Science: The Fleeting Flesh

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About 98% of the atoms in the human body are renewed each year. This surprising fact is discussed by Dr. Paul C. Aebersold of Oak Ridge in the latest Annual Report of the Smithsonian Institution. Dr. Aebersold based his conclusion on experiments with radioisotopes, which trace the movements of chemical elements in and out of the body.

The fastest-changing component, says Dr. Aebersold, is water. It forms about 70% of the body, and about half the water molecules are replaced every eight days. Other fleeting elements are carbon, sodium and potassium. The calcium and phosphorus in bones and teeth stay put longer, but even they are not permanent. "Bones are quite dynamic." says Dr. Aebersold. The little crystals in bones are continually dissolving and reforming. In the process, some of the atoms are lost.

Some of the brain tissues, though soft and juicy, are more permanent than bone. One of the most permanent elements is iron; the same iron atoms stay in the body for a long time. Dr. Aebersold believes that 2% of the body's substance is an ample allowance for the part that sticks around for as long as a year. A human body, he says, should not be considered permanent in a material sense. It is more like a famous old regiment, all of whose members have changed many times over, while the regiment retains its organizational identity. In a larger sense, says Dr. Aebersold. the atoms that each human body is made of were once parts of other living things−e.g., dogs, whales, birds. The atoms that made up Plato and Henry VIII are still floating around as parts of people now living.