L Carnitine Side Effects & Safety: A Complete Guide The Science of Good Health Trimethylaminuria (Fish Odor Syndrome or TMAU) A recurring fishy odor could indicate a larger medical problem. 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. Carnitine Rich Foods: Understanding L Carnitine and Its Benefits Loyal 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.