L-Carnitine is a type of vitamin B, and its structure is similar to that of amino acids. It is mainly used to help transport long-chain fatty acids to provide energy and to prevent fat from collecting in the heart, liver, and skeletal muscles. Carnitine can prevent disordered fat metabolism due to diabetes, fatty liver disease and heart disease, and it can reduce heart damage, lower blood triglyceride, aid in weight loss, and increase the antioxidant effects of vitamin E and C. Meats and giblets are high in carnitine. Artificially synthesized carnitine includes L-carnitine, D-carnitine, and DL-carnitine, and only L-carnitine has physiological activities. On the other hand, D-carnitine and DL-carnitine competitively inhibit the activity of carnitine acetyltransferase (CAT) and carnitine palmitoyltransferase (PTC) to prevent cells’ fat metabolism, thus harming human nutrition.