[1][3][4] Despite common carnitine supplement consumption among athletes for improved exercise performance or recovery, there is insufficient high-quality clinical evidence to indicate it provides any benefit.

[3] The total body content of carnitine is about 20 grams (0.71 oz) in a person weighing 70 kilograms (150 lb), with nearly all of it contained within skeletal muscle cells.

[1] TML is then hydroxylated into hydroxytrimethyllysine (HTML) by trimethyllysine dioxygenase (TMLD), requiring the presence of ascorbic acid and iron.

[1] Gamma-butyrobetaine is then hydroxylated by gamma butyrobetaine hydroxylase (a zinc binding enzyme[10]) into l-carnitine, requiring iron in the form of Fe2+.

[12] Carnitine plays a role in stabilizing acetyl-CoA and coenzyme A levels through the ability to receive or give an acetyl group.

[1] The tissue distribution of carnitine-biosynthetic enzymes in humans indicates TMLD to be active in the liver, heart, muscle, brain and highest in the kidneys.

The pyrophosphate, formed from the hydrolysis of the two high-energy bonds in ATP, is immediately hydrolyzed to two molecules of Pi by inorganic pyrophosphatase.

In the second step, the thiol group of a cytosolic coenzyme A attacks the acyl-adenylate, displacing AMP to form thioester fatty acyl-CoA.

The carnitine molecule formed is then shuttled back into the intermembrane space by the same cotransporter (CACT) while the fatty acyl-CoA enters β-oxidation.

Lower levels of malonyl-CoA disinhibit carnitine acyltransferase 1, allowing fatty acid import to the mitochondria, ultimately replenishing the supply of ATP.

[13] PPARα functions as a transcription factor in two cases; as mentioned before when there is an increased demand for energy from fat catabolism, such as during a fast between meals or long-term starvation.

Plasma levels of acylcarnitine in newborn infants can be detected in a small blood sample by tandem mass spectrometry.

Primary carnitine deficiency is a genetic disorder of the cellular carnitine-transporter system that typically appears by the age of five with symptoms of cardiomyopathy, skeletal-muscle weakness, and hypoglycemia.

[15][8] Despite widespread interest among athletes to use carnitine for improvement of exercise performance, inhibit muscle cramps, or enhance recovery from physical training, the quality of research for these possible benefits has been low, prohibiting any conclusion of effect.

[3][21][22] 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.

[1] Carnitine has been studied in various cardiometabolic conditions, indicating it is under preliminary research for its potential as an adjunct in heart disease and diabetes, among numerous other disorders.

[1][24] 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.

[1] Carnitine has no effect on most parameters in end-stage kidney disease, although it may lower C-reactive protein, a biomarker for systemic inflammation.

[1][3] Strict vegetarians obtain little carnitine from dietary sources (0.1 μmol/kg of body weight daily), as it is mainly found in animal-derived foods.

[1] Treatment with the anticonvulsants valproic acid, phenobarbital, phenytoin, or carbamazepine significantly reduces blood levels of carnitine.

[4] When taken in the amount of roughly 3 grams (0.11 oz) per day, carnitine may cause nausea, vomiting, abdominal cramps, diarrhea, and body odor smelling like fish.