Srila Siddhaswarupananda Paramahamsa

Let me ask you a few simple questions: Do you exist at this moment? Did you exist five years ago? Are you your body? Most people would answer “yes” to all three questions. But if you identify your body as yourself, and simultaneously accept that you exist now and also existed five years ago, then you have a problem: The body you had five years ago does not exist today. There is a dynamic turnover of atoms and molecules which make up your body. There isn’t a single particle of matter—not one atom—present in your body today that was present five years ago. The body you have today is not the same body you had five years ago. It’s not that the body you had still exists but has now changed somewhat. No. The body you had is gone. That collection of atoms appearing as flesh, bone, blood, hair, and so on no longer exists. Yet you still exist.
Science of Identity Foundation – Siddhaswarupananda

The physical body is made up of countless numbers of tiny material particles called molecules and atoms. These material particles are constantly being replaced by material particles from the outside environment (in the form of food, water, air, and so on). Over a period of five to seven years, this process of metabolism brings about a complete change of the matter that makes up your body.
Science of Identity Foundation – Siddhaswarupananda

Recent studies at the Oak Ridge Atomic Research Center have revealed that about 98 percent of all the atoms in a human body are replaced every year. You get a new suit of skin every month and a new liver every six weeks. The lining of your stomach lasts only five days before it’s replaced. Even your bones are not the solid, stable, concrete-like things you might have thought them to be: They are undergoing constant change. The bones you have today are different from the bones you had a year ago. Experts in this area of research have concluded that there is a complete, 100 percent turnover of atoms in the body at least every five years. In other words, not one single atom present in your body today was there five years ago.*
Science of Identity Foundation – Siddhaswarupananda
* Taken from Guy Murchie, The Seven Mysteries of Life (Boston: Houghton-Mifflin, 1978), pp. 321-22.

Many people believe that a person is the brain or some part of the brain. You may be one of them. If so, the following should boggle your mind:
Recent studies on the turnover of the molecular population within a given nerve cell have indicated that ... their macromolecular contingent is renewed about ten thousand times in a lifetime.*
In other words, the matter making up each brain cell is completely renewed every three days.
Your brain—that mass of matter which is contained in your skull today—is not the same brain that was in your skull last week.
Science of Identity Foundation – Siddhaswarupananda
*Paul Weiss, “The Living System: Determinism Stratified,” in Arthur Koestler and J.R. Smythies, eds., Beyond Reductionism (London: Hutchinson, 1969), p. 13.