for National Geographic News
How long can humans conceivably live? In most developed countries, life expectancy has grown steadily to an average of 75 years.
But scientists are exploring ways to extend lifespan to lengths that seem inconceivable nowperhaps 120 years and beyond.
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Ideally, future centenarians who avail themselves to life-prolonging advances won't suffer the familiar frailties of old age. The goal is for them to retain their youthful vitality, rather than add extra years of decline.
Several studies show lifespans can be stretched far beyond normal limits. In one example, Cynthia Kenyon, a professor at the University of California in San Francisco, has doubled the lifespans of simple roundworms from two weeks to a month by altering the function of a single gene, known as daf-2.
Even near death, these mutated worms look better than normal worms half their age. Their bodies are smooth and plump, and they wriggle along like much younger worms.
"It's amazing that they look so healthy," Kenyon said.
The daf-2 gene has two counterparts in mammals, both conserved during evolution. One is the insulin receptor, or tiny cell structure, that controls levels of blood sugar. By inactivating this receptor in mouse fat cells, scientists can increase the rodents' lifespans by up to 18 percent.
Even longer lifespans are achieved by changing the daf-2 genes's other mammalian counterpart, known as insulin-like growth factor, or IGF-1. Reduce this gene's function and mice can live up to 33 percent longer than average.
Why? Preliminary evidence suggests the mice suffer less cell and organ damage from naturally corrosive oxidants, which play a role in aging. What's more, the animals are less susceptible to age-related diseases, including cancer.
"They're more youthful, and so naturally more resistant to these diseases," Kenyon said.
Diets and Aging
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