06:07am Monday 17 December 2018

No finite limit to the human lifespan, says statistics professor

s there a finite limit to the human lifespan? Among researchers of this issue, there is a lot of heated discussion. According to Holger Rootzén, Professor of Mathematical Statistics, there is not. His research has produced the surprising observations that survival rates after 110 years are seemingly unaffected by lifestyle or genetics – and that in the next 25 years, we may see a human live to 128 years old.

​In October 2016, a paper published in the scientific journal Naturesuggested that there was a hard limit to human lifespan. Looking at global demographic data, the researchers claimed that the natural limit for a human life was around 115 years.
However, among the research community there was some serious doubt as to the accuracy of this claim. Holger Rootzén was contacted by the scientific journal Extremes, and asked to investigate the paper further. Together with his colleague Dmitrii Zholud, postdoctoral researcher at the Department of Mathematics, Holger published his work last year, concluding that the original authors had analysed the data incorrectly.

The original paper analysed data from 1968 – 2006, looking at the maximum age through the years. They saw that maximum lifespan increased from 1968, peaked in the 1990s, and then declined slightly towards 2006. The researchers concluded therefore, that an upper limit had been reached. But Holger and Dmitrii saw that this conclusion was false, and based on misinterpretation of limited data.
“In the mid-90s, they had data from 4 countries, which they combined. But for the earlier and the later periods, they only had data from one or two countries. So of course, in the middle there was a higher number of supercentenarians, with fewer at the beginning and at the end,” explains Holger. “It’s like playing darts – if you throw 10 darts at a board, compared to if you throw 1000 – the best score of your 1000 tries will surely be better than the best score of your 10 tries. Likewise, for the second-best hit,” he continues.

Risk of dying ‘plateaus’ at extremely high ages

Now, another paper recently published in the journal Scienceseems to support Holger’s research. Holger was subsequently invited by the Washington Post and Live Science to comment on the findings, and will very shortly publish a rejoinder article in Extremes, summarising the debate so far, and responding to points raised.
One of the biggest challenges for investigating the secrets of extremely long life is finding enough reliable, verifiable data. Claims of long-life are often prone to exaggeration, and lack of evidence. The new paper in Science made use of a brand-new dataset, of all the individuals in Italy aged 105 or older, between 2009 and 2015. All of the people in the data had accurate birth and death certificates (or were still alive), making it a very trustworthy source. Holger and Dmitrii’s data covered individuals aged 110 and older, from 15 countries.
What Holger and the Italian researchers both agreed on, was that although the risk of dying increases as we get older, after a certain point the mortality rate actually levels off. In other words, the chance of living from 110 to 111 is the same as living from 111 to 112 – about 50%. Holger’s work observed this plateauing effect from the age of 110, but the Italian researchers, with their new data, saw it occur earlier, after 105.

This indicates that we could yet see the human lifespan extend beyond that of Jeanne Calment, a French woman who died in 1997, at the age of 122 years and 164 days.

Survival chance after 110 years remains the same for all groups

Further to this, Holger and Dmitrii made another startling observation. Beyond the age of 110, there seemed to be very few identifiable factors that influenced survival rates. For example, women generally live longer than men, but, after the age 110, this difference disappears. There is also no appreciable difference in survival rates between different countries or regions – supercentenarians from Japan, northern and southern Europe, and the USA all have the same mortality rate after 110 years. Additionally, so did people from throughout the entire period 1968 – 2006.
“This is quite surprising and interesting. You would expect genetics and lifestyle to play a role, as they surely do at earlier ages. But after 110, it seems you are equally likely to survive regardless,” says Holger.

So will we see Jeanne Calment’s record broken anytime soon? Holger and Dmitrii’s paper offers their prediction for highest human lifespan in the next few decades.
“We need to know two things: first, we need to know the survival rate after 110. Second, we need to know how many people reach 110 in the first place. Based on these two factors, we came up with a distribution of somewhere between 119 and 128 years old. We would expect the oldest person in the next 25 years to be somewhere between those two ages. Assuming there isn’t a big war!”

 

CHALMERS UNIVERSITY OF TECHNOLOGY


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