[Harri (OP)]

As I approach 65 years of age I was wondering just how much of my body I was born with is still original? I know my dna remains the same but does the whole of the body continually lose and regenerate new cells? For instance, will any of the original heart I was born with remain with me now?

[Colin2B]

I know my dna remains the same

I wouldn’t be so sure about that. DNA does mutate as it passes from cell to cell and we age.

For instance, will any of the original heart I was born with remain with me now?

I would say not a lot, if any, of the original cells. Usual quoted cell replacement is between 7 & 10 years, but can be shorter for important organs.

[Halc]

Every cell split, only half the dna atoms are 'original', and the other half (both cells) are made from atoms from the environment.

All in all, probably under 0.1% (probably well under) of your birth atoms are still in you.
You birth atoms are also not original since you did an awful lot of growing before you were born.

[evan_au]

Some cells are not replaced during your lifetime. This applies to the lens of your eye, and brain cells.
- Most parts of the cell are generated from instructions in the DNA, using new raw materials from your diet
- But if the cell does not divide after birth (eg brain), it can continue to use the same DNA with which you were born
- Some parts of the brain do generate new cells (eg hippocampus), but this does not apply to most of the brain
- There will be spot repairs to DNA when it is damaged by natural radioactivity or metabolic stress, and these repairs would be made from "new" atoms in your diet.

I understand that some of these DNA ages were determined by measuring the radioactive content of DNA from different tissues. The isotopic mix in the diet changed noticeably in the years that atmospheric nuclear testing was underway.
https://www.sciencefocus.com/the-human-body/what-cells-in-the-human-body-live-the-longest/

[Harri (OP)]

It might surprise you to know that the question originated in my head not through any particular scientific field, but because I was gathering some thoughts together for a poem. I was writing about my relationship with my mother and wondered just how much of her baby remains today 65 years on. Your replies and that very helpful article answer my question. The poem is still processing.

[Petrochemicals]

Teeth are all I believe that is not replaced, they are used as a locator for isotopes in the water in the region that you grew up in.

[alancalverd]

So apart from bits of brain  (not teeth - they all fell out and were replaced when you were a child!) and I think a few nerves, what is "you". Is the lump of stuff that people call by your name, really the same person as on your birth or marriage certificate? If not, how can anyone be held liable for past actions? Is a 10-year passport really a  valid document?

[Bored chemist]

(not teeth - they all fell out and were replaced when you were a child!)

The replacements may have already been in place, but it's probably beside the point.

I'm not saying this is an accurate mechanism to answer the question but it does give some indication.
In a typical year you can expect to eat something like 1000 Kg of food. That's about 7 times your weight.
The stuff food is made from isn't very different from the stuff you are made from.
So each year your body takes in 7 times its mass in "stuff".

If that were properly mixed then only 1/7 of you would be still there at the end of the first year.
So it's 1 in 49 after 2 years and by the time you are 65 it's pretty close to bugger all.

Obviously some bits are better mixed than others.
The chip in my front tooth that I have been carrying round since school is still there- more like 45 years than 65, but the point remains.
Most of that stuff is pretty much fixed. Skeletal bone is turned over much faster than that, but still pretty slowly.
My skeleton is something like 10Kg
It contains something like 1400 grams of Calcium
"The calcium content of bone at maturity is approximately 1,200 g in women and 1,400 g in men"
from
https://www.ncbi.nlm.nih.gov/books/NBK56060/
and it turns over about 0.5 grams per day
https://www.medscape.com/answers/241893-20078/how-much-calcium-turnover-occurs-normally-each-day
so that's about 180 grams per year or (very roughly) the whole lot gets rebuilt every 7 or 8 years.
Some bits deep in the middle might date back to when I was a teenager.
At best, very little of it will be original.
"At full-term birth, the human infant has accrued about 26 to 30 g of calcium, most of which is in the skeleton."

Just a thought; if we are what we eat, why are we not "new and improved"?

[alancalverd]

The replacements may have already been in place, but it's probably beside the point.

I doubt it. They are much bigger than "milk" teeth and couldn't possibly hide in a neonatal jaw.