The study's authors examined just the ends of DNA strands. Called telomeres, these act something like the plastic caps on shoelaces, preventing the DNA in chromosomes from unraveling.
Previous research has shown that older people have shorter ends than younger folks. Indeed, biologists say they shrink every time a cell divides.
How does this lead to overall decrepitude? Eventually it stops your cells from dividing and replenishing themselves, said Emmanuel Skordalakes, a researcher at the Wistar Institute in Philadelphia.
"When the telomeres become short, then you start cutting into actual chromosomes where there are genes essential for our body," he said.
To prevent the fraying DNA in all those aging cells from seeding malignant tumors, Skordalakes said, the body turns them dormant. "Your body shuts down more and more cells every day and you become old."
Not everyone's DNA ages at the same rate. Some people may start off with sturdier telomeres than others, or perhaps longer ones, researchers said.
To try to separate the influences of heredity and lifestyle, researchers at King's College in London studied more than 2,401 sets of twins.
The length of the twins' telomeres was directly related to their activity levels, the researchers found. People who did a moderate amount of exercise — about 100 minutes a week of activity such as tennis, swimming or running — had telomeres that on average looked like those of someone about five or six years younger than those who did the least — about 16 minutes a week. Those who did the most — doing about three hours a week of moderate to vigorous activity — had telomeres that appeared to be about nine years younger than those who did the least.
"It's another jigsaw piece in trying to understand why exercise is important in longevity," said Stephen Coles, who studies aging at the University of California at Los Angeles. But Coles and others stressed that much more research is needed to definitively establish a causal relationship between exercise and aging.
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