Hi, Longevity Enthusiast,
How old is your body?
It sounds like an easy question.
If you're 45, your body is 45.
Right?
Not even close.
Some of the cells inside you are only a few days old.
Others have been with you for months.
Some tissues are being dismantled and rebuilt while you read this.
And parts of you may have been there since around the time you were born.
Your body doesn't have one age.
It contains many.
Some of you is only days old
Start with your gut.
The intestinal lining sits at one of the busiest boundaries in the human body - exposed constantly to food, microbes, digestive chemicals, and mechanical stress.
So it doesn't simply last.
Much of the intestinal epithelium turns over remarkably quickly - on the scale of just a few days.
The cells lining you today are not necessarily the ones that were there last week.
Already, the idea of having one biological age starts to fall apart.
Your blood lives on a different clock
Now look inside your bloodstream.
A typical red blood cell circulates for about 120 days before being removed and replaced.
And the scale is extraordinary.
Researchers estimate that roughly 170 billion red blood cells are renewed every day.
You don't feel any of this happening.
There is no sensation of an old red blood cell leaving.
No sensation of a new one arriving.
Yet an enormous replacement operation is running continuously inside you.
Even your skeleton isn't as permanent as it looks
Bone feels like the most permanent material in the body.
But bone is alive.
Specialized cells continuously remove old bone while others lay down new tissue.
This process - bone remodeling - continues throughout life.
So the skeleton you imagine as your body's permanent frame is actually under continual renovation.
But then we reach the other extreme.
Some brain cells may be your age
Scientists found an ingenious way to determine the age of human cells.
Above-ground nuclear testing in the 1950s and early 1960s caused a sharp rise in atmospheric carbon-14.
That carbon eventually entered plants, animals...
and humans.
Decades later, researchers realized they could use this historical carbon-14 signature almost like a timestamp inside DNA.
And when they examined neurons from the human cerebral cortex, they found something remarkable:
The neurons dated back to around the birth of the person.
While surrounding non-neuronal cells showed turnover, cortical neurons did not show detectable adult replacement at meaningful levels in those studies.
Think about that.
Some cells involved in the way you experience the world today may have been there when you were a baby.
But there is something even stranger.
You are looking through something older than almost everything else in your body
Inside the lens of your eye are proteins called crystallins.
Unlike most proteins in the body, crystallins in the central lens undergo extraordinarily little - and for important fractions effectively no - turnover.
Radiocarbon dating has shown that much of the crystallin formation in the lens nucleus occurs around the time of birth.
These proteins have to remain functional for decades.
No regular replacement cycle.
No fresh copy every few months.
Some of the molecular machinery helping you see the words on this screen has been with you essentially your entire life.
And the lens itself keeps adding new fibers around its older core as life goes on.
Old remains.
New gets added.
Layer upon layer.
So what about the “new body every 7 years”?
You've probably heard some version of this:
Every seven years, every cell in your body has been replaced.
It's a great story.
It's also wrong.
There is no master seven-year replacement clock.
Your gut and blood turn over rapidly.
Your bones remodel much more slowly.
Many cortical neurons can last for essentially your lifetime.
And some proteins in the center of your eye lens are extraordinarily long-lived.
Which leads to a much stranger picture of what a human body actually is.
You are old and new at the same time
Right now, you contain:
cells that arrived days ago,
cells that have been working for months,
tissues being continuously dismantled and rebuilt,
neurons that may have been with you since infancy,
and proteins that have survived almost your entire life.
There is no single age inside you.
Your body is more like a city.
Some structures are constantly replaced.
Others are slowly renovated.
And a few have been standing almost since the beginning.
Perhaps that's one of the strangest things about aging:
Every day, you are getting older.
And every day, parts of you are becoming new again.
Stay curious,
David
Founder, Longevity Enthusiasts