Map of life expectancy at birth from Global Education Project.

## Saturday, August 20, 2011

### Life span vs. span of life

To clarify in response to CC, and because this is important, here's how to parse the following terms: Life expectancy, Life span, Length of life/age at death.

Life expectancy is a fictitious number, derived as follows. Consider a person of a given age -- the moment of birth, or age zero, is the most common, but you can start at any point. Now look at all similarly situated persons, for example you might want to consider life expectancy at birth for U.S. males, or some such category.

Now look at all the people who died last year (or as recently as you can get the data) who were in that category. A small percentage died in their first year of life, an even smaller percentage age 1-5, 6-10, etc. Making the assumption that the baby just born will have the same odds of dying during each interval, figure out how old he'll have to be before there's a 50% chance he's dead. That's his life expectancy.

In reality, he won't have the same experience as people living today. We're accustomed to life expectancy increasing over time, but it doesn't have to. We'll see.

Now, life span has to do with an incurable genetic disease with which every one of us is born, called aging. Even assuming we don't experience misfortune such as being hit by a bus or sent off to Afghanistan to be blown up, and we stay super duper healthy, we won't live forever because our cells are programmed to be able to divide a finite number of times after which the tissues cannot renew themselves and we'll just die of plain old old age. Right now the maximum human life span for the genetically extremely lucky - and we're talking very rare outliers here - is about 120 years. We used to think of the life span as more like 70 years but that's because hardly anyone stayed healthy enough to make it to the true human life span, which for most of us is probably more like 90+ years. In other words, we all died of something other than old age, but nowadays we don't necessarily. (Where Alzheimer's fits in the aging process vs. disease dichotomy is maybe a little unclear.)

How long you actually live is self-explanatory, I think. So what happened with the mice in the experiment described yesterday is that they lived longer than fat mice usually do, because fat mice tend to die of heart disease just like fat humans. But their life span was not extended, in fact they didn't even make it to the typical age of svelte mice. So the drug wasn't slowing the aging process at all, it was just counteracting some of the badness of obesity.

Does that make sense? If not, keep asking.