Health Benefits of Acetyl-L-Carnitine
A Critical Molecule in Fatty Acid Metabolism
Oct 14, 2009 Stephen Allen Christensen
Carnitine is a vitamin-like nutrient that is biosynthesized in the body from two amino acids, lysine and methionine. It functions primarily to transport fatty acid molecules to the interior of the mitochondria, where they can be burned for energy.
Carnitine is also instrumental in removing the metabolic waste that results from fatty acid catabolism.
Carnitine is primarily produced in the kidneys and liver and then transported to other tissues; it is found in high concentrations in the skeletal muscle, heart, brain, and testis.
Approximately 25% of the body’s needs for carnitine are fulfilled through biosynthesis; the remaining 75% comes from the diet. Protein-rich foods, particularly red meats, contain high levels of carnitine.
L-carnitine and acetyl-L-carnitine are forms of carnitine that are more readily absorbed from the gastrointestinal tract than the native carnitine molecule; both L-carnitine and its acetylated form are available as supplements.
Carnitine Deficiency
Although the human body can usually obtain the carnitine it needs from biosynthesis and dietary sources, there are conditions that lead to either inadequate levels of carnitine in the tissues or to impairment in the cells’ ability to use the carnitine that is present. Some of these conditions are caused by inherited enzyme abnormalities, but relative carnitine deficiency can occur in other situations:
- Vegetarian diet
- Tissue hypoxia (angina, vascular claudication, etc.)
- Prolonged fasting
- Liver or kidney disease
- Hemodialysis
- Severe diarrhea
- Medications (diuretics, valproate [Depakene], zidovudine, etc.)
- Severe burns or sepsis
- Aging
Since carnitine plays a pivotal role in energy metabolism, symptoms of deficiency are manifold and may include:
- Lethargy
- Muscle weakness
- Poor exercise tolerance
- Weight loss
- Poor heart function (congestive failure, dysrhythmias, etc.)
- Dementia
- Confusion
- Coma
- Seizures
- Infertility
Supplementing with Carnitine is Beneficial for Certain Conditions
As is often the case with supplements, numerous benefits are ascribed to carnitine, and not all of them are supported by scientific evidence:
- Primary and secondary carnitine deficiencies: Clearly, whenever clinical syndromes (sepsis, burns, hereditary enzyme deficiencies, prolonged intravenous feeding, etc.) point to carnitine deficiencies, supplementation is indicated.
- Weight loss: Carnitine – especially acetyl-L-carnitine, which is purported to increase energy levels – is often promoted for weight loss. It seems intuitive that a molecule which promotes fat metabolism would be helpful for eliminating unwanted body fat. Unfortunately, studies do not consistently document weight loss in overweight individuals who supplement with carnitine. (Villani R, et al. L-carnitine supplementation combined with aerobic training does not promote weight loss in moderately obese women. Int J Sport Nutr Exerc Metab. 2000;10:199-207)
- Hemodialysis: Since dialysis removes carnitine from the circulation, intravenous L-carnitine is indicated to address weakness, fatigue, lipid abnormalities, and anemia that are often seen in dialysis patients. (Guarnieri G, et al. Carnitine metabolism in uremia. Am J Kidney Dis 2001;38(4 Suppl 1):S63-67)
- Intermittent claudication: Activity-induced pain in the extremities – typically the result of peripheral vascular disease – appears to respond to carnitine supplementation. This benefit was more pronounced in people with more severe atherosclerosis, and propionyl-L-carnitine was more effective than L-carnitine. (Hiatt W. Carnitine and peripheral arterial disease. Ann N Y Acad Sci 2004;1033:92-98)
- Age-related memory decline: Aging leads to decreases in mitochondrial function and reduced tissue levels of carnitine in humans and other animals. Rats fed acetyl L-carnitine demonstrated improved mitochondrial function and memory, but it isn’t known if the same benefit is conferred in humans. (Liu J, et al. Memory loss in old rats is associated with brain mitochondrial decay and RNA/DNA oxidation: partial reversal by feeding acetyl-L-carnitine and/or R-alpha -lipoic acid. Proc Natl Acad Sci U S A 2002;99(4):2356-2361)
- Alzheimer’s disease, congestive heart failure, angina, HIV, myocardial infarction (heart attack): According to the Linus Pauling Institute, studies showing benefit of carnitine supplementation for these conditions are promising, but inconclusive. Interestingly, the FDA has approved a form of L-carnitine, Carnitor, for treating congestive heart failure.
- Athletic performance: Most studies attesting to carnitine’s beneficial effects on athletic performance have been poorly designed (i.e., small numbers of participants, short duration, no control groups). Thus, no conclusive evidence supports claims of improved athletic prowess following carnitine supplementation. Indeed, one trial showed that L-carnitine supplementation failed to increase intramuscular levels of carnitine, a necessary condition for enhancing muscular performance. (Brass E. Carnitine and sports medicine: use or abuse? Ann N Y Acad Sci 2004;1033:67-78)
Individuals who wish to supplement with carnitine should take acetyl-L-carnitine in doses of 500 – 1000 mg daily. Since increased energy production is accompanied by a greater generation of free radicals, many advocates of carnitine supplementation recommend the addition of 200 – 400 mg of alpha-lipoic acid.
Mitochondria, Where Carnitine Does Its Work
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