Posted on 25 August 2021
Would you rather have an extra year of life, or an extra year free of cancer? In the Greek legend, Eos asks Zeus to grant her lover Tithonus eternal life, but forgets to ask for eternal youth. Thus, while unable to die, Tithonus becomes ever older and weaker. Is modern medicine making the same mistake? Humanity experienced a massive increase in life expectancy during the 20th century – global average lifespan more than doubled between 1900 and 2000, rising from around 32 to 66. Most of this increase was due to reduced infant mortality and improved ability to treat infectious diseases. Now, most deaths are caused by age-related diseases. Between 1991 and 2011, the number of people who reached age 50 only increased by a percentage point or so, while the number of people reaching age 85 increased by about 20%. Much of this is down to our improving ability to keep people with chronic diseases alive. However, life expectancy doesn’t tell the whole story.
While improvements in our ability to treat the likes of diabetes, cancer and heart disease shouldn’t be understated, it is important to recognise that those years of life gained from treatment are often not healthy years of life. Compared to life expectancy, the average number of years lived free of disease or disability (‘health expectancy’) has not increased much over the last 50 years or so. While the number may vary depending on exactly how disease and disability are defined, the American Heart Association puts health expectancy in the United States at 66 years of age, meaning the average American now lives 12-13 years of their life with an an age-related disease or disability.
Aside from the individual suffering caused by age-related diseases, the increasingly large population living in poor health puts strain on societies and economies, a problem that will only worsen as the world’s population continues to grow while birth rates decline. What if we shifted our focus from simply extending human lifespan to delaying the onset of age-related diseases, or even slowing or reversing the course of ageing itself? Whether living a shorter but healthier life is preferable to living a longer one is something only the individual can answer. One thing that science can attempt to address, however, is the economic impact of these approaches. Is it more cost-effective to increase life expectancy or health expectancy? Should we focus our efforts on individual diseases or on ageing itself? By how much would we need to slow ageing down for it to be worthwhile?
In this article published in Nature, Professor of genetics David Sinclair and Professors of economics Martin Ellison and Andrew Scott attempt to answer these questions by calculating the economic value of these different approaches to human health.
To estimate the monetary value of changes in healthspan or lifespan, the authors use a methodology called ‘value of statistical life’. This does not refer to the value of some lives being greater than others, but rather is used to calculate the economic value of a change in the likelihood of death. Suppose you asked 100 people how much they would be willing to spend for a 1% absolute reduction in their risk of dying over the course of the next year (for example, to take their risk of death from 5 in 100 to 4 in 100). We would expect such a reduction to save one life in a sample of 100 people. Therefore, if those 100 people had paid $100 000 between them, that would be the value of one statistical life.
In the model used in this paper, individuals make decisions each year about consumption, work and leisure based on their knowledge of remaining life expectancy and current and probable future health. Based on this, the authors modelled the year-by-year ‘willingness to pay’ for improvements in life expectancy and health. This means that the economic value of each year of life declines with worsening health. More years of life means more years of economic value, while improved health expectancy means that years of life are ‘worth’ more for longer.
Using this model, they then estimated the willingness to pay and value of statistical life under 4 scenarios:
The results suggest that an increase in healthy lifespan is much more valuable than an increase in lifespan itself. While an increase in lifespan means more years in which to consume and participate in leisure, those years are worth less due to declining health. According to the models used, increasing the remaining life expectancy of an 80 year-old by one year is worth around US$300 000, while increasing the remaining healthy lifespan of an 80 year-old by one year is worth over twice that, between around $650 000 and $690 000. In fact, even a uniform slowing down of the ageing process (the Peter Pan case) that extended lifespan by one year was not as valuable as a one year increase in healthspan. Even though the Peter Pan case increases both life and health expectancy, gains in healthspan are lower than gains in lifespan in this scenario.
Metformin is an approved drug for the treatment of type II diabetes that may potentially protect against a range of other age-related diseases, with some evidence suggesting that diabetics taking metformin may actually live longer than non-diabetics on average. The authors attempted to estimate what the economic benefit of metformin treatment beginning at the age of 75 might be, based on existing evidence of metformin’s effectiveness for preventing age related diseases. With the stipulation that the efficacy of metformin for targeting ageing still needs to be confirmed in large-scale trials, the findings here suggest that metformin treatment could be extremely valuable, with benefits possibly even ”matching or exceeding those from the complete eradication of cancer, dementia or cardiovascular diseases”. This would suggest that drugs that can extend healthspan by delaying a range of age-related diseases are more beneficial than tackling individual diseases one by one, or ‘whack-a-mole medicine’ as some have termed it.
Finally, the authors aim to estimate the total benefits across society for a slowing of ageing according to the Peter Pan scenario, including for the unborn generations, using US census data on the US population, its age structure and birth rates. Based on this, they estimate that if ageing could be slowed by enough to extend life expectancy by just one year, then for the population as it stood in 2017 it would be US$37.6 trillion. For a 10-year increase, that number would increase to $366.8 trillion. These calculations don’t take into account health inequalities across the population, meaning that these figures are likely to underestimate the total gains from slowing ageing. However, they also don’t take into account income inequality, which would limit access to anti-ageing treatments depending on their cost.