Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids from the cytosol into mitochondria to be oxidized for free energy production, and also participates in removing products of metabolism from cells. Both are biologically active, but only -carnitine naturally occurs in animals, and -carnitine is toxic as it inhibits the activity of the -form. At room temperature, pure carnitine is a whiteish powder, and a water-soluble zwitterion with relatively low toxicity. Derived from amino acids, carnitine was first extracted from meat extracts in 1905, leading to its name from Latin, "caro/carnis" or flesh.
- fatty acid transport across the mitochondrial membrane by forming long-chain acylcarnitine esters which are shuttled into the mitochondria, where they undergo β-oxidation to produce ATP, the cell's main energy currency;
Biosynthesis and metabolism
Physiological effects in humans
As an example of normal biosynthesis of carnitine in humans, a person would produce 11–34 mg of carnitine per day.) into -carnitine, requiring iron in the form of Fe<sup>2+</sup>.
Fatty acid transport
Carnitine is involved in transporting fatty acids across the mitochondrial membrane, by forming a long chain acylcarnitine ester and being transported by carnitine palmitoyltransferase I and carnitine palmitoyltransferase II.
Acetyl-CoA stabilization
Carnitine plays a role in stabilizing acetyl-CoA and coenzyme A levels through the ability to receive or give an acetyl group.
The first reaction of the carnitine shuttle is a two-step process catalyzed by a family of isozymes of acyl-CoA synthetase that are found in the outer mitochondrial membrane, where they promote the activation of fatty acids by forming a thioester bond between the fatty acid carboxyl group and the thiol group of coenzyme A to yield a fatty acyl–CoA. and facilitate recovery after such performance, but their results are inconclusive, since various studies used various regimens of carnitine supplementation and intensity of exercise. At supplement amounts of per day over a month, there was no consistent evidence that carnitine affected exercise or physical performance on moderate-intensity exercises, whereas on high-intensity exercises results were mixed. There is no evidence that L-carnitine influences fat metabolism or aids in weight loss.
Male fertility
The carnitine content of seminal fluid is directly related to sperm count and motility, suggesting that the compound might be of value in treating male infertility. and has no significant effect on blood lipids.
Although there is some evidence from meta-analyses that L-carnitine supplementation improved cardiac function in people with heart failure, there is insufficient research to determine its overall efficacy in lowering the risk or treating cardiovascular diseases.
The kidneys contribute to overall homeostasis in the body, including carnitine levels. In the case of renal impairment, urinary elimination of carnitine increasing, endogenous synthesis decreasing, and poor nutrition as a result of disease-induced anorexia can result in carnitine deficiency. Carnitine blood levels and muscle stores can become low, which may contribute to anemia, muscle weakness, fatigue, altered levels of blood fats, and heart disorders. Vegans get noticeably less (about 10–12 mg) since their diets lack these carnitine-rich animal-derived foods. Approximately 54% to 86% of dietary carnitine is absorbed in the small intestine, then enters the blood.
L-Carnitine, acetyl--carnitine, and propionyl--carnitine are available in dietary supplement pills or powders, with a daily amount of 0.5 to 1 g considered to be safe.
It is also a drug approved by the Food and Drug Administration to treat primary and certain secondary carnitine-deficiency syndromes secondary to inherited diseases.