Article in Science

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" How Much Can Human Life Span Be Extended? --- Couzin

When Jeanne Calment died in a nursing home in southern France in

1997, she was 122 years old, the longest-living human ever

documented. But Calment's uncommon status will fade in subsequent

decades if the predictions of some biologists and demographers come

true. Life-span extension in species from yeast to mice and

extrapolation from life expectancy trends in humans have convinced a

swath of scientists that humans will routinely coast beyond 100 or

110 years of age. (Today, 1 in 10,000 people in industrialized

countries hold centenarian status.) Others say human life span may be

far more limited. The elasticity found in other species might not

apply to us. Furthermore, testing life-extension treatments in humans

may be nearly impossible for practical and ethical reasons.

Just 2 or 3 decades ago, research on aging was a backwater. But when

molecular biologists began hunting for ways to prolong life, they

found that life span was remarkably pliable. Reducing the activity of

an insulinlike receptor more than doubles the life span of worms to a

startling--for them--6 weeks. Put certain strains of mice on near-

starvation but nutrient-rich diets, and they live 50% longer than

normal. Some of these effects may not occur in other species. A

worm's ability to enter a " dauer " state, which resembles hibernation,

may be critical, for example. And shorter-lived species such as worms

and fruit flies, whose aging has been delayed the most, may be more

susceptible to life-span manipulation. But successful approaches are

converging on a few key areas: calorie restriction; reducing levels

of insulinlike growth factor 1 (IGF-1), a protein; and preventing

oxidative damage to the body's tissues. All three might be

interconnected, but so far that hasn't been confirmed (although

calorie-restricted animals have low levels of IGF-1).

Can these strategies help humans live longer? And how do we determine

whether they will? Unlike drugs for cancer or heart disease, the

benefits of antiaging treatments are fuzzier, making studies

difficult to set up and to interpret. Safety is uncertain; calorie

restriction reduces fertility in animals, and lab flies bred to live

long can't compete with their wild counterparts. Furthermore,

garnering results--particularly from younger volunteers, who may be

likeliest to benefit because they've aged the least--will take so

long that by the time results are in, those who began the study will

be dead.

That hasn't stopped scientists, some of whom have founded companies,

from searching for treatments to slow aging. One intriguing question

is whether calorie restriction works in humans. It's being tested in

primates, and the National Institute on Aging in Bethesda, land,

is funding short-term studies in people. Volunteers in those trials

have been on a stringent diet for up to 1 year while researchers

monitor their metabolism and other factors that could hint at how

they're aging.

Insights could also come from genetic studies of centenarians, who

may have inherited long life from their parents. Many scientists

believe that average human life span has an inherent upper limit,

although they don't agree on whether it's 85 or 100 or 150.

One abiding question in the antiaging world is what the goal of all

this work ought to be. Overwhelmingly, scientists favor treatments

that will slow aging and stave off age-related diseases rather than

simply extending life at its most decrepit. But even so, slowing

aging could have profound social effects, upsetting actuarial tables

and retirement plans.

Then there's the issue of fairness: If antiaging therapies become

available, who will receive them? How much will they cost?

Individuals may find they can stretch their life spans. But that may

be tougher to achieve for whole populations, although many

demographers believe that the average life span will continue to

climb as it has consistently for decades. If that happens, much of

the increase may come from less dramatic strategies, such as heart

disease and cancer prevention, that could also make the end of a long

life more bearable. "

Rodney.