I remember reading a book (Protector, by Larry Niven[0]), in which the protagonist (a "proto-Belter"), eats a sweet potato, turns into an ugly-ass duck, and lives forever.

After about 25,000 years or so, his brain becomes "full," and he can't remember stuff, anymore.

[DISCLAIMER: It's been a long time since I read the book, so some liberty has been taken in the Cliff's Notes version]

[0] https://en.wikipedia.org/wiki/Protector_(novel)