4.8 years sooner than women with longer telomeres; men with shorter telomeres died four years earlier than those with longer ones.
Elizabeth Blackburn, a UC San Francisco molecular biologist and telomere researcher, said the study is the first she knows of that compares telomere lengths at specific ages and links them to mortality.
But she said the study does not necessarily mean that shorter telomere length causes earlier deaths. For instance, telomere length may shorten over time as a consequence of aging, not as a cause.
Cawthon noted that the association is not strong enough to provide a test that would allow individual people to know how long they are likely to live. Even if it could, he said, it would be of little use because there is no way to lengthen someone's telomeres.
"I have never had my telomeres measured," he said. "The way I look at it, if there's no treatment, I'm not interested."