Carnitine is a non-essential amino acid and a quaternary ammonium compound. Carnitine is also classified as an alcohol (specifically, a trimethylated carboxy-alcohol). Carnitine exists as one of two stereoisomers (the two enantiomers D-carnitine and L-carnitine. Both are biologically active, but only L-carnitine naturally occurs in animals, and D-carnitine is toxic as it inhibits the activity of the L-form. Carnitine is involved in the metabolism in most mammals, plants, and some bacteria.  Carnitine plays a key role in lipid metabolism and beta-oxidation. It is used to transport long-chain fatty acids into the mitochondria to be oxidized for energy production. This is done by forming a long chain acetylcarnitine esters which are then transported by carnitine palmitoyltransferase I and carnitine palmitoyltransferase II. Carnitine also participates in removing products of metabolism from cells. Given its key metabolic roles, carnitine is concentrated in skeletal and cardiac muscle as well as other tissues that metabolize fatty acids as an energy source. A normal 70 kilogram person typically produces 11‚Äì34 mg of carnitine per day. Adults eating mixed diets of red meat and other animal products ingest 60‚Äì180 mg of carnitine per day, while vegans consume about 10‚Äì12 mg per day. Most carnitine obtained from the diet is absorbed in the small intestine before entering the blood.[3] The total body content of carnitine is about 20 grams in a person weighing 70 kilograms, with nearly all of it contained within skeletal muscle cells. Carnitine is so important in providing energy to muscles (including the heart) that some researchers are now recommending carnitine supplements in the diet, particularly for people who do not consume much red meat (the main food source for carnitine). Carnitine has been described as a vitamin, an amino acid, or a metabimin (i.e. an essential metabolite). Like the B vitamins, carnitine contains nitrogen and is very soluble in water. However, most animals, including humans, make their own carnitine; thus, carnitine cannot be considered to be a vitamin. In certain circumstances, such as methionine deficiency, lysine deficiency, vitamin C deficiency or kidney dialysis, carnitine shortages can develop. Under these conditions, carnitine must be absorbed from food, and for this reason, it is sometimes referred to as a "metabimin" or a conditionally essential metabolite. In humans, about 25% of carnitine is synthesized in the liver, kidney, and brain from lysine and methionine. Most of the carnitine in the body comes from dietary sources such as red meat and dairy products. Inborn errors of carnitine metabolism such as Reye‚Äôs syndrome can lead to brain deterioration gradually worsening muscle weakness, Duchenne-like muscular dystrophy, and extreme muscle weakness with fat accumulation in muscles. Carnitine is an essential nutrient for pre-term babies and individuals who are unable to eat a normal diet (e.g. non-ketotic hypoglycemics, kidney dialysis patients) (PMID: 115309). In conditions such as kwashiorkor, cirrhosis, and heart muscle disease (cardiomyopathy) as well as in inborn errors of metabolism such as type IV hyperlipidemia and propionic aciduria, carnitine is essential to life and carnitine supplements are critically important. Carnitine therapy may also be useful in a wide variety of clinical conditions. Carnitine supplementation has improved some patients who have angina secondary to coronary artery disease. Carnitine supplements may also be useful in many forms of metabolic liver diseases and heart muscle disease. Hearts undergoing severe arrhythmia quickly deplete their stores of carnitine. Athletes, particularly in Europe, have used carnitine supplements for improved endurance. Carnitine may improve muscle building by improving fat utilization and may even be useful in treating obesity. Carnitine may be of value in treating pregnant women, hypothyroid individuals, and male infertility due to the low motility of sperm. Carnitine deficiency is noted in abnormal liver function, renal dialysis patients, and severe to moderate muscular weakness with associated anorexia. Carnitine is a biomarker for the consumption of meat. In fact, carnitine was first extracted from meat extracts in 1905, leading to its name from Latin, "caro/carnis" or flesh.