[RBerenguel]

Fat tissue is a collector of some kinds of toxins (don't remember exactly which now) and some heavy metals (heh). Going into fat burning actually releases these into your system (which should be bad, though)

[Pete_D]

Are you thinking of https://en.wikipedia.org/wiki/Persistent_organic_pollutant ?

[RBerenguel]

Yup that was it, thanks!