Never Mind the Bullocks, Here's the Science (15 page)

Seven-Year Regrowth
(A myth in its prime)

The number seven has long had special significance in our culture. For example, there are seven days in the week; the seven deadly sins are counterbalanced by the seven virtues; there are also the seven wonders of the world; and ancient astronomers identified and named seven planets. Even today, motivational books will often use the number seven in the title, e.g.
The Seven Habits of Highly Effective People, The Seven Principles for Making Marriage Work
and
The Seven Pillars of Health.

But the number seven’s other Special Claim to Fame is the very widely held belief that our bodies are renewed every seven years. This claim comes from many sources including new-age therapies, beauty products, health foods and motivational speakers. This sevenish Claim to Fame has also been linked to the incorrect assertion that women’s bodies have a major hormonal shift every seven years.

However, new research has finally been able to measure the life span of various human cells in different body organs. And, in doing so, it has exposed the simple lie of the seven-year renewal of the human body.

Seven is (not) the Magic Number

There is a widely held belief that our bodies are renewed every seven years. BUT, the research of scientists has told us that cells in different parts of the body get renewed at different rates.

On average, most of your body is less than 10 years old – probably around 5.6 years.

How Old Are You?

This new research is helping us answer the age-old question: How old are you? In actual fact, this is a very difficult question to answer. You see, this new research tells us that we are all older than our bodies—if we start the clock ticking when we are born.

There are many mysteries about ageing. For example, why do human beings live so much longer than most other mammals? Why does our memory fade as we get older? How does our brain remember things?

And there is yet another mystery. At what rate do different parts of the human body renew or replicate themselves?

The key to solving the renewal-rate mystery involves a simple fact, which is that every living creature on Earth takes in carbon from its environment.

Carbon from the atmosphere reacts with oxygen to form carbon dioxide. This carbon dioxide can then enter plants via photosynthesis. Animals, including human beings, then consume these plants.

At 18.5%, carbon is the second most common element (by mass) in the human body, after oxygen.

Carbon-14

Carbon is the sixth lightest element, so it has – in its core or nucleus – six protons. An isotope (of any element) has the same number of protons, but a variable number of neutrons. Carbon has approximately 15 isotopes, most of them extremely rare and uncommon.

A carbon nucleus with six neutrons is the isotope carbon-12, which accounts for about 99% of the carbon found in the biosphere. This isotope is not radioactive.

A carbon nucleus with seven neutrons is the isotope carbon-13, which accounts for about 1% of carbon. This isotope is also not radioactive.

A nucleus with eight neutrons is the isotope carbon- 14, and accounts for about a one-trillionth part of carbon. Carbon-14 is radioactive.

Carbon-14 was discovered during World War II, on 27 February 1940, at the University of California Radiation Laboratory in Berkeley. Its discoverers were Martin Kamen and Sam Ruben.

Radioactive Carbon 101

Carbon atoms come in a few different varieties or ‘isotopes’. Practically all of the carbon that occurs in the biosphere consists of the non-radioactive isotope carbon-12 (C-12). But if you look very carefully, you will find that for every million million (trillion) atoms of carbon-12, there is one atom of radioactive carbon-14 (C-14).

C-14 is continually being made naturally, whenever a neutron associated with a cosmic ray from Outer Space slams into a
nitrogen atom. Nitrogen makes up about 80% of our atmosphere—the stuff that we breathe. So there’s lots of nitrogen available to be hit by cosmic rays. (In the following equation, ‘n’ is a neutron, while ‘p’ is a proton. Cosmic rays provide the energy to ‘pump up’ a nitrogen atom into a radioactive carbon atom.)

n +
¹⁴
₇
N →
¹⁴
₆
C + p

Being radioactive, C-14 also decays naturally, with a half-life of about 5,730 years. It decays back to stable, non-radioactive nitrogen. (In the following equation, ‘e
^-
’ is an electron, while ‘
’ is an electron antineutrino.)

¹⁴
₆
C →
¹⁴
₇
N + e
^-
+

For billions of years there has been a balance between C-14’s natural production and its natural decay. This balance gave the environment low (and fairly constant) background levels of C-14. (As mentioned above, there is about one atom of C-14 for every trillion atoms of ‘regular’ C-12.)

The C-14 level in the atmosphere mirrors fairly closely the C-14 levels in living creatures (because we are always taking in and excreting—to put it politely—carbon).

So all living creatures take in some kind of food from the environment—and this food always has some carbon in it. But of course, once an animal dies, it stops eating. It no longer takes up any carbon and, consequently, no more C-14. So once it dies, its levels of C-14 halve every 5,730 years. This concept is used in the technique called radiocarbon dating in archaeology to determine the age of material remains that were once living, e.g. trees and animals.

However, in the 1940s, human beings invented nuclear weapons. When nukes explode, they also make C-14. So our nuclear weapons added extra C-14 into the atmosphere. By 1963, when the testing of nuclear weapons in the atmosphere was stopped thanks to the Partial Test Ban Treaty, the background levels of C-14 in living creatures had already doubled (to about two atoms of C-14 for every trillion atoms of ‘regular’ C-12).

The scientists realised 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 on cell and tissue dating.

Your Food is Radioactive

All the food you eat is – very slightly – radioactive. That’s because out of every trillion atoms of carbon, just one is the radioactive carbon-14.

Half-Life

The ‘half-life’ of a radioactive element is the time taken for half of the atoms to decay into another element. The term was first used in 1907 and, at the time, was known as a ‘half-life period’. But the word ‘period’ was dropped in the early 1950s, and today we just call it a ‘half-life’.

For example, radon-222 has a half-life of less than four days, while uranium-238 (not the isotope that goes ‘bang’) has a half-life of 4.47 billion years. It’s 704 million years for uranium-235 (the isotope that does go ‘bang’).

At the short end of half-lives is carbon-8. It has a half-life of about two-thousandths of a billionth of a billionth of a second.

Carbon-14 has a half-life of 5,730 years.

Suppose we have a million atoms of C-14. We know that after about 5,730 years, we will have only half a million atoms of C-14 (the rest will have decayed back to nitrogen). After a further 5,730 years, we will be left with only 250,000 atoms of C-14. And so on.

Or to express a half-life in another way, if you have a gram of carbon (with all the carbon isotopes in their respective proportions), in each second there will be just 14 disintegrations or decays of C-14 atoms.

Now here’s something really weird about the half-life of an element and radioactive decay.

We have no way of telling
which
individual atoms will decay. We also have no way of telling
when
(in that 5,730-year window) any of those atoms will decay.

Radioactive decay is totally random – one of the very few events in our Universe that is.

The Age of Your Brain

By the way, in the cells in your body, most of the molecules are in a state of constant flux and exchange with the environment. One unique exception is your DNA—this interacts with the environment only when it divides. So the C-14 levels in DNA can be used to establish the birth date of the cells that the DNA belongs to.

Dr Jonas Frisen from Sweden and Dr David Fink from the Australian Nuclear Science and Technology Organisation (ANSTO) 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 to count the number of C-14 atoms in the DNA in neurons from human brain cells. This allowed Dr Fink to test samples as small as ten-millionths of a gram. This corresponds to the DNA from as few as five million cells.

The numbers are quite astonishing. In the DNA of a single cell, there are about 64 billion carbon atoms. But you have to look at one thousand billion carbon atoms to find a single C-14 atom. This means you need to look at the DNA of 15 cells to find just one C-14 atom. And we can do this—aren’t human beings wonderful!

Their amazing discovery was that the neurons (or nerve cells) in most of your brain are as old as you are. It makes sense that the nerves in your vision and memory centres preserve, for your whole life, the knowledge of how you see and how you remember.

But the cells in your cerebellum—which are concerned with balance and coordination—are about 2.9 years younger than you. This also makes sense, as balance and coordination develop during the years of infancy.

However, in the brain, there are cells that are not nerve cells—these are called glial cells. They are about ten times as common as
nerve cells, with some people believing that glial cells act as ‘support’ or ‘supply’ cells for the neurons. (On the other hand, Albert Einstein had an abnormally high percentage of glial cells.) On average, you have about 100 billion glial cells in your brain, but only ten billion neurons. Anyhow, glial cells are 4.9 years younger than your birth age.

Brain Cells Do (Not) Replace?

When I went to medical school (in the 1980s), we were taught that brain cells
never
repair themselves and
never
replicate.

Today, it is quite well established that some brain cells in some parts of the brain
do
replicate. These regions include the hippocampus and the area around the ventricles. This replication happens throughout life in all the mammals that have been studied. But most of the human brain does not replicate.