What is L-Carnitine and How does it work?

L-Carnitine is a naturally occurring amino acid which plays a vital role in the metabolism of fat. It functions as a transporter of fatty acids into the mitochondria, the metabolic furnace of the cell.
L-Carnitine was discovered in Russia, and one year later in Germany. Its structural formula was decoded in 1927, and its physiological significance was understood by the 1960's. L-Carnitine is synthesized in the liver and kidneys, from two essential amino acids, lysine and methionine. This synthesis requires the presence of vitamins niacin, B6, and iron.
It has been established that an adequate supply of L-Carnitine is necessary for the maintenance of good health. L-Carnitine aids in the release of stored body fat, tryglycerides, into the bloodstream for energy. Tryglycerides are the major source for the production of energy in the heart and skeletal muscles. Access to L-Carnitine is believed to increase energy levels for long-term aerobic activity. L-Carnitine is also responsible for muscle contraction, regulation of protein balance and maintenance of a healthy heart. 
Effects on bone-mass:
In the course of human aging, carnitine concentration in cells diminishes, affecting fatty acid metabolism in various tissues. Particularly adversely affected are bones, which require continuous reconstructive and metabolic functions of osteoblasts for maintenance of bone mass. Administration of a carnitine mixture or propionyl-L-carnitine is capable of increasing serum osteocalcin concentrations.

Heart conditions:
L-Carnitine is primarily used for heart-related conditions. Several clinical trials show that L-Carnitine and propionyl-L-carnitine can be used along with conventional treatment for angina to reduce medication needs and improve the ability of those with angina to exercise without chest pain. There is little evidence about a positive effect of the use of carnitine after a heart attack. Some studies suggest that people taking L-Carnitine may be less likely to suffer a subsequent heart attack or experience chest pain and abnormal heart rhythms.

Kidney disease and dialysis:
Because kidneys produce carnitine, kidney disease may lead to the deficiency of carnitine in the body. Thus, carnitine may be prescribed to those with kidney disease.

Effect in male infertility:
The use of carnitine showed some promise in a controlled trial in selected cases of male infertility by improving sperm quality. L-Carnitine supplementation has also shown to have beneficial effects in the treatment of varicocele, a major cause of male infertility.

As a weight loss supplement:
L-Carnitine has been marketed as a weight-loss supplement, some studies show that oral carnitine reduces fat mass, increases muscle mass, and reduces fatigue. All of these effects may contribute to weight loss. In 2011, researchers using L-Carnitine L-tartrate supplementation for 6 months in a well controlled study demonstrated not only increased muscle carnitine in subjects without carnitine deficiencies, but also an impact on muscle metabolism and performance.

Regular supplements of L-Carnitine contribute to energy metabolism and improved neurotransmitter function in the brain in elderly patients.