Oct 06 2016

The Longevity Debate

lifespan-tableA recent study that concludes human lifespan is approaching its maximum (around 115 years) has created some interesting debate. It’s the good kind of scientific debate that focuses on how to interpret data and infer the implications of limited data.

The authors, Dong, Milholland and Vijg, examined the mortality database, which includes death statistics from 38 countries. They looked at the age of the oldest person to die in each country by year. They also looked at the change in life-expectancy by age group.

To clarify one point of terminology that is often confused: life expectancy refers specifically to the number of years a person is statistically likely to live from their current age. Usually when you see life expectancy figures quoted, those are life expectancy from birth.

For example, a person born in 2012 in the US has a life expectancy of 78.8 years. If you were 65 in 2012 in the US your life expectancy was 84.5 (both are slightly higher for women and slightly lower for men).

Lifespan is the maximal number of years a species can theoretically live, given optimal health and being free from accidents or disease. It is lifespan, not life expectancy, that is the focus of this research.

The oldest recorded age at death for a human was Jeanne Calment, who died in 1997 at the age of 122. She, however, was an outlier (there’s always an outlier with any large-enough data set).

The researchers looked at the age of the oldest person to die in each country, and they found that this age increased from 1970 to 2000, reaching an average of about 115 years. From 2000 to present, however, this figure has peaked, and in fact trended down slightly.

They also looked at the age that had the greatest annual increase in survival. If both life expectancy and lifespan were increasing then this number should be increasing. They found that this number was also increasing from 1920 to about 1980, but then plateaued at around 100 years, and has only slightly crept up since then (101 for men, 102 for women).

The authors conclude that these two trends indicate that we are pushing up against the maximal lifespan for humans of about 115 years. Obviously there is going to be a Bell curve around this average maximal lifespan, with Calment representing the far right tail at 122. They think it is very unlikely anyone will break her record.

No one is disagreeing with their data. These trends are pretty clear, they involve a large data set, and their analysis is fairly straightforward. Where there is disagreement is about the interpretation.

The authors argue that the data suggest we are approaching the maximal theoretical human lifespan at 115 years. We can continue to push life expectancy closer to this maximum, but the upper limit has plateaued. There are simply biological constraints and the human machine is not sustainable beyond that point.

Longevity enthusiasts disagree. Aubrey de Grey does a good job of representing this side:

Biomedical gerontologist Aubrey de Grey, chief science officer of the SENS Research Foundation Mountain View, California, which develops and promotes rejeuvenation biotechnologies, is more hopeful. “Unlike a dam, the pressure on the so-far-unplugged leaks actually diminishes as one plugs more and more of them,” he says. “The result in this paper is absolutely correct, but it says nothing about the potential of future medicine, only the performance of today’s and yesterday’s medicine.”

This statement is correct, but does not provide any argument that future medicine will accomplish what modern medicine has not. Aubrey de Grey obviously believes that it will, that is essentially his life’s work.

My take is that this evidence is entirely demographic. This tells us what is happening with human lifespan, and clearly shows that our modern medical interventions have increased lifespan to about 115 years, but have peaked without any significant increase in the last two decades. Therefore, increasing lifespan is not going to be a relentless effect of improving medical technology. We have already reaped the benefits of modern medical care.

The study, however, is not any kind of analysis of the biological limits of the human lifespan, or of current theories about how to increase it. The answer there is – we simply don’t know at this point.

Some argue that animal research suggests there is huge potential to increase lifespan. However, animal models are a poor analogy to humans when it comes specifically to longevity. Research in mice or other animals typically does not translate to humans’ longevity. Further, you might argue that humans have already reaped those benefits – we already have an extended lifespan, beyond the animal models we are using, even relatively speaking. We are now trying to push it even further and we just don’t have an animal model for that.

What I think this research is saying is that we are at the limits of our current approaches to human health. If we are going to push past this barrier we need entirely new approaches, although ones that are already theoretically possible. For example, it may take human genetic engineering to significantly increase lifespan.

One approach that is already getting a lot of attention is treatments that increase telomere length. Telomeres are the caps at the ends of chromosomes. They tend to shorten over time and this limits the age of cells. Lengthening them does increase longevity, but we do not yet have a proven safe and approved treatment for this.

We may also be able to freshen up organs with stem cell therapy. There are other theoretical interventions to help cells survive longer, like clearing out cellular debris.

My sense is that the high tech interventions that will push past the 115 year lifespan limit are possible, but decades away at best.

So, in a sense both sides of this debate are correct. The evidence does strongly suggest that we are approaching the limits of human lifespan at 115 years. That is the best our current approaches to health care will provide. But, that does not rule out that new technologies can be game-changers. It does mean, that is what it will take – radical new approaches that change the nature of the game of slowing aging.

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