Data on the functionalities of L-carnitine on obesity, diabetes, and as an ergogenic aid are summarized as follows: Obesity: Total lipid, triglyceride, and total protein increased during the 3T3-L1 cell differentiation. However, nonesterified carnitine (NEC), acid-soluble acylcarnitine (ASAC), and acid-insoluble acylcarnitine (AIAC) concentrations were lower in the differentiated 3T3-L1 cells. In addition, the exogenously added carnitine inhibited the increases in triglyceride and total lipid levels. In an animal study, L-carnitine supplementation reduced serum leptin and abdominal fat weight caused by high-fat diet in C57BL/6J mice. Diabetes: In an animal study, streptozptpcin-induced diabetic rats had markedly lower IGFBP-3 than normal rats, and IGFBP-3 was increased by L-carnitine treatment, demonstrating that L-carnitine treatment of diabetic rats modulates the IGFs/IGFBPs axis. A study of Korean diabetics indicated that there is a remarkable abnormality in lipid and carnitine metabolism in Korean diabetic patients. Ergogenic aids: We investigated the separate and combined effects of L-carnitine and antioxidant supplementation on carnitine and lipid concentrations in trained and non-trained animal and humans. Supplementation of L-carnitine and antioxidants improve lipid profiles and exercise ability in exercise-trained rats. Also, both exercise training and supplementation of carnitine and antioxidants improved lipid profiles and carnitine metabolism in humans, suggesting that carnitine and antioxidant supplementation may improve exercise performance.