The human body requires L-carnitine for fat metabolism. This amino acid is manufactured in the liver and kidneys and stored in the muscles, heart, brain, and sperm.1 L-carnitine is made from two other essential amino acids—lysine and methionine—in a process requires niacin, vitamin B6, and iron. After synthesis, the majority of L-carnitine is stored in skeletal muscles for later use.2 Although most L-carnitine is made in the body, food sources augment the supply. Foods rich in this amino acid include red meat, dairy products, and avocado, but levels vary widely.3A well-balanced diet in a healthy individual provides the building blocks for up to 75% of the required levels of L-carnitine, but illness and other stressors can make it difficult to maintain this balance.3
In 1986, the FDA approved L-carnitine for use in cases of primary carnitine deficiency.4 Studies attempting to link supplementation of L-carnitine to cures for a variety of illnesses show ambiguous results. On average, people in developed countries consume adequate amounts of meat and dairy products, and, if tested, would most likely exceed the recommended daily requirement.
On a cellular level, L-carnitine functions by carrying fats into the intracellular mitochondria to be burned for energy and used to combat cellular oxidative stress.5 L-carnitine also acts as a signal to the body to release stored body fat into the bloodstream for energy.5 The amino acid boosts energy by stimulating the body’s burning of triglycerides as fuel and attempts to spare the supply of glycogen stored in the liver for heavier exertion. During exercise, the body burns fat at a rate of 75%-80% of maximum exertion, resulting in less glycogen from carbohydrates being used up. In other words, L-carnitine allows the body to burn more fat, save more glycogen, and ultimately boost stamina and endurance.
L-carnitine supplementation has shown the most promise in treating cardiovascular disease, particularly as an addition to conventional treatment for stable angina. Several clinical trials indicate that L-carnitine and propionyl-L-carnitine can help reduce symptoms of angina and improve angina patients’ ability to exercise without chest pain.6 A few small studies have suggested that L-carnitine can help reduce symptoms of heart failure (HF) and improve exercise capacity in people with HF.7 More comprehensive research is needed to verify any benefit. Studies have also shown that L-carnitine can help reduce symptoms of intermittent claudication and increase the distance that individuals with this disease can walk.8
Researchers have eagerly pursued clinical trials to determine L-carnitine’s efficacy in such areas as athletic endurance and weight loss. A survey of the supplement shelves at the grocery store confirms that L-carnitine is often taken to improve exercise performance, but there is no evidence supporting that use.9 Although it has also been marketed as a weight-loss supplement, there is no scientific evidence backing up that claim either.10
The evidence is mixed as to whether L-carnitine is useful in treating Alzheimer’s disease (AD). Several early studies indicated that it might help delay the progression of AD, relieve depression related to senility and other forms of dementia, and improve memory in the elderly.11 Larger clinical trials found no benefit.
Kidney disease could lead to a deficiency of L-carnitine in the body. Patients with end-stage renal disease who are on renal replacement therapy have been studied for any benefit of L-carnitine supplementation. Small trials confirmed a clinical benefit, but these were not large enough or controlled enough to validate the results.12
Safety, dosage, how supplied, cost
Overall, significant drug interactions are few. One of the more concerning ones is related to the anticonvulsant medication valproic acid (Depakene, Depakote), which may lower blood levels of L-carnitine and can cause deficiency.1 The FDA recommends 1 g of L-carnitine t.i.d. for two to four months to treat L-carnitine deficiency, but no actual recommended daily allowance has been established.4
Side effects are mild and include body odor, rash, and increased appetite. People with cardiovascular disease, diabetes, or renal diseases should talk to a clinician before taking L-carnitine.
Given current data, it is impossible to say that L-carnitine has any proven efficacy as an OTC supplement. Clinical trials have demonstrated no significant benefit of supplementation. On the other hand, there seem to be no major side effects, so unless the patient has a contraindicating illness or is taking prescription medication with adverse interaction, routine L-carnitine supplementation is not likely harmful.
6. Fugh-Berman A. Herbs and dietary supplements in the prevention and treatment of cardiovascular disease. Prev Cardiol. 2000;3:24-32.
7. Witte KK, Clark AL. Micronutrients and their supplementation in chronic cardiac failure. An update beyond theoretical perspectives. Heart Fail Rev. 2006;11:65-74.
8. Carrero JJ, Grimble RF. Does nutrition have a role in peripheral vascular disease? Br J Nutr. 2006;95:217-229.
9. Brass EP. Carnitine and sports medicine: use or abuse? Ann N Y Acad Sci. 2004;1033:67-78.
10. Villani RG, Gannon J, Self M, Rich PA. 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.
11. Malaguarnera M, Cammalleri L, Gargante MP, et al. L-carnitine treatment reduces severity of physical and mental fatigue and increases cognitive functions in centenarians: a randomized and controlled clinical trial. Am J Clin Nutr. 2007;86:1738-1744.
All electronic documents accessed December 15, 2010