Trimethylaminuria: FMO3 mutations that cause malodorous body odor Treatments of trimethylaminuria: where we are and where we might be heading ScienceDirect Trimethylaminuria, or fish odour syndrome is an uncommon condition that causes an unpleasant, fishy smell. The body is unable to turn a strong smelling chemical called trimethylamine produced in the gut when Did You Know? Some people report a fishy smell after supplementing with L carnitine. This happens because certain gut bacteria convert carnitine to trimethylamine, a malodorous compound found in rotting fish.