Comment by Enginerrrd 12 hours ago
In case anyone was curious like me: the standard deviation of lifespan is ~12-15 years in developed countries.
So environmental effects, sleep, diet, lifestyle, etc (I.e. modifiable factors) maybe account for half of that, so like 6-7.5 years of variance. Which… sounds about right to me.
Yup.
I'm not really afraid of getting old, but I'm afraid of becoming decrepit.
My grandma has been decrepit for over 5 years now. She can't walk and has no bladder or bowel control, so she just sits on the couch and shits herself all day. She's not living, she's merely surviving. She was living with my mom for a while, but my mom decided she couldn't handle it anymore and put her in an assisted living facility.
If I get to the point where I couldn't cook my own meals and wipe my own ass, just put a bullet in me. I do not fear dying, but I do fear spending years of my life not being able to actually do anything.
My dad died at the end of last year, and was not too different from your grandma. For him the main problem was chronic pain from his failing body. Even fairly powerful opioids from a pain management doctor only helped a bit. Basically all he could do was sleep, eat meals, and sit in his chair in pain.
I feel similar to you, but I wonder if it's one of those those things where age changes your perspective. Dad was in assisted living and had several stints in rehab/nursing home facilities, and in both there were quite a few people with what I'd call poor quality of life who were still holding on to life.
Something we youngsters (I'm 69) may not realize is that people in assisted living still have friends and frequently even sex lives while they are there. They read, play games, and watch movies, just like us. They might not be able to do all the things they could when they were younger, but their lives are not necessarily over.
It is probably more than half the story. Health span is strongly correlated to life span, although not completely. The median "health span gap" is about 10 years, and has widened by roughly one year over the past 20 years. However, this is probably just due to an aging population and not necessarily from any factors you can control fully.
I wouldn't be surprised if "health span" (although defining it is difficult) exactly mirrors the inheritability pattern of mortality.
> The median "health span gap" is about 10 years
It depends on the definition, if you're even just 20kg overweight you're living a wildly different life than you'd have if you were fit, you're closing so many doors by default and making a bunch of things much harder than they should be, But you're still considered "healthy" here
My Dad (age 81) tore his rotator cuff splitting wood recently. It's slow to heal and he's in a lot of pain which (along with his Alzheimer's) is really getting him down.
Maybe even if you're still fit and strong in your 80s you should let someone else split your wood for you
Yeah, been working in IT since forever (sitting work all day), but started lifting recently and it already made remarkable improvements in my wellbeing. Should've started sooner of course, but I'm still well in time.
This plus stretching / yoga has been amazing as I'm entering my 40s. For a while I was just lifting and I had strong muscles but they were short and tight. Not everyone has that problem, but just noting strong muscles are half the picture, being strong and flexible makes life feel effortless and years of being a desk jockey.
One of the most consistent health research findings Ive heard in recent years is the benefits of weight training for older adults. Hopefully the message is being received.
It's a remarkable tragedy how many people don't understand your point.
https://en.wikipedia.org/wiki/Disability-adjusted_life_year
Too many people think your life is a binary 'living or dead' when thats not the case at all. I didn't even understand it fully till I was hit by a car.
It is almost never reasonable to assume normality and make calculations like this. This is particularly the case when you are dealing with lifespan, which isn't normally-distributed even in the slightest. The actual ranges are likely smaller than you are stating here, and variance is just not a very practical or interpretable metric to use when dealing with such a skewed distribution.
We should be stating something like a probability density interval (i.e. what is the actual range / interval that 95% of age-related deaths occur within), and then re-framing how much genetic variation can explain within that range, or something like it. As it is presented in the headline / takeaway, the heritability estimate is almost impossible to translate into anything properly interpretable.
https://biology.stackexchange.com/questions/87850/why-isnt-l...
This is a nice example/re-stating of what the heritability % "means" here.
I'm curious, with something like smoking/drinking, how you can be confident that you've untangled genetic predispositions to addiction or overconsumption from those "modifiable factors". I guess that's just captured within the 50% heritability? And if you could confidently untangle them, you might find heritability is higher than 50%?
Heritability is a pretty funky concept because it's contextual to a certain point in time, environment, and population, effectively.
An example I like is that if you measured the heritability of depression in 2015, and then you measured the heritability of depression in 2021, you would likely see changes due to environmental effects (namely, there's the pandemic/lockdowns and this could conceivably cause more people to experience depressive symptoms). Let's assume we make those measurements and the rate of depression did increase, and we could tie it causally to the pandemic or related events.
In that scenario, the heritability of depression would have decreased. I don't think anyone would argue there were massive genetic changes in that 6 year time period on a population scale, but the environment changed in a way that affected the population as a whole, so the proportion of the effect on the trait which is genetically explained decreased.
For something like lifespan in the above example, you can imagine that in a period of wartime, famine, or widespread disease the heritability would also decrease in many scenarios (if random chance is ending a lot of lives early, how long the tail of lifespan is influenced genetically is much less important).
Given that note, it's generally tricky to talk about whether heritability increases or decreases, but with more accurate estimates of how genetic predispositions form you could see the heritability of certain traits increase with the environment held stable, as there's certainly ones that may be underestimated or genetic factors that aren't currently accounted for in many traits.
*edit: I realized I never mentioned the other thing I wanted to mention writing this! since you mentioned what the percent heritability means here, I think the best way to think of it is just "the proportion of phenotypic variation for this trait in a measured population which is explained by genetic variation." So it's dependent on the amount of variation in several aspects (environmental, genetic, phenotypic).
Some epigenetic effects are semi-heritable too, eg maternal exposure can be transmitted. That's in addition to environmental effects like you mentioned. Two otherwise identical cohorts can inherit the same genetic predisposition for depression where one manifests and another does not entirely due to their circumstances.
Evolution is just super super messy.
> the standard deviation of lifespan is ~12-15 years in developed countries.
That seems rather higher than I would have expected, at least if one corrects for preventable accidents and other such things (that I would expect to shift the results away from a normal distribution).
> at least if one corrects for preventable accidents and other such things
You can't really correct for these. Yes there are genuine accidents that will kill you under any circumstances, but for a lot of things both your odds of having an accident and the odds of surviving it are strongly linked to age. As a simple example, despite driving significantly less, the elderly get into more car accidents and suffer worse injuries in those accidents than people earlier in life. Only the age range of 15-24 has higher car accident fatality rates.
There is no such thing as death by old age. At most there are deaths in the elderly that don't get attributed to a specific cause (typically because of so many different things going on at once and no desire to cut up grandma after the fact to see which straw broke her back) which we tend to refer to as "died of old age" but it's not a recognized medical cause of death. People die of diseases, injuries, and various other things, many of which are strongly influenced by age but also heavily influenced by other factors.
You can set a cutoff point and say these things don't count as age related deaths whereas these others do. As long as you're consistent with these choices, you can learn something useful. But a wide enough net that is widely agreed to cover what we think of as aging is going to include a lot of other maladies, whereas a narrower selection criteria is probably going to yield wildly different results from one analysis to the next.
Lifespan is a quite skewed distribution, so the SD looks large because it is in fact a poor summary of the bulk of the distribution. The actual part we care about for age-related mortality is narrower than such an SD would imply if we had a normal distribution (simple image example: https://biology.stackexchange.com/a/87851).
Lifespan is not even half the story though, health span is much more important. Your life is completely different if you can ski or split your own wood at 80+ vs being barely able to use stairs at 50. Both might die at 90 but one "lived" 30 years more