L-carnitine
Meaning of L-carnitine: Made in the body and eaten in meat, it is the ferry without which long chain fats never reach the furnaces inside a cell.
Definition of L-carnitine
L-carnitine is a compound the body builds from two amino acids, mainly in the liver and kidneys, and also takes in from meat, fish and dairy foods. Its job is to carry long chain fatty acids into mitochondria, the compartments inside cells where fuel is burned, a crossing those fats cannot make without it. It is sold as a supplement and also prescribed as a medicine.
What does it actually do inside a cell?
Fat arrives at a cell as fatty acids, but the ones with long carbon chains cannot cross the inner wall of a mitochondrion by themselves. L-carnitine latches onto such a fatty acid outside, carries it through, releases it for burning and returns for the next. Short and medium chain fatty acids slip across unaided, which is why they are still handled normally when carnitine runs short. The shuttle runs in reverse as well, carrying unwanted leftover fragments out of the mitochondrion so they can be cleared, and that second job explains much of what goes wrong in a shortage. Skeletal muscle and heart muscle hold most of the body's supply, since both lean heavily on fat as fuel. The compound sits at a junction of fat and energy metabolism rather than acting as a stimulant of any kind.
When is it given as a medicine?
| Situation | What lies behind it |
|---|---|
| Primary deficiency | An inherited fault in the transporter that pulls carnitine into cells |
| Secondary deficiency | Long term dialysis, inherited disorders of fat breakdown, or certain medicines that drain the supply |
| Under study | Fatigue, heart conditions, fertility and nerve pain after chemotherapy, none of them settled |
Genuine deficiency is uncommon and is diagnosed by measuring blood levels, not assumed from symptoms. It is a separate matter from the far more common use of the supplement by people whose levels are normal, where the body simply makes less of its own to compensate.
Why the L in the name?
Carnitine exists as two mirror image forms, and only the L form works in the body. The D form is inactive and competes with the active one, so products and prescriptions specify L. A related ester, acetyl-L-carnitine, reaches brain tissue more readily and is studied for different purposes. One older label still causes confusion: carnitine was once called a vitamin, but a vitamin by definition cannot be made by the body, and healthy adults synthesize enough of this one on their own.
Also known as
Used in a sentence
L-carnitine levels were measured as part of the evaluation for a suspected metabolic disorder.
Good to know
Supplement use of L-carnitine by people with normal levels is a separate matter from carnitine deficiency, an uncommon diagnosed condition in which the body does not produce or retain adequate carnitine on its own.
Word origin
Where “L-carnitine” comes from
Formed from Latin caro, carnis, meaning flesh, reflecting the compound's discovery in an extract of meat.