Carnitine is a very popular and highly valuable supplement that can be used for many health benefits. It comes in many forms though, and establishing the differences between the structure, role and benefits of each can become a bit confusing. In this article I would like to cover the differences, the uses and the benefits of the two most well-known forms, L- Carnitine and Acetyl L- Carnitine, in an attempt to clear some of this confusion up.
Carnitine is a compound of the amino acids Lysine and Methionine. It exists in two different forms: L- Carnitine and the inactive D- Carnitine.
Carnitine is largely sourced from diet but can also be made in the liver and kidneys. An omnivorous diet including a mix of meats, dairy and poultry can obtain between 60 – 180mg of carnitine per day, compared to a vegan diet that would average between 10 – 12mg in a day (Rebouche, 2004). Dietary carnitine is absorbed through the small intestine and along with synthesised carnitine, is stored in various tissue including skeletal muscle, heart muscle and can also be found in the nervous system.
Carnitine plays a very important role in fatty acid metabolism. It functions as a transport mechanism between Acyl Coenzyme A long chain fatty acids outside of the mitochondria and Acetyl Coenzyme A - the starting metabolite of the Krebs cycle, which is responsible for energy production for the body – within the mitochondria.
Outside of the mitochondria (the powerhouse of the cell) carnitine starts as L- Carnitine and after a process with carnitine acyltransferase I –an enzyme that helps transfer acyl groups from CoA – Carnitine then takes on an acyl compound. The Acetyl group is a specific type of acyl compound and these types of molecules make up organic acids which are commonly found widespread across the body. Acyl L- Carnitine then crosses the mitochondria outer wall to interact with carnitine acyltransferase II, a second enzymethat cleaves the acyl group from the carnitine to go through Beta Oxidation – the process where fatty acids are broken down to produce more Coenzyme A – and continue into the Krebs cycle. Carnitine leaves the mitochondria to begin the process again in the cytosol, the fluid portion that makes up most of a cell (Talwar & Srivastava, 2003).
Put simply, the carnitine functions like a key for the acyl group to get into the mitochondria since CoA cannot pass through the mitochondrial wall. Once Acetyl L- Carnitine is in the Mitochondria, it separates and the L- Carnitine is free to leave the mitochondria to begin the process again.
Because of this process it can be seen that both L- Carnitine and Acetyl L- Carnitine can be useful for fatty acid metabolism. Acetyl L- Carnitine has other uses however.
Acetyl L- Carnitine can be found in high concentrations in the brain and nervous system. Through facilitated transport across the blood brain barrier (Inano, et al., 2003) Acetyl L- Carnitine can reach cells that supplemented L- Carnitine cannot.
Because of this, Acetyl L- Carnitine has been studied for its many effects on the brain and nervous system. The acetyl component is seen to improve acetylcholine levels and function as a precursor (White & Scates, 1990), while Acetyl L- Carnitine as a whole may help improve function in the brain, assisting with age related cognitive decline, depressive disorders and Alzheimer’s disease (Pettegrew, Levine, & McClure, 2000)
Outside of this, Acetyl L- Carnitine is used for many clinical conditions including; Diabetic Neuropathy (nerve pain), Peyronie’s Disease, Male Infertility, Age related Testosterone deficiency in men. There are also cases where Acetyl L- Carnitine has been used in conjunction with HIV treatment to relieve muscle weakness.
As mentioned above, Carnitine can be made in the body and obtained from diet but in supplement form it is most common to see doses between 2g – 3g daily.
As a summary the list below details the differences between L- Carnitine and Acetyl L- Carnitine.
· Helps transport fatty acid for energy
Acetyl L- Carnitine
· Helps transport fatty acid for energy
· Can cross the mitochondrial walls
· Can cross the blood brain barrier
· Precursor for acetylcholine
· Helps with brain function
· Helps with a variety of chronic illnesses
For general lifestyle uses like weight loss, L- Carnitine may prove to be useful. For weight loss and potential improved brain and nervous system function Acetyl L- Carnitine may be the better option.
Inano, A., Sai, Y., Nikaido, H., Hashimoto, N., Asano, M., Tsuji, A., & Tamai, I. (2003). Acetyl-L-carnitine permeability across the blood–brain barrier and involvement of carnitine transporter OCTN2. Biopharmaceutics & Drug Disposition, 357 - 365.
Pettegrew, J. W., Levine, J., & McClure, R. J. (2000). Acetyl-L-carnitine physical-chemical, metabolic, and therapeutic properties: relevance for its mode of action in Alzheimer's disease and geriatric depression. Molecular Psychiatry, 616 - 632.
Rebouche, C. J. (2004). Kinetics, Pharmacokinetics, and Regulation of l-Carnitine and Acetyl-l-carnitine Metabolism. Annals of the New York Academy of Sciences, 30 - 41.
Talwar, G. P., & Srivastava, L. M. (2003). Textbook of Biochemistry and Human Biology. India: Prentice Hall India.
White, H. L., & Scates, P. W. (1990). Acetyl-l-carnitine as a precursor of acetylcholine. Neurochemical Research, 597 - 601.