"L-carnitine" is deeply involved in lipid metabolism, and is an essential substance for incorporating fatty acid into mitochondria and converting it into energy.
How L-carnitine works
L-carnitine is a very small molecule that weighs 161.21 Da; and it is produced through the biosynthesis of the essential amino acids lysine and methionine.
L-carnitine is found in our muscle cells (mainly "skeletal muscle" and "myocardium"), and it is an essential substance for lipid metabolism (conversion of lipids to energy), which takes place in the mitochondria of cells. However, lipids cannot cross the membranes of mitochondria alone. When lipids enter the body, they are broken down into fatty acids, which are transported into mitochondria by the combination of “fatty acids” + “L-carnitine.” Thus, lipids are converted into energy and used in the body. While sugar is responsible for instantaneous energy production, fatty acid is responsible for sustainable energy production and effectively operating the muscles and the heart.
In other words, L-carnitine is the component that transports fatty acid into mitochondria (Fig. 1).
L-carnitine is an amino acid analog, which is biosynthesized in our body as a precursor of lysine and methionine, mainly through liver and kidney by a five-step reaction process.
L-carnitine is consumed in the body, and for most Japanese people, 1/4th of L-carnitine is produced in the body through biosynthesis, while the remaining 3/4th is to be supplemented from foods. As shown in Table 1, L-carnitine is particularly abundant in meats and therefore, active intake of meats is effective in supplementing L-carnitine.
Changes in the amount of L-carnitine stored in the body
With regards to L-carnitine in the body, as shown in Table 2, the concentration of L-carnitine (free carnitine, acetyl carnitine) in muscle decreases due to age-related decline of biosynthetic ability and reduced food consumption. Further, recently, it has been said that L-carnitine tends to run short even in young people due to weight loss and unbalanced diet and therefore, active intake of L-carnitine is considered necessary regardless of age.