Does every atom in your body get replaced every X years?

[Mr. Billion]

Chow me if this has been asked before, but:

I keep hearing different versions of a "well-known fact" to the effect that every year (or every ten years or something) you replace every atom in your body. I seem to recall it was mentioned in a Discworld novel, in a philosophy course, and in a few other places. Is it true? How would such a thing be discovered and proven?

Googling a bit, I find this claim using the "fact" to attempt to prove the transcendence of consciousness: "If you have replaced your entire body down to the last atom in less than a year, you are certainly not your body." A similar claim appears here and in this blog post. (I also turned up this interesting but silly discussion.)

No place where I've encountered this factoid has provided an actual citation, though those first two links do sort of try to give a plausible explanation. The factoid seems pretty suspect to me.

However, it does seem reasonable that the atoms in one's body would eventually have to be replaced. Wouldn't they?

[Mycroft]

Originally Posted by Mr. Billion

However, it does seem reasonable that the atoms in one's body would eventually have to be replaced. Wouldn't they?

That is exactly opposite to my reasoning, specifically concerning hard structures, ie once teeth have grown I see no way of replacing enamel, so the elementsin there are not replaced periodacaaly. It is also accepted that some cells (eg neurons) are not replaced, wheras others (eg red blood cells last around 4 months before being replaced)

[tribrats]

My step-Mom believes that rumor. She is convinced that every 7 year all your cells are replaced making you essentially a "new" person. This is her reasoning for develop new allergies at any time of your life.

[Cervus]

Originally Posted by tribrats

My step-Mom believes that rumor. She is convinced that every 7 year all your cells are replaced making you essentially a "new" person. This is her reasoning for develop new allergies at any time of your life.

My mom told me something similar - that every seven years your "body chemistry" changes, making you susceptible to new allergies.

If it were true that every atom or cell in your body was replaced periodically, nobody would be afflicted with permanent diseases, tooth decay, cancer, brain damage, etc. Maybe we'd even regenerate missing limbs.

[Oxnard Lobsterman]

A lot of this talk goes back to an ancient logical conundrum that bedevils the concept of "identity." Imagine you have a wooden ship, the traditional paradox goes, and as you make your travels planks and ropes and such slowly wear out. As each small component becomes unfit it is replaced. Eventually, no original material remains, yet the ship has been in continuous use. Is it still the same ship?

And to make things even more complicated, imagine someone has been following behind and collecting all the old pieces, eventually hammering them into a (supposedly terrible-looking) complete vessel. Is this then the original ship?

Scenarios like this one force us to inspect the underpinnings of "identity." What makes something itself, and what could make something the same as itself under other circumstances?

(I don't have the answers. I'm just a crustacean.)

Originally Posted by Class Bravo

I had a biology teacher in middle school who told us that we essentially become a new person every 7 years because in that amount of time our bodies replace every single cell. She went on to say that it would be a good murder defense if you could stay on the run for 7 years because then you could claim that it truly wasn't you who committed the crime since you had become a new person within the seven years you were on the run. I don't think she was joking. It was about then that she lost me for the rest of the year.

Maybe you could be tried as an accomplice after the fact. Or, even worse, you could be tried for the murder of your past self! Oh no!

Anyway, that's a silly idea because it's not the physical materials of someone's body we hold culpable for their behavior. Instead, we assess their personhood, their animated consciousness, which is an emergent property of those materials (given their configuration as a human being). If, to gruesomely recapitulate the above scenario, the cops were able to collect all those skin cells and such that a killer had sloughed off after seven years, would we consider justice done? Of course not. That skin, spit, and sh*t isn't a person. It's byproduct. The real person is inherent in the dynamic system of those chemicals' interaction.

[surfcitydogdad]

Ah, this reminds me of the debate mentioned in Star Trek: Enterprise regarding whether by using the transporter we actually travel or are simply replaced by copy or our self; and what of the soul?

[Mr. Billion]

Originally Posted by Oxnard Lobsterman

A lot of this talk goes back to an ancient logical conundrum that bedevils the concept of "identity." Imagine you have a wooden ship, the traditional paradox goes, and as you make your travels planks and ropes and such slowly wear out. As each small component becomes unfit it is replaced. Eventually, no original material remains, yet the ship has been in continuous use. Is it still the same ship?

Yep, that's what prompted* this post. I had a philosophy professor last year mention this factoid while outlining the problem of the Ship of Theseus, although he called it the Ship of Thebes.

*well, it wasn't prompted very promptly. The class was a year ago. I was just thinking about it recently and realized that Snopes might be a good place to get to the bottom of whether or not the factoid is true.

Originally Posted by Oxnard Lobsterman

Maybe you could be tried as an accomplice after the fact. Or, even worse, you could be tried for the murder of your past self! Oh no!

Anyway, that's a silly idea because it's not the physical materials of someone's body we hold culpable for their behavior. Instead, we assess their personhood, their animated consciousness, which is an emergent property of those materials (given their configuration as a human being). If, to gruesomely recapitulate the above scenario, the cops were able to collect all those skin cells and such that a killer had sloughed off after seven years, would we consider justice done? Of course not. That skin, spit, and sh*t isn't a person. It's byproduct. The real person is inherent in the dynamic system of those chemicals' interaction.

Very well put, though granted I didn't need any convincing. However, I think anyone who seriously believed that the OP would actually make a good defense for a crime would probably not be much of an audience for your explanation.

Originally Posted by tribrats

My step-Mom believes that rumor. She is convinced that every 7 year all your cells are replaced making you essentially a "new" person. This is her reasoning for develop new allergies at any time of your life.

I had a biology teacher in middle school who told us that we essentially become a new person every 7 years because in that amount of time our bodies replace every single cell. She went on to say that it would be a good murder defense if you could stay on the run for 7 years because then you could claim that it truly wasn't you who committed the crime since you had become a new person within the seven years you were on the run. I don't think she was joking. It was about then that she lost me for the rest of the year.

[Troodon]

I have heard the "seven years" number mentioned, although not in an academic context. I suppose that one could calculate the rate at which cells are replaced and then calculate how long it would take to replace every cell in the body at that rate, and that number may be seven years. It wouldn't be particularly meaningful because obviously some cell types replace themselves quickly while others are much slower (or never replace themselves at all).

The number may be interesting in the sense that, to some degree, aging is not a result of individual cells "wearing out" because many cells are frequently replaced. Rather, some components of aging may be due to mistakes or non-optimal organization of cell replacement.

Edit: as for atoms, some of them are in very stable structures (such as DNA) while others are in things like sugar that are chemically used up and expelled relatively rapidly. I have no idea how a number for the rate of atom exchange with the environment could be accurately measured, and I've only heard the "seven years" number with regard to cells.

[Chloe]

As my sister once said "Seven doesn't sounds like a very scientific number."

[Little Pink Pill]

The way I understand it is that different types of cells divide at different rates, so after 7 years in a young adult, you eventually have all "new" cells in that the old cells have died off and been replaced by their own daughter cells. The rate is more rapid in children, every month or so in a baby, and as you age, it decreases to much longer than 7 years.

But they are still all your cells, copies of copies of copies. The argument that you are a different person because of it is just silly.

cell division

[Skeptic]

Originally Posted by tribrats

My step-Mom believes that rumor. She is convinced that every 7 year all your cells are replaced making you essentially a "new" person. This is her reasoning for develop new allergies at any time of your life.

I've heard this as the reason that children, (including myself) outgrew asthma.

[tribrats]

Originally Posted by Skeptic

I've heard this as the reason that children, (including myself) outgrew asthma.

She claims that as the reason I developed asthma. Although after looking over my history my doctor believes that I may have had mild asthma all my life. But that doesn't change her mind at all.

Hmmmm, maybe you could catch "ammonia" by being exposed to too much ammonia in cleaning supplies? A good argument for not cleaning the stove? I'll have to try that one...

Bee

I joke that she "talks dyslexic".

[Mr. Billion]

Originally Posted by Mycroft

That is exactly opposite to my reasoning, specifically concerning hard structures, ie once teeth have grown I see no way of replacing enamel, so the elementsin there are not replaced periodacaaly. It is also accepted that some cells (eg neurons) are not replaced, wheras others (eg red blood cells last around 4 months before being replaced)

Ah, good point.

neurons...are not replaced

At this moment, this assertion is being challenged.

[Elbe]

I've always heard it as every cell in your body gets replaced (which isn't completely true). While I guess it's possible that all the atoms are being replaced I don't think there is a need for it. Cells live and die, but atoms will continue being whatever element they are unless a chemical reaction gets involved. I imagine there are places in your body where an element of compound can continue being that element or compound for years without any reactions changing things around.

I guess, and this is just my crazy idea, that many atoms of rarer elements (i.e. things we don't ingest often) just get shuffled around inside us. But who knows, I sure don't.

[Bassist]

Originally Posted by Elbe

Cells live and die, but atoms will continue being whatever element they are unless a chemical reaction gets involved. I imagine there are places in your body where an element of compound can continue being that element or compound for years without any reactions changing things around.

Atoms don't change elements due to chemical reactions (the combining of different atoms to form molecules, or the disassembly of said molecules). The only way that an atom can become a different element is through a nuclear reaction.

Okay - I know it's extremely nit-picky, but I felt the need to interject.

[Elbe]

Originally Posted by Bassist

Atoms don't change elements due to chemical reactions (the combining of different atoms to form molecules, or the disassembly of said molecules). The only way that an atom can become a different element is through a nuclear reaction.

Okay - I know it's extremely nit-picky, but I felt the need to interject.

Oi, I know that but I was tired when I wrote that. Atoms can, though, become energy during chemical reactions, and that was what was in my head when I wrote it.

[Delta-V]

Originally Posted by Elbe

Oi, I know that but I was tired when I wrote that. Atoms can, though, become energy during chemical reactions, and that was what was in my head when I wrote it.

(nitpick)
Actually that still requires a nuclear reaction.
During a chemical reaction, energy can be released (or absorbed) by breaking or forming molecular bonds between atoms. But nothing 'becomes' energy, the atoms are still there, same as before, just in different formations.
(/nit-pick)