Author Topic: How much mass will have been me?  (Read 907 times)

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Online DanDanDan

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How much mass will have been me?
« on: February 25, 2019, 10:28:59 PM »
This question is directly related to the Ship of Theseus thought experiment, as applied to a person.

I heard somewhere that the vast majority of the atoms that my body is comprised of will be replaced in X number of years. I'm wondering what X actually is, and I'm wondering how much stuff, in tons or kgs or whatever, the average person is made of, in total, in their lifetime.

I'm sure that the answer will depend a lot on arbitrary definitions, supposition, etc. Personally, I wouldn't count the stuff that just passes right through, that is inhaled and immediately exhaled, etc.

So does anybody know if this question has ever been addressed by an academic or some such?
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Offline brilligtove

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Re: How much mass will have been me?
« Reply #1 on: February 25, 2019, 11:04:55 PM »
Another way to think of this is, "How long can I live in a holodeck, consuming and breathing holomatter, before my entire body is a hologram?"

Why YES I am a nerd. ;)
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Offline bachfiend

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Re: How much mass will have been me?
« Reply #2 on: February 26, 2019, 12:24:46 AM »
It’s an interesting question.  My first thought was using tritium, the radioactive isotope of hydrogen.  Hydrogen is present in all tissues, including bone, which isn’t rapidly turned over.

Tritium has a half life of 12 years, but a biological half life of around 10 days in humans, reflecting mainly absorption and excretion.  If you ingest a dose of tritium labelled water, most of it will be excreted rapidly in urine.  But some will be taken up by tissues, rapidly overturning tissues, and then quickly excreted when the tissue is turned over.

But they’ve done studies in rats with giving tritium labelled water for a sufficiently long period to reach equilibrium, and then sacrificing the animals at various time and measuring the tritium level in various tissues:

http://www.jbc.org/content/223/2/795.full.pdf

There’s a graph on page 11 which shows the decline in various tissues, slowest in collagen (which would be similar to bone), next slowest in brain, and fastest in liver.  Table III shows a more detailed breakdown.

So, as you note, the turnover in various tissues varies.

I suppose my answer after all this is that I don’t know.  Sorry.
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Online DanDanDan

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Re: How much mass will have been me?
« Reply #3 on: February 26, 2019, 05:24:06 AM »
I suppose my answer after all this is that I don’t know.  Sorry.

No reason to be sorry in the least. It's awesome that you were able to find that! I assumed that certain tissues turn over faster than others, but now I actually know which. It's interesting to think that of the tissues on that graph, the vast majority of human (or rat) tissue that has ever existed was liver tissue.

I figured from the outset that there was probably not an answer to this, since I think I would have heard of it. It would be one of those fascinating bits of trivia in the vein of the how much water a person drinks in a lifetime, the number of neurons in the brain, the number of heartbeats in a lifetime, etc. Puts things into perspective.

That's the history of the human biomass for you folks. Ugly bags of mostly liver. (Why yes, I am also a nerd!)
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Offline daniel1948

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Re: How much mass will have been me?
« Reply #4 on: February 26, 2019, 10:30:56 AM »
Fascinating question. When you finally get enough data to work it out, don't forget that some atoms will have been part of your body more than once. So it's not just the turnover rate for all the tissues, it's also the probability of a given atom coming back for a second go.

If I'm inhaling an atom from Caesar's last breath with every inhalation of my own (on average) then I'm also (on average) inhaling an atom of every one of my own previous breaths with each new breath, which suggests that lots of the atoms that are part of me and eventually discarded will come back to become part of me again.
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Offline Billzbub

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Re: How much mass will have been me?
« Reply #5 on: February 26, 2019, 10:46:38 AM »
This sounds like a job for Randall Munroe of XKCD fame.

https://what-if.xkcd.com/
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Offline daniel1948

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Re: How much mass will have been me?
« Reply #6 on: February 26, 2019, 01:43:17 PM »
This sounds like a job for Randall Munroe of XKCD fame.

https://what-if.xkcd.com/

Thanks for posting that!
Daniel
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Online DanDanDan

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Re: How much mass will have been me?
« Reply #7 on: February 26, 2019, 02:42:15 PM »
This sounds like a job for Randall Munroe of XKCD fame.

https://what-if.xkcd.com/

Done! Thanks so much for suggesting that!
Logic goes first, ideas go later, because ideologies can all go awry.

Online arthwollipot

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Re: How much mass will have been me?
« Reply #8 on: February 26, 2019, 08:28:10 PM »
This sounds like a job for Randall Munroe of XKCD fame.

https://what-if.xkcd.com/

Done! Thanks so much for suggesting that!

Please come back and post here if you do get an answer. I follow xkcd, but I don't follow what-if as closely.
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Offline fred.slota

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Re: How much mass will have been me?
« Reply #9 on: February 26, 2019, 09:56:47 PM »
Stupid calculation.

2,000 cal/day, if pure sugar, would be about 0.5 kg of sugar.
If we take a lifespan of 82+ years (30,000 days), then that would be 15,000 kg of sugar.

sucrose C12H22O11 342.3 g/mol
15,000 kg sucrose = 43,821 mol

C12H22O11 + 12 O2 -> 12 CO2 + 11 H2O

525,852 mol CO2 and 482,031 mol H2O
55.346 mol in 1 liter of H2O



A lifetime metabolism produces about 8,709 L of H2O, about 20 m x 20 m x 20 m, or a little more than 12 Jacuzzis.

Offline brilligtove

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Re: How much mass will have been me?
« Reply #10 on: February 27, 2019, 07:40:56 AM »
You have confused "stupid" with "quite clever".
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Offline Billzbub

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Re: How much mass will have been me?
« Reply #11 on: February 27, 2019, 12:41:23 PM »
Stupid calculation.

2,000 cal/day, if pure sugar, would be about 0.5 kg of sugar.
If we take a lifespan of 82+ years (30,000 days), then that would be 15,000 kg of sugar.

sucrose C12H22O11 342.3 g/mol
15,000 kg sucrose = 43,821 mol

C12H22O11 + 12 O2 -> 12 CO2 + 11 H2O

525,852 mol CO2 and 482,031 mol H2O
55.346 mol in 1 liter of H2O



A lifetime metabolism produces about 8,709 L of H2O, about 20 m x 20 m x 20 m, or a little more than 12 Jacuzzis.

No wonder there is so much of the cancer.  I can't even pronounce dihydrogen monoxide, and people eat so much sugar that they fill their bodies with 12 Jacuzzis of it!?
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gleefully altering one’s beliefs to accommodate new information should be a badge of honor

Online DanDanDan

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Re: How much mass will have been me?
« Reply #12 on: February 28, 2019, 04:30:30 PM »
This sounds like a job for Randall Munroe of XKCD fame.

https://what-if.xkcd.com/

Done! Thanks so much for suggesting that!

Please come back and post here if you do get an answer. I follow xkcd, but I don't follow what-if as closely.

Will do.
Logic goes first, ideas go later, because ideologies can all go awry.

Online DanDanDan

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Re: How much mass will have been me?
« Reply #13 on: February 28, 2019, 04:37:25 PM »
Stupid calculation.

2,000 cal/day, if pure sugar, would be about 0.5 kg of sugar.
If we take a lifespan of 82+ years (30,000 days), then that would be 15,000 kg of sugar.

sucrose C12H22O11 342.3 g/mol
15,000 kg sucrose = 43,821 mol

C12H22O11 + 12 O2 -> 12 CO2 + 11 H2O

525,852 mol CO2 and 482,031 mol H2O
55.346 mol in 1 liter of H2O



A lifetime metabolism produces about 8,709 L of H2O, about 20 m x 20 m x 20 m, or a little more than 12 Jacuzzis.

Thanks! But now comes the hard part. How much of that actually gets incorporated into the body? If I have a mouth full of food, I don't consider the food to be a part of me. Same for a butt load of crap, if you'll pardon the expression.
Logic goes first, ideas go later, because ideologies can all go awry.

Offline brilligtove

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Re: How much mass will have been me?
« Reply #14 on: February 28, 2019, 04:54:52 PM »
All of it, I think. I mean, that calculation is based on total lifetime calories needed for your metabolism to run. There wouldn't be any undigested substance. IRL you'd need a variety of other nutrients. Also poop can carry a lot of undigested calories out of you. But as a baseline? Neat calculation.
evidence trumps experience | performance over perfection | responsibility – authority = scapegoat | emotions motivate; data doesn't