Atomic Tune-Up: How the Body Rejuvenates Itself


The human body replaces 98 percent of its atoms yearly, but the makeover prompts wider questions.

From NPR news, this is All Things Considered. I am Debbie Elliott. Time now for our look at some of the elemental questions of life in our segment Science Out of the Box.

Many have heard someone say I am not the person I was. Well, that maybe more true than you know. NPR science correspondent David Kestenbaum has this tale.

Okay, we all get haircuts, but have you ever thought that when we do, we lose atoms. That hair that was once part of us goes its own way. So, um, at the end of the day, what happens to my hair on the floor here?

Well, they will get swept up and put into the trash.

Those are my atoms.

How much of me is really with me throughout my life? Mean my skin rubs off. I trim my fingernails. Part of me is eroding, and presumably being rebuilt, but how much is it? One percent? Is it ten percent? I called around to biologists and chemists and no one seemed to know the answer.

I'm Logan McCarty. I teach chemistry at Harvard University.

Had you ever thought about, to what extent, we're just spare parts being replaced all the time?

I guess it never really occurred to me. You know, who I was is not, the atoms that I have now are not the same atoms that I had, you know, yesterday or the week before.

McCarty did some research. And he found this article from a Smithsonian Institution Publication from 1953. So this is the beginning of the atomic age and the article described the six experiments where researchers fed people radioactive atoms or they injected them with radioactive atoms and then using radiation detectors they could watch the atoms as they moved around. So they watched them go up one arm into the heart and down the other arm.

You can follow it through their body. Does it get excreted through urine or is it excreted through their sweat or through feces or, you know, what happens to it, does it end up in their fingernails or in their eyeballs or, you know, so you can follow where these atoms go.

And where do they (go)?

And by doing that. Well, they end up in, in all of your tissues.

A lot of the atoms get incorporated into our bodies. The article says the atomic turnover is quite rapid and quite complete.

In a year, 98% of the atoms in us now will be replaced by other atoms that we take in, in our air, food and drink. So that means 98% of me is new every year. So this is the very profound rule of nature: you are what you eat.

Yes, absolutely. If you eat a hamburger one day, then the atoms and molecules in that hamburger will end up making up your cell walls, and different organs and tissues.

So my friend, I have a friend of friend who eats soup everyday for lunch. So he is mostly made up of soup or largely.

He is mostly made up of soup, yes.

But don't be sad. McCarty says this constant replacement of parts is actually what makes life so robust and so adaptable, so so lively.

Life is remarkable in that it requires always a flow of energy and matter through the system. So that if the system isn't constantly bringing in new energy, and bringing in new matter then it is basically dead.

Still, this means that in a very real sense, we are not the people that we were a year ago with this collection of atoms that hang out together for a while and they go on to do other things sort of a momentary crowd of organization. So what is me? Am I still me if my parts have been replaced?

Well, of course, the, the question goes way back to ancient philosophy.

This is Daniel Dennett. He is a philosopher at Tufts University.

Remember he says the old joke about Abe Lincoln's axe.

There it is, in the glass case. And it says This is Abe Lincoln's axe. Someone says is that really his axe. He says, Oh, yes. But of course, the head has been replaced twice and the handle three times.

There is also a modern atomic version of this puzzle. That really gets to the heart of things.

We imagine that, your rocketship has landed on Mars. You have to get back from Mars to Earth by teleporter.

Here is how the teleporter works. It dismantles you atom by atom, deluuuu, you know. Records the precise location, every carbon, every hydrogen every phosphors. And it sends that information to earth. Uuuu where receiving transporter reconstructs you, blurrrrrrrr out of new atoms.

And you step out of the teleporter receiver on Earth. Is that really you? I say of course it's you.

Okay! That is clear enough. But now imagine he says instead the teleporter on Mars doesn't take your parted, doesn't disassemble you. It just scans your atom, dudududud. Leaving you intact.

So now you are, there is a you that's stranded on Mars. And there is a you that's back on Earth, which is the really you.

Well, it's pretty clear to me. Then there is, then I have David One and David Two.

Yeah, and, and, and does one of them have some sort of special priority, is one of them sort of realer, more you than the other.

Then what does my wife do?

Exactly, yes.

I can tell you what my wife would do. My wife would groan if two of me showed up.

It turns out there are some atoms that are with us for our entire lives. This comes from a research in Sweden. And the atoms are actually in some interesting places. They are deep in the DNA, of some cells in our brain and in our heart and also some atoms in our teeth. So brain, heart and teeth. Don't forget to brush.

That is the temporary collection of atoms currently known as our science correspondent David Kestenbaum.