I read a few weeks ago about a study where vitamin D supplementation reduced all-cause mortality rates by 6%. How many years would that add to life expectancy? I wondered. 6% of a 75-year life span would mean 4½ extra years, I thought, naïvely. I pulled up a mortality table (from the Social Security Admin) and did the calculation in a spreadsheet. The two lines were barely distinguishable. A 6% drop in mortality only increases life expectancy by 7 months.
If the death rate did not increase with age, then it would be true that subtracting 6% from mortality would add about 6% to life expectancy. That's where the intuition came from about 4½ years. But with a death rate that increases with age, you "have to work a lot harder" to get an improvement in life expectancy. And in reality, the mortality curve doesn't just rise with age - it rises at an accelerating rate.
Once I had set up the spreadsheet, it's easy enough to ask the general question: How much does life expectancy improve for a given change in mortality? The answer I found was: very slowly. ... To add just 5 years to life expectancy, we would need to slash the mortality rate by more than 40%. This is a counter-intuitive statistic - and a discouraging one.
There is another perspective: [in] medical research, we are working piecemeal to chip away at the mortality rate from one disease and another. But if the fundamental rate of aging can be slowed, this will push the curve not down but to the right. This will have as much benefit as many decades of progress in cancer and heart disease.