Carnitine Deficiency

Excerpt

Carnitine (beta-hydroxy-gamma-trimethylammonium butyrate) is an indispensable water-soluble molecule derived from amino acids. In non-vegetarians, dietary intake is the primary source of carnitine and accounts for almost three-fourths of the total body stores. The main dietary source of carnitine is red meat, poultry, and dairy products. The bioavailability of dietary carnitine is between 54% to 87%. The remaining one-fourth of the carnitine pool can be produced endogenously from lysine and methionine mostly by the liver and kidneys. Vegetarians have a relatively lower plasma carnitine level than non-vegetarians. In strict vegetarians, most of the carnitine (>90%) is produced endogenously. Despite variabilities in dietary carnitine intake, the plasma carnitine level is maintained within the normal range by an efficient renal reabsorption system while excessive carnitine is promptly excreted in the urine. About 90% to 99% of the filtered carnitine is usually reabsorbed by the renal tubules.

Carnitine plays a substantial physiological role in lipid metabolism and intermediary metabolic pathways. Through the carnitine shuttle, carnitine helps in transporting the long-chain fatty acids from the cytoplasm to the mitochondrial matrix for subsequent degradation for beta-oxidation, which is detailed in the pathophysiology section (figure). Plasma carnitine accounts for approximately 0.5% of the total body stores, and the remaining vast majority is found within the cells. As carnitine is essential for fueling the exercising muscle through fatty acid oxidation and energy production via the Krebs’s cycle, more than 95% of total body carnitine is found in skeletal muscle. The liver, heart, and kidneys have the rest of the carnitine stores.