Carnitine is not an amino acid in the strictest sense (it is actually a substance related to the B vitamins). However, because it has a chemical structure similar to that of amino acids, it is actually considered together with them.      

Unlike true amino acids, carnitine is not used for protein synthesis or as a neurotransmitter. Its main function in the body is to help transport long-chain fatty acids, which are burned within the cells, mainly in the mitochondria, to provide energy. This is a major source of energy for the muscles. Carnitine, thus increases the use of fat as an energy source. This prevents fatty buildup, especially in the heart, liver, and skeletal muscles. Carnitine may be useful in treating chronic fatigue syndrome (CFS), because a disturbance in the function of the mitochondria may be a factor in fatigue. Studies have shown decreased carnitine levels in many people with CFS.        

Carnitine reduces health risks posed by poor fat metabolism associated with diabetes; inhibits alcohol-induced fatty-liver; and lessens the risk of heart disorders.         

Studies have shown that damage to the heart from cardiac surgery can be reduced by  treatment with carnitine.  According to the American Journal of Cardiology, one study showed that proprionyl-L-carnitine, a carnitine derivative, helps to ease the severe pain of intermittent claudication, a condition in which a blocked artery in the thigh decreases the supply of blood and oxygen to the leg muscles, causing pain especially with physical activity. Carnitine has the ability to lower blood triglyceride levels, aid in weight loss, improve the motility of sperm, and improve muscle strength in people with neuromuscular disorders.  It may be useful in treating Alzheimer's disease. Conversely, carnitine deficiency may be a contributor of certain types of  muscular dystrophy, and it has been shown that these disorders lead to losses of carnitine in the urine. People with such conditions need greater-than-normal amounts of carnitine. Carnitine has also been shown to reduce fatigue, which is common in many diseases. In studies, people with celiac disease (an autoimmune disorder of the small intestine) and people with cancer had more energy with carnitine supplementation. 

Daily supplementation with acetyl-L-carnitine (ALC) has been shown to significantly slow the progression of Alzheimer's disease, resulting in less deterioration of memory, attention,  language, and spatial abilities. It can also be used to treat other cognitive disorders, as well as depression.  ALC provides numerous other benefits to many of the body's systems.  It helps to limit damage caused by oxygen starvation, enhance the immune system, protect against oxidative stress, stimulate the antioxidant activity of certain enzymes, protect membranes, slow cerebral aging, prevent nerve disease associated with diabetes and sciatica, modulate hormonal changes caused by physical stress, and increase the performance-enhancing benefits of branched-chain amino acids.