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.

[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.

[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.

[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.

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

Originally Posted by Chloe

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

Not a very scientific number?! It's exactly one-sixth of the answer to the ultimate question of life, the universe and everything!

[Chloe]

See? It's more of a philosophical number, really.

Agreed. And it's not nearly bad as that sneaky bastard phi.

[tribrats]

Originally Posted by Cervus

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.

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.

Originally Posted by Chloe

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

Unfortunately there isn't much that can be said to convince her. She can't even begin to explain the reasoning to that or where she even heard it. She is a sweet woman but very naive. She also believes that going out in the cold will cause a cold and you can catch "ammonia" that way too.

Originally Posted by tribrats

Unfortunately there isn't much that can be said to convince her. She can't even begin to explain the reasoning to that or where she even heard it. She is a sweet woman but very naive. She also believes that going out in the cold will cause a cold and you can catch "ammonia" that way too.

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

[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".

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.