iAge: Life Expectancy

Life expectancy is akin to the median rather than an average age of death. The median is a statistic, that defines when half a given population is expected to die before that age, and the other half will survive beyond that period. In physics it is close to the half-life of elements. This statistic is not an average (mean), it is a-mid point. Most confuse life expectancy with the average age of death. But it is best described as a mid-point. Gerontologists use life expectancy to define aging of populations. But there are nuances—both statistical and biological—that heed caution about interpreting historical life expectancy data. The problem is that life expectancy is equated with lifespan, and the two are not related.

In 2002, Jim Oeppen and James Vaupel from the Max Planck Institute for Demographic Research showed that life expectancy in some of the world’s developed countries (and Chile) has been increasing steadily by about 2.5 years per decade since the mid-19th century. Although they leave out contradictory evidence from across the world, including a large country like Russia—this argument that life expectancy is constantly improving also ignores latest life expectancy figures from the USA. For Blacks/African Americans in the United States life expectancy is declining. Despite these realities, there is no denying that long-term stable decline in mortality suggests a continued rise in life expectancy. The dispute is not with life expectancy but with lifespan.

Source: Luke Pamer/https://unsplash.com/

Although life expectancy in some selected countries is increasing—and it has been for some years after the second world war—this does not mean that such increases are linear or that the end point has moved—lifespan has remained static. Even though there will be more centenarians both in terms of numbers—prevalence, because we have a larger population—but also in terms of percentage—incidence, because of improved public health—centenarians are exceptional beings. The reality is that human biology will preclude survival to age 100 for most people. Even for those that live to 100, the likelihood that they survive to become supercentenarians (110 years old) is 1 in 6 million. As Fanny Janssen and his colleagues in the Netherlands reported, at some point there will be a wall. A wall that is both biological and psychological.

Studies that show continuing increases in life expectancy cannot be used to argue that there is no lifespan, or that the lifespan can be increased. Life expectancy is an aggregate statistic. In the case where life-expectancy increases and the maximum number of years that you can live remains the same, means that more people reach the end, but they still reach an end. Although life-expectancy algorithm is different from median, and can reflect outliers, changes in life expectancy just tells us that more people are living longer but not to what age. In reality life-expectancy is still an estimate based on earlier survival probabilities. This is the reason it is used in gerontology because it provides an indication of the average person and ignores those exceptional people that live up to and beyond 100 years of age—1 per 25,000—and those who die in infancy—6.15 per 1,000. Especially in the past when there was a high rate of child mortality and very low rate of centenarians, life expectancy was a good estimate. Nowadays it is not. A more appropriate statistics is the modal age of death--the most common age at which people die. What we find is that through time, life expectancy and modal age are converging.

That is just what Juliana da Silva Antero-Jacquemi from the Institute of Biomedical and Epidemiology Research in Sport, France, and her colleagues used. They analyzed 19,012 Olympian competitors and 1,205 supercentenarians—who lived up to 110 years—that died between 1900 and 2013. Although most Olympians had longer life expectancy than most general population, they did not live as long as supercentenarians. However, what they identified is that there was a common death trend between Olympians and centenarians—indicating a similar mortality pressures over both populations that increase with age. The authors argue that mortality trend is better explained by a biological “barrier” model—that there is a static lifespan.

The issue of whether there are limits to life expectancy—a lifespan—received theoretical backing from demographers who argue that fundamental limits to life expectancy are likely. And that this is similarly to be determined, in part, if not on the whole, by our genes which drives an intense search for longevity genes in both animal models and humans. Human family studies have indicated that a modest amount of the overall variation in adult lifespan (approximately 20–30%) is accounted for by genetic factors. Genetic influences on lifespan are minimal prior to age 60 but increase thereafter. Although these studies look at monozygotic twins—identical twins—there might be other confounding factors.

There is a problem with estimating age at very old age. In 1986, given continued reports of claims of extreme age, Norris and Ross McWhirter, the editors of the Guinness Book of World Records, noted the need to validate such assertions when they repeatedly stated that there is no single subject is more obscured by obfuscation than the extremes of human longevity. And the inaccuracy increases with the older the person is reported to be. Stephen Coles reports how the U.S. Census Bureau dropped its estimate of centenarians from 2,700 in 1990 to 1,400 centenarians in 2000 after checking the dates of birth with the claimed ages at the Social Security Administration. However, even this conservative number was found to be inflated as there were only 139 persons aged 110 or older. And then, even this number is likely to be exaggerated since the true number, based on physician uncertainty about their age, is more likely to be between 75 and 100 persons.

One of the classic example of such uncertainty occurred in the 1973 issue of National Geographic when Alexander Leaf gave a detailed account of his journeys to regions of purported long-living people: the Hunzas in Pakistan, the Abkhazians in the Soviet Union, and Ecuadorians in Vilcabamba. According to this article, there were ten times more centenarians in these countries than in most Western countries despite poor sanitation, prevalence of infectious diseases, high infant mortality, illiteracy, and a lack of modern medical care. Unfortunately in 2009, a fantastic age claim by Sakhan Dosova of Kazakhstan, age “130 years” was supported in an issue of Scientific American, despite the lack of early-life documentation.

These inaccuracies in reporting extreme long age have received a lot of attention from demographers. Eventually a resurgence of longevity myths in the 1970s were finally debunked which lead to Alexander Leaf himself acknowledging that people lied to him in order for them to improve their social status and to promote local tourism. More recently demographers have become increasingly concerned with the accuracy of unprecedented growth of extreme longevity in developed countries. As a consequence more careful checks are being implemented which has resulted in a systematic refutation of numerous cases of extreme age since they appeared to be undocumented or exaggerated. One such example was when in 1999, Sardinian data was presented showing extreme male longevity. This pushed demographers to assess the validity of the data and lead to the development of a robust methodology for asserting the true age of participants.

Life expectancy vs. Lifespan

One of the most persuasive arguments that lifespan is separate from life expectancy is that even if we eliminate most diseases associated with age, we will still die. Of course, we can only do this statistically. Kenneth Manton and his colleagues from Duke University eliminated one disease at a time in their statistical modeling. What they found is that if we eliminate all of age-related diseases we expect to see those over 87 years of age to live an addition 5.7 years for males (estimated for 1987) and 6.5 years for females. This is about the same improvement in life expectancy at 65 in the last 100 years in the USA (5.7 years.) If you are 65 years old today, you have a 50/50 chance of living an additional 5.7 years than if you were living in the 1900s. In the last hundred years, the great improvement in life expectancy is not amongst older adults, but among newborns and infants and have very little to do with clinical care at later ages. Statistically, as we have shown before, if we live longer than the life expectancy, the statistic of life expectancy at birth will not change, and life expectancy at other ages will only improve slightly.

Most older adults suffer from not just one, but multiple, health conditions. So if we conjecture that we can cure one disease, say cancer, we will still be faced—sooner rather than later—with another disabling disease that might kill us slower. Douglas Manuel with the Institute for Clinical Evaluative Sciences, Toronto, Canada, and his colleagues calculated what happens when they eliminated specific killer diseases from their data. They reported that by eliminating cancer they predicted that one ﬁfth of the years of life gained would be spent in poor health—and increased cost. On the other hand, eliminating musculoskeletal conditions, would result in a year of good health for women and under half a year for men. And that is what we are finding across the world. Even if we eliminated all diseases we might improve the life expectancy but not lifespan. Life expectancy and lifespan, despite their close association are separate statistical and theoretical constructs.

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Note: Life expectancy is a statistical formula to enumerate the average life left. Although it is different from median, the two are in reality very similar and produce very similar results. It is much easier to see life expectancy as a median, a mid-point because it illustrates how we can have an increase in life-expectancy and yet our maximum life that we can attain remains static.

Friday, March 27, 2015

Life Expectancy

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