L Carnitine: Benefits, Uses, and Side Effects Lifespan Research Institute 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. PDF) [L carnitine treatment and fish odor syndrome: an unwaited adverse effect]. Does L Carnitine Work for Fat Loss? A Dietitian Explains