L-Carnitine

L-carnitine is an amino acid derivative — chemically, (R)-3-carboxy-2-hydroxy-N,N,N-trimethylpropan-1-aminium inner salt — often sold under the old name "vitamin BT." Your body synthesizes it in the liver and kidneys from lysine and methionine, so it is not strictly an essential nutrient.

The total body pool is roughly 20 grams, and over 95% of it sits in cardiac and skeletal muscle. Dietary sources are dominated by red meat and dairy (goat meat is the richest at about 2.1 g/kg); plant foods contain almost none, which is why vegetarians and vegans typically carry lower muscle carnitine stores.