“You may think of your body as a fairly permanent structure, but most of it is in a state of constant flux as old cells are discarded and new ones are generated.”
Even your bones, which you may think of as static and non-living, are full of cells called osteoclasts and osteoblasts that are forever (respectively) breaking down and rebuilding your bones as they adapt to the stresses they’re subjected to. (This is why bed-rest or being in weightlessness results in bone loss).
A fascinating article from the New York Times highlights the work of Doctor Jonas Frisén, of the Karolinska Institute in Stockholm, who hit on a method to estimate the average age of tissues. The method involves measuring the amount of Carbon-14 in the DNA of various tissues.
A pulse of artificially-generated Carbon-14 was created in the atmosphere in the years that above-ground nuclear testing was allowed, and since then the level (which was double the normal background level) has been slowly returning to normal. This Carbon-14 entered the food chain as plants absorbed it and incorporated it into their bodies, and from there it percolated into all living things, including us. The amount of Carbon-14 in the DNA of our tissues reveals the average age of the cells making up those tissues, since the DNA is fabricated at the birth of the cell.
Frisén’s work, and that by other researchers, tells us the following:
- “The epithelial cells that line the surface of the gut have a rough life and are known to last only five days. Ignoring these surface cells, the average age of those in the main body of the gut is 15.9 years.”
- “Red blood cells, bruised and battered after traveling nearly 1,000 miles through the maze of the body’s circulatory system, last only 120 days or so on average before being dispatched to their graveyard in the spleen.”
- “The epidermis, or surface layer of the skin, is recycled every two weeks or so.”
- “An adult human liver has a turnover time of 300 to 500 days.” This means that you have a new liver just about every year!
- “The entire human skeleton is thought to be replaced every 10 years or so in adults.”
- “The average age of all the cells in an adult’s body may turn out to be as young as 7 to 10 years.”
About the only pieces of the body that last a lifetime, on present evidence, seem to be the neurons of the cerebral cortex, the inner lens cells of the eye and perhaps the muscle cells of the heart. But even there, it’s the age of the DNA that Frisén’s method measures. There are ongoing processes that replace individual components of the cells around that DNA, meaning that even in the cerebral cortex and other “static” tissues, there may be considerable change taking place. And other work has shown that new neurons are in fact generated in the cerebral cortex, although presumably not in numbers sufficient to show up in Frisén’s work. Additionally, although cells in the brain may be long-lived, they still change; brain cells have a life-long ability to develop new connections with each other. This is how learning takes place.
The upshot is that the body is continually changing.
And yet we are attached to our bodies. This brings up the question, how can we be attached to something that is constantly changing? How can we cling to something that doesn’t remain the same from one moment to the next? Well, we can’t. We identify with our bodies, thinking that if the body fails, we fail. We think that when others judge the body, they judge us. And so clinging (or trying to cling) to something impermanent leads to suffering.
The Buddhist tradition encourages us to regard all things as being like mist, or flowing water, or a mirage — what the mind takes to be solid, substantial, and graspable is actually ever-changing and characterized by impermanence. The Six Element Practice — the subject of my new book, Living as a River, is a way of developing an experiential appreciation of the transience of the body. This helps us to let go, stop identifying the body with the self, to suffer less, and to experience a profound sense of freedom.