December 31, 2019
Life expectancy is a metric for the average age of death in a population. In 1800, regardless of place of birth, a newborn could expect to live fewer than 35 years. By 1950, the global average for life expectancy was still only 46 years, but with a much broader distribution spanning from 68 years in the richest countries to only the low 30s in the poorest countries. Today, it ranges from 53 (in the Central African Republic) to 85 (in Japan), with a global average of 71 years. (More information about life expectancies over time and geography is available here).
There is much to work still to do to address the persistent life expectancy gap between richer and poorer countries. But recently I heard an observation about life expectancy that I had never thought about before and which points toward another set of questions to be asked and challenges to be addressed:
"If average life expectancy for most of our existence as a species has been between 30 and 50 years, then most of the biology of older age today is largely unprecedented and free from the influence of evolution."
To put the increases in life expectancy noted above in an evolutionary context, the human life expectancy at birth life expectancy doubled over a span of about 300,000 generations from the great ape ancestor that we share with chimpanzees. Then life expectancy doubled again in <10 generations over the last 200 years during industrialization (see Finch, PNAS, 2009), with the greatest jump occurring in just four generations over the last 100 years.
A major problem with increasing life expectancy, though, is that it also increases morbidity merely due to people living long enough to get more age-related disease and dysfunction. Many serious diseases have increased prevalence with age, including cancer, heart disease, stroke, respiratory disease, kidney disease, dementia, arthritis, and osteoporosis. For example, 30% of the population over 60 in the UK now become demented before they die, and this proportion is projected to grow as a result of the overall population continuing to age.
The increase in average lifespan shows no signs of slowing down, though, and there is no agreed biological reason why an absolute maximum lifespan should exist. Aging is not programmed or selected for during evolution—because for most of our history as a species we died prior to significant aging—but rather seems to be a result of accumulating wear and tear. So while rapid improvements in food supply, hygiene, healthcare, child mortality, and life styles can be credited with dramatic increases in life expectancy, they have unfortunately had no impact on aging-related morbidity, and there has been no proportional increase in quality of life for the elderly during the recent period of increasing life expectancy.
The economic costs of an older, but sicker, population are already high and continue to increase rapidly. In the past, people died young and relatively quickly, while people now tend to die old and slowly from degenerative diseases preceded by years of multiple morbidity. This trend places increasing pressure on the economy and social systems in most countries. Current public expenditure on the retired population is already nearly 25% of GDP in the EU, and it is expected to rise substantially in the future. Thus, if we continue our march to eliminate causes of death without concomitantly reducing the rate and health impact of aging, then pension and healthcare costs will become unsustainable.
These trends raise important questions about what type of biomedical research we should prioritize in the coming years—either to further reduce mortality, thereby increasing life expectancy, or to reduce aging-related morbidity in order to improve quality of life while diminishing economic dependency among the elderly.
These issues are explored in more detail in a review paper authored by Guy Brown. His conclusion should be provocative to decision-makers allocating our scarce public and private biomedical resources:
"Should public policy favour increasing the quantity or quality of life? I would advocate a managed compression of morbidity. This would entail switching most medical research funding from tackling the causes of death to the causes of ageing and age-related morbidity until the quality of life at the end of life is sufficiently high to make it worth extending lifespan further. Medical research funding therefore needs to be shifted from the main causes of death of the elderly, such as cancer and heart disease, towards the main causes of morbidity of the elderly, such as dementia, depression, arthritis and ageing itself. Doing so is likely to be cost neutral in the short term and cost beneficial in the long term by reducing healthcare and pension costs. More importantly, it will reduce the chances of degenerative disease, disability, dementia and extreme ageing for ourselves and our children, hopefully enabling a better quality of life and end of life for us all."
– Geoffrey W. Smith
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