Comparison of short term and long term effects of peroral L carnitine intake: clinical implications of elevated TMAO levels in cardiovascular complications Breakthrough in studying the enzyme that ultimately produces fish odour syndrome Effect of dietary fiber on trimethylamine N oxide production after beef consumption and on gut microbiota: MEATMARK a randomized cross over study European Journal of Clinical Nutrition 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.