Regrowth mystery reborn
If you're only as old as you feel then are you as old as your body? The idea that body cells regrow every seven years should be an accurate guide, right? Wrong! Dr Karl can sense it in his bones.
Seven has long been a special number in our culture — there are seven days in the week and seven deadly sins.
Even today, the motivational books will often use the number seven in the title.
But seven's special claim to fame is the very widely held belief that our bodies are renewed every seven years. This claim appears in many forms, for example, the promotion of body products and health foods, and in new age literature and in motivational speeches.
But new research has finally been able to measure the life span of various human cells in different body organs — and in doing so, put the lie to this idea of the blanket seven-year renewal of the human body.
This new research is helping us answer the age-old question of 'How old are you?'.
It turns out that it's actually a very difficult question to answer. You see, this new research tells us that we are all older than our bodies, if you start the clock ticking when we were born.
There are many mysteries about aging. For example, why humans live so much longer than most other mammals, or why only humans and the pilot whale go through menopause, and at what rates do different parts of the human body renew themselves.
The key to solving the renewal rate mystery is the fact that every living creature on Earth takes in carbon from its environment.
Carbon atoms come in a few different varieties or isotopes.
Practically all of the carbon in the biosphere is in the form of the non-radioactive isotope carbon-12 (C-12). But for every million million atoms of carbon-12, there is one atom of radioactive carbon-14 (C-14).
C-14 is continually being made naturally, whenever a cosmic ray from outer space slams into a nitrogen atom.
Being radioactive, C-14 also naturally decays, with a half-life of about 5730 years.
For billions of years there was a balance between C-14's natural decay and its natural production. This balance gave all living creatures low background levels of C-14.
But once an animal dies, it no longer takes up any C-14. Indeed, its levels of C-14 halve every 5730 years.
But then we humans invented nuclear weapons which, when they explode, also make C-14.
By 1963, when the testing of nuclear weapons in the atmosphere was stopped, the natural background levels of C-14 in living creatures had already doubled.
The scientists realized that this short-lived spike of radioactive C-14 from nuclear weapons could be used as a clock to measure cell life — and that led to the big breakthrough for this new research.
Dr Jonas Frisen from Sweden and Dr David Fink from the Australian Nuclear Science and Technology Organisation were among the authors of the paper 'Neocortical Neurogenesis in Humans is Restricted to Development'.
Dr Fink and his team used an ultra-sensitive technique called Accelerator Mass Spectrometry. This let him test samples as small as 10-millionths of a gram. This corresponds to the DNA from as few as five million cells.
Their amazing discovery was that nerve cells in most of your brain are as old as you are.
It makes sense for the nerves in your vision and memory centres to preserve how you see and remember for your whole life.
But the cells in your cerebellum (which are concerned with balance and coordination) are 2.9 years younger than you, which again makes sense, as balance and coordination develop during the years of infancy.
Other scientists' research has told us that cells in different parts of the body get renewed at different rates.
Your gut runs some 10 metres from your mouth to your anus. The cells that line your gut are replaced every five days. On the other hand, the cells in your gut that are not part of the lining get replaced every 15.9 years.
The turnover time for skin cells is about two weeks. Red blood cells are recycled every 120 days, while muscle cells hang around for 15.1 years.
Bone is rebuilt every 10 years. The carbon in the enamel in your teeth is never replaced, which makes teeth a clock accurate to 1.6 years.
We still don't know how many bodies you use up in your life. But we do know that the bland claim 'your cells are replaced every seven years' is as wrong as the claim that a cat has nine lives.
Published 18 November 2008
© 2013 Karl S. Kruszelnicki Pty Ltd