肉碱 (carnitine)，或音译卡尼丁，是一种类氨基酸，属于季铵阳离子复合物，可以透过生物合成方法从赖氨酸及蛋氨酸两种氨基酸合成产生。肉碱存在有两个立体异构：包括有生物活跃的L-肉碱，以及其非生物活跃的对映异构体D-肉碱。以化学方式合成，同时存在L和D两种肉碱的化合物，则一般以「DL-肉碱」的形式标示。在中国大陆，L-肉碱有一个更常见的商品名称：左式肉碱或左旋肉碱。

左式肉碱最初是以黄粉虫的生长因子而被发现，当时曾被命名为「维生素Bt」。在生物的细胞里，当脂肪新陈代谢产生能量时，左式肉碱是把脂肪酸从胞质溶胶运送到线粒体内所必需的，以防止脂肪酸积聚在细胞内。在世界各地，左式肉碱经常都被包装成为营养补充剂而售卖，并声称能够帮助燃烧脂肪，帮助减肥。

Carnitine is an amino acid derivative and nutrient involved in lipid (fat) metabolism in mammals and other eukaryotes. It is in the chemical compound classes of β-hydroxyacids and quaternary ammonium compounds, and because of the hydroxyl-substituent, it exists in two stereoisomers, the biologically active enantiomer L-carnitine, and the essentially biologically inactive D-carnitine. Both are available through chemical synthesis, and the L-form is continuously biosynthesized in eukaryotic organisms from the proteinogenic amino acids lysine and methionine. In such eukaryotic cells, it is specifically required for the transport of fatty acids from the intermembraneous space in the mitochondria into the mitochondrial matrix during the catabolism of lipids, in the generation of metabolic energy. Carnitine was originally found as a growth factor for mealworms and labeled vitamin B_T, although carnitine is not by biochemical definition a true vitamin. It is used efficaciously, clinically, in the treatment of some conditions, e.g. systemic primary carnitine deficiency, and it is available over the counter as a nutritional supplement, though its efficacy for most conditions for which it is advertised is controversial or not yet established.