Carnitine Deficiency: What Is It, Causes, Symptoms, and More Osmosis 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]. Trimethylaminuria: Definition, symptoms, and more Optimizing endurance performance in elite swimmers: effects of L carnitine supplementation and hydration strategies Applied Water Science Springer Nature Link