What percentage of my atoms do I keep?

07 February 2017

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Question

Adel - Assuming an average western life expectancy, what percentage of the atoms we are born with, do we die with?

Answer

Chris Smith answered this question from Adel, after first asking the team.

Richard - Well, it’s that great philosophical question, isn’t it, that if we regenerate our cells are we still us anymore? It’s a question you could apply to the Flying Scotsman, for example, to bring it round to trains.

The Flying Scotsman locomotive, built in the 30s, rebuilt so many, so many times. Completely rebuilt recently - is it still the same engine as was originally built because so many pieces have been replaced?

Chris - In that great comedy “Fools and Horses,” I remember Trigger saying - he shows Del Boy his broom and says “this is my Great Grandfather's broom.” And Rodney goes “what the real one?” And he says “well it’s had one or two new handles and heads since then but yeah, yeah, it’s the original broom.”

Danni - do you want to speculate how many of your atoms are the ones you were born with?

Danni - Killed a lot of mine off in the first few years at university I think. But I think what you’re saying about the brain accounting for about 5 percent of your weight, and your eyes and all the rest, I’d say about 3 percent.

Chris - So we’ve got a 3 percent from Danni. Tim any thoughts?

Tim - I think it has to be a low percentage. We know quite a few different cells die and then you replace those cells, and this happens at different rates throughout your body. I think this happens for most of your cells, so by the time you’re dead, I’m thinking you’ve replaced quite a few of them.

Chris - I think it’s actually quite a difficult one to know the precise answer to this. But making it into the simplest question possible, you could argue how much does a newborn baby weigh? About seven and a half pounds or in new money, three and half kilo or so. How much does the average adult human weigh? Seventy kilo. Therefore your birth weight as a proportion of your grown adult weight is 3.5 divided by 70 times 100. That means you weigh about 5 percent at birth of what you will as an adult. Therefore, if you’re growing from 5 percent to your full size, only 5 percent of the atoms in your body can, by definition, be the ones you're born with. Therefore, if the answer to this question cannot be more than 5 percent, I would argue. Therefore, it’s got to be less than 5 percent but it’s not zero percent.

Now we know that things like your brain and the nervous system, those cells, and some muscle cells are also there for life. Your heart cells, for instance, last you a lifetime. Therefore, the answer probably is what proportion is the brain of your total body mass? It’s a couple of percent. So it’s probably somewhere between 2 percent and 5 percent of the atoms that you are born with last you a lifetime. Because you’ve got literally cells in your brain that are lasting you a lifetime, and the DNA in those nerve cells because the cells are not dividing, the DNA in those cells is not being replaced. So I’d say the numbers close to about two and a half to three percent. That would be my guess.

Richard - Yes. I like the way you show your workings there.

Chris - You get marks for that.

Richard - Excellent. Very good.

Comments

I like your calculation - based on the fact that brain cells don't get renewed as you grow. However according to https://faculty.washington.edu/chudler/facts.html, a baby has a brain weight of around 350-400g, so relative to the body weight of an adult at death, that would give us an upper bound of atoms kept through out your life, of around 60kg, giving us roughly ,6%

My workings also assume that the whole mass of a cell is DNA and that the DNA does not change over a lifetime. In fact, cells are constantly repairing and replacing atoms in the DNA, so this is a furthe simplification on my part. However, regarding brain weight, neurogenesis is largely complete by the time of birth.

The growth of the brain is mainly the creation and revision of connections between cells, and an increase in the numbers of non-neuronal cells. This is particularly the case with myelin-producing cells called oligodendrocytes. These wrap around nerve fibres to "insulate" them and improve nerve cell communication. But because they take up space the developing brain foregoes myelination of many non-essential pathways until after birth; adding this myelination is a major part of the growth, development and maturation of the nervous system. But because the cells involved are not lifelong entities, I discounted them in my (extremely simplified) estimation...

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