Old questions, new answers! 인간의 수명은?
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Death rates in later life flatten out and suggest there may be no fixed limit on human longevity, countering some previous work.
Emma Morano, an Italian supercentenarian who died in 2017 at the age of 117, was the world’s last surviving person born in the 19th century.Credit: Antonio Calanni/AP/REX/Shutterstock
There might be no natural limit to how long humans can live — at least not one yet in sight — contrary to the claims of some demographers and biologists.
That’s according to a statistical analysis published Thursday in Science1on the survival probabilities of nearly 4,000 ‘super-elderly’ people in Italy, all aged 105 and older.
A team led by Sapienza University demographer Elisabetta Barbi and University of Roma Tre statistician Francesco Lagona, both based in Rome, found that the risk of death — which, throughout most of life, seems to increase as people age — levels off after age 105, creating a ‘mortality plateau’. At that point, the researchers say, the odds of someone dying from one birthday to the next are roughly 50:50 (see ‘Longevity unlimited’).
“If there is a mortality plateau, then there is no limit to human longevity,” says Jean-Marie Robine, a demographer at the French Institute of Health and Medical Research in Montpellier, who was not involved in the study.
That would mean that someone like Chiyo Miyako, the Japanese great-great-great-grandmother who, at 117, is the world’s oldest known person, could live for years to come — or even forever, at least hypothetically.
Researchers have long debated whether humans have an upper age limit. The consensus holds that the risk of death steadily increases in adulthood, up to about age 80 or so. But there’s vehement disagreement about what happens as people enter their 90s and 100s.
Some scientists have examined demographic data and concluded that there is a fixed, natural ‘shelf-life’ for our species and that mortality rates keep increasing. Others have looked at the same data and concluded that the death risk flattens out in one’s ultra-golden years, and therefore that human lifespan does not have an upper threshold.
Age rage
In 2016, geneticist Jan Vijg and his colleagues at Albert Einstein College of Medicine in New York City rekindled the debate when they analysed the reported ages at death for the world’s oldest individuals over a half-century. They estimated that human longevity hit a ceiling at about 115 years — 125 tops.
Vijg and his team argued2 that with few, if any, gains in maximum lifespan since the mid-1990s, human ageing had reached its natural limit. The longest known lifespan belongs to Jeanne Calment, a French super-centenarian who died in 1997 at age 122.
Experts challenged the statistical methods in the 2016 study, setting off a firestorm into which now step Barbi and Lagona. Working with colleagues at the Italian National Institute of Statistics, the researchers collected records on every Italian aged 105 years and older between 2009 and 2015 — gathering certificates of death, birth and survival in an effort to minimize the chances of ‘age exaggeration’, a common problem among the oldest old.
They also tracked individual survival trajectories from one year to the next, rather than lump people into age intervals as previous studies that combine data sets have done. And by focusing just on Italy, which has one of the highest rates of centenarians per capita in the world, they avoided the issue of variation in data collection among different jurisdictions.
As such, says Kenneth Howse, a health-policy researcher at the Oxford Institute of Population Ageing in the United Kingdom, “these data provide the best evidence to date of extreme-age mortality plateaus in humans”.
Ken Wachter, a mathematical demographer at the University of California, Berkeley, and an author of the latest study, suspects that prior disputes over the patterns of late-life mortality have largely stemmed from bad records and statistics. “We have the advantage of better data,” he says. “If we can get data of this quality for other countries, I expect we’re going to see much the same pattern.”
Robine is not so sure. He says that unpublished data from France, Japan and Canada suggest that evidence for a mortality plateau is “not as clear cut”. A global analysis is still needed to determine whether the findings from Italy reflect a universal feature of human ageing, he says.
Off limits
The world is home to around 500,000 people aged 100 and up — a number that’s predicted to nearly double with each coming decade. Even if the risk of late-life mortality remains constant at 50:50, the swelling global membership in the 100-plus club should translate into a creep upwards in the oldest person alive by about one year per decade, says Joop de Beer, a longevity researcher at the Netherlands Interdisciplinary Demographic Institute in The Hague.
Many researchers say they hope to better understand what’s behind the levelling off of mortality rates in later life. Siegfried Hekimi, a geneticist at McGill University in Montreal, Canada, speculates that the body’s cells eventually reach a point where repair mechanisms can offset further damage to keep mortality rates level.
“Why this plateaus out and what it means about the process of ageing — I don’t think we have any idea,” Hekimi says.
For James Kirkland, a geriatrician at the Mayo Clinic in Rochester, Minnesota, the strong evidence for a mortality plateau points to the possibility of forestalling death at any age. Some experts think that the very frail are beyond repair. But if the odds of dying don’t increase over time, he says, interventions that slow ageing are likely to make a difference, even in the extremely old.
Not everyone buys that argument — or the conclusions of the latest paper.
Brandon Milholland, a co-author of the 2016 Nature paper, says that the evidence for a mortality plateau is “marginal”, as the study included fewer than 100 people who lived to 110 or beyond. Leonid Gavrilov, a longevity researcher at the University of Chicago in Illinois, notes that even small inaccuracies in the Italian longevity records could lead to a spurious conclusion.
Others say the conclusions of the study are biologically implausible. “You run into basic limitations imposed by body design,” says Jay Olshansky, a bio-demographer at the University of Illinois at Chicago, noting that cells that do not replicate, such as neurons, will continue to wither and die as a person ages, placing upper boundaries on humans’ natural lifespan.
This study is thus unlikely to be the last word on the age-limit dispute, says Haim Cohen, a molecular biologist at Bar-Ilan University in Ramat-Gan, Israel. “I’m sure that the debate is going to continue.”