What is Carnitine? L Carnitine is not a magic fat burner , It wont melt fat, build muscle, or replace hard work., The only thing it burns is your money , Stick to the basics Nutrition, Training & Consistency. Trimethylaminuria: Definition, symptoms, and more L carnitine supplements can cause a fishy odor in the breath, sweat, and urine. This is likely due to the formation of trimethylamine (TMA). TMA is also responsible for the pungent smell of 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.