Friday, September 18, 2009

Life Expectancy, Priorities and Aging Research

There are many different ways to arrive at a list of the top priorities a society should set for itself. One could set priorities based on the intuitions or "gut instincts" people happen to have at any given time. Or, alternatively, one could base priorities on the empirical data we have concerning what harms individuals and societies and what the magnitude of the benefits of mitigating such harms would be. I prefer the latter approach.

Indeed it is my preference for the latter that leads me to be a strong advocate for aging research rather than an advocate for tackling terrorism, trying to control the climate, etc. Most people alive today will most likely develop, and die from, one of the chronic diseases of aging like cancer, heart disease or stroke.

So tackling chronic disease is the greatest challenge of this century. However, unlike infectious diseases, eliminating any one chronic disease will have a minimal impact on a population's health prospects. Why? Because of the fact of co-morbidity. "Now that comorbidity has become the rule rather than the exception, even if a “cure” was found for any of the major fatal diseases, it would have only a marginal effect on life expectancy and the overall length of healthy life" (source)

To drive home the reality of co-morbidity, and the challenges it raises for promoting the health of aging populations, take a look at the United States life tables for eliminating certain causes of death here (it's a bit dated (1990)... if anyone can find a more recent version of this life table with gains for eliminating causes of death please let me know).

Let's start first with Table A. "Probability at birth of eventually dying from specified causes of death by race and sex: United States, 1989–91". The probability (for the total population at birth) of dying from infectious and parasitic diseases in the US is 2%. Death in car accident 1.4%. Homicide 0.7%. Sudden infant death syndrome 0.13%. The highest probabilities are the chronic diseases that mostly afflict the aged. Probability of dying from cancer is 22%, and for cardiovascular diseases (e.g. heart disease and stroke) the probability is 47%.

These statistics make vivid what is the greatest threat to the health prospects of a population- chronic diseases- and who is most at risk (i.e. those over the age of 60).

But suppose we eliminate a specific disease. How much would this increase life expectancy at birth? Scroll down to Table 22. Gain in expectation of life due to elimination of specified causes of death, by exact age for the total population for the answer. If newborns in the United States were born into a society with 0% risk of infectious disease life expectancy would increase by 164 days. With 0% risk of ever developing cancer life expectancy would increase by 3.36 years. With 0% risk of Alzheimer's disease life expectancy for the coherent born today would increase by 18 days. For a baby born today who would survived long enough to die from AD, they would live an extra 6.8 years if AD was eliminated. And with no major cardiovascular diseases to kill you life expectancy would increase by 6.7 years, which is 1.5 years more than the difference in life expectancy between being born female rather than male.

The striking thing about these gains in life expectancy is how low they really are. We just assume a world with no cancer would dramatically increase life expectancy, but this is not so. Of course a cure for an early onset disease would result in large benefits to the life expectancy of those who would die from that specific disease (and those stats are also there). But the reason the numbers are not bigger is that eliminating one cause of death in late life simply delays the time one will most likely develop another disease of aging (the reality of co-morbidity). So if you don't die from cancer in your 80's you most likely have a stroke or heart disease, etc.

These kinds of data should be the basis of a rational approach to health extension. And that is why aging research ought to be a top priority today. Retarding aging would help us delay all age-related disorders simultaneously, thus yielding healthy dividends that far exceed what a cure for cancer or AD or stroke could yield. So one of our top priorities should be to increase the health prospects of humans in late life. Thus aging research ought to be at the top of our priorities.