Do You Really Replace Every Cell Every 7 Years? The Biology and the Ship of Theseus (opens in new tab)

TL;DR: No, you do not replace every cell every seven years. Your body turns over roughly 330 billion cells a day, about 1 percent of its ~30 trillion cells, but this is dominated by blood and gut, which renew in days to months, not on any tidy seven-year clock. A few structures are never replaced at all: the neurons of the cerebral cortex, the crystallin proteins at the core of the eye lens, and tooth enamel last for life. The upshot for identity is that you are less like the matter you are m...