In a related question, Are all cells of the human body completely replaced every seven to ten years?, it is explained that there are cells in the human body, like neurons, that are never replaced.
However, are the atoms within those neurons replaced?
Some examples of people claiming they are:
Every ~16 days nearly 100% of the water is exchanged in a healthy body. Heavy elements like carbon, sodium and potassium take occupancy far longer perhaps 8 months - 11 months.
In about a year every atom in your body would have been exchanged. Not a single atom in your body resides there forever and there is a 100% chance that 1000s of other humans through history held some of the same atoms that you currently hold in your body. -
There was a study done at Oak Ridge Lab. by Paul C. Aebersold in 1953 that found that 98 percent of all the atoms in a person’s body change out every year, and that within five years all the atoms had changed.
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.