His initial statement, "It takes seven years for each and every atom in your body to be replaced by another." sounds like an urban myth, and he provides zero documentation for it.
I don't know any specific place to point to, but I have heard this from scientists that I know.
None the less the point I was trying to make was more that the body is in flux not simply a static thing.
Regrettably, I can't find a good citation on that, although it's widely cited as the origin for the superstition that breaking a mirror causes seven years of bad luck.
Back in the 1500's and up, when mirrors were smoothed silver on glass, they were only for royalty. And as per royalty, there were servants. The "bad luck" was actually punishment of 7 years of indentured servitude to pay for a broken mirror.
Oh what the things you learn by being in the SCA.