Carnitine Overview

Carnitine is a substance used by the body uses to turn fat into energy. It is not normally considered an essential nutrient because the body can manufacture all it needs. However, supplemental carnitine could in theory improve the ability of certain tissues to produce energy. This has led to the use of carnitine for various muscle diseases as well as heart conditions.

There is no dietary requirement for carnitine. However, a few individuals have a genetic defect that hinders the body's ability to make carnitine. In addition, diseases of the liver, kidneys, or brain may inhibit carnitine production. Certain medications, especially the antiseizure drugs valproic acid (Depakene) and phenytoin (Dilantin), may reduce carnitine levels; however, whether taking extra carnitine would be helpful has not been determined. ^{1 2 3 4 5 6 7 8 9 10} Heart muscle tissue, because of its high energy requirements, is particularly vulnerable to carnitine deficiency.

The principal dietary sources of carnitine are meat and dairy products, but to obtain therapeutic dosages a supplement is necessary.

Typical adult dosages for the diseases described here range from 500 mg to 1,000 mg three times daily. For children, one study used 50 mg/kg twice daily, up to a maximum of 4 g daily. ¹¹ Carnitine is taken in three forms: L-carnitine (for heart and other conditions), propionyl-L-carnitine (for heart conditions), and acetyl-L-carnitine (for Alzheimer's disease). The dosage is the same for all three forms.

Angina

Carnitine might be a good addition to standard therapy for angina . In one controlled study, 200 individuals with angina (the exercise-induced variety) took either 2 g daily of L-carnitine or were left untreated. All the study participants continued to take their usual medication for angina. Those taking carnitine showed improvement in several measures of heart function, including a significantly greater ability to exercise without chest pain. ¹² They were also able to reduce the dosages of some of their heart medications (under medical supervision) as their symptoms decreased.

Unfortunately, the results of this study can't be fully trusted because researchers didn't use a double-blind protocol. (For more information on why double-blinding is so important, see Why Does This Database Rely on Double-blind Studies? ) Another trial did use a double-blind, placebo-controlled, design tested L-carnitine in 52 people with angina and found evidence of benefit. ¹³ In addition, several small studies (some of them double-blind) tested propionyl-L-carnitine for the treatment of angina, and also found evidence of benefit. ^{14 15 16 17}

Intermittent Claudication

People with advanced hardening of the arteries, or atherosclerosis , often have difficulty walking due to lack of blood flow to the legs, a condition called intermittent claudication . Pain may develop after walking less than half a block. Although carnitine does not increase blood flow, it appears to improve the muscle's ability to function under difficult circumstances. ¹⁸ A 12-month, double-blind, placebo-controlled trial of 485 patients with intermittent claudication evaluated the potential benefits of propionyl-L-carnitine. ¹⁹ Participants with relatively severe disease showed a 44% improvement in walking distance as compared to placebo. However, no improvement was seen in those with mild disease. Another double-blind study followed 245 people and also found benefit. ²⁰ Similar results have been seen in most but not all other studies of L-carnitine or propionyl-L-carnitine. ²¹ Propionyl-L-carnitine may be more effective for intermittent claudication than plain carnitine.

Congestive Heart Failure

Several small studies have found that carnitine, often in the form of propionyl-L-carnitine, can improve symptoms of congestive heart failure . ²² In one trial, benefits were maintained for 60 days after treatment with carnitine was stopped. ²³

After a Heart Attack

L-carnitine has shown inconsistent promise for use after a heart attack .

A double-blind, placebo-controlled study that followed 101 people for 1 month after a heart attack found that use of L-carnitine, in addition to standard care, reduced the size of the infarct (dead heart tissue). ²⁴ In the months following a severe heart attack, the left ventricle of the heart often enlarges, and the pumping action of the heart becomes less efficient. Some evidence suggests that L-carnitine can help prevent heart enlargement, but that it does not improve heart function. In a 12-month, double-blind, placebo-controlled study of 472 individuals who had just undergone a heart attack, use of carnitine at a dose of 6 g/day significantly decreased the rate of heart enlargement. ²⁵ However, heart function was not significantly altered.

A 3-month, double-blind, placebo-controlled study of 60 individuals who had just undergone a heart attack also failed to find improvements in heart function. ²⁶ (Heart enlargement was not studied.)

Results consistent with those of the studies above were seen in a 6-month double blind, placebo-controlled study of 2,330 people who had just had a heart attack. ²⁷ Carnitine failed to produce significant reductions in mortality or heart failure (serious decline in heart function) over the 6-month period. However, the study did find reductions in early death. (Unfortunately, for statistical reasons, the meaningfulness of this last finding is questionable. Reduction in early death was a secondary endpoint rather than a primary one.)

Note:Carnitine is used along with conventional treatment, not as a substitute for it.

Diabetic Neuropathy

High levels of blood sugar can damage the nerves leading to the extremities, causing pain and numbness. This condition is called diabetic peripheral neuropathy . Nerve damage may also develop in the heart, a condition called cardiac autonomic neuropathy. Acetyl-L-carnitine has shown considerable promise for diabetic peripheral neuropathy and some promise for cardiac autonomic neuropathy.

Two 52-week double-blind, placebo-controlled studies, involving a total of 1,257 people with diabetic peripheral neuropathy, evaluated the potential benefits of ALC taken at 500 mg or 1000 mg daily. ²⁸ The results showed that use of ALC, especially at the higher dose, improved sensory perception and decreased pain levels. In addition, the supplement appeared to promote nerve fiber regeneration.

A small study found some potential benefits for cardiac autonomic neuropathy. ²⁹ For more information, including full dosage and safety issues, see the full Carnitine article.

Male Sexual Function

Carnitine has shown promise for improving male sexual function . One double-blind, placebo-controlled study of 120 subjects compared a combination of propionyl-L-carnitine (2 g per day) and acetyl-L-carnitine (2 g per day) against testosterone for the treatment of male aging symptoms (sexual dysfunction, depression, and fatigue). ³⁰ The results indicated that both testosterone and carnitine improved erectile function, mood, and fatigue, as compared to placebo. However, no improvements were seen in the placebo group. This is an unusual occurrence in studies of erectile dysfunction, so it casts some doubt on the study results.

A double-blind study of 40 men evaluated propionyl-L-carnitine (2 g per day) in diabetic men with erectile dysfunction who had not responded well to Viagra. ³¹ The results indicated that carnitine significantly enhanced the effectiveness of Viagra.

In another double-blind study, a combination of the propionyl and acetyl forms of carnitine enhanced the effectiveness of Viagra in men who suffered from erectile dysfunction caused by prostate surgery. ³²

Male Infertility

Growing evidence suggests that L-carnitine or acetyl-L-carnitine or their combination may be helpful for improving sperm quality and function, thereby benefiting male infertility . ³³ For example, in one double-blind, placebo-controlled study of 60 men, use of combined L-carnitine (2 g per day) and acetyl-L-carnitine (also at 2 g per day) significantly improved sperm quality. ³⁴

Chronic Obstructive Pulmonary Disease (COPD)

Evidence from three double-blind, placebo-controlled studies enrolling a total of 49 people suggests that L-carnitine can improve exercise tolerance in COPD , presumably by improving muscular efficiency in the lungs and other muscles. ^{35 36}

Alzheimer's Disease

Numerous double- or single-blind studies involving a total of more than 1,400 people have evaluated the potential benefits of acetyl-L-carnitine in the treatment of Alzheimer's disease and other forms of dementia. ³⁷ However, while early studies found evidence of modest benefit, two large and well-designed studies failed to find acetyl-L-carnitine effective at all.

The first of these was a double-blind, placebo-controlled trial that enrolled 431 participants for 1 year. ³⁸ Overall, acetyl-L-carnitine proved no better than placebo. However, because a close look at the data indicated that the supplement might help people who develop Alzheimer's disease at an unusually young age, researchers performed a follow-up trial. This 1-year, double-blind, placebo-controlled trial evaluated acetyl-L-carnitine in 229 patients with early onset Alzheimer's. ³⁹ Unfortunately, no benefits were seen here either.

One review of the literature concluded that acetyl-L-carnitine may be helpful for mild cases of Alzheimer’s disease, but not more severe cases. ⁴⁰

Mild Depression

A double-blind study of 60 seniors with dysthymia (a mild form of depression ) found that treatment with 3 g of carnitine daily over a 2-month period significantly improved symptoms as compared to placebo. ⁴¹ Positive results were seen in two other studies as well, one of depression and one of dysthymia. ^{42 43}

Hyperthyroidism

Enlargement of the thyroid (goiter) can be due to many causes, including cancer and iodine deficiency. In some cases, thyroid enlargement occurs without any known cause, so-called benign goiter.

Treatment of benign goiter generally consists of taking thyroid hormone pills. This causes the thyroid gland to become less active, and the goiter shrinks. However, there may be undesirable effects as well. Symptoms of hyperthyroidism (too much thyroid hormone) can develop, including heart palpitations, nervousness, weight loss, and bone breakdown.

A double-blind, placebo-controlled trial found evidence that use of L-carnitine could alleviate many of these symptoms. This 6-month study evaluated the effects of L-carnitine in 50 women who were taking thyroid hormone for benign goiter. ⁴⁴ The results showed that a dose of 2 g or 4 g of carnitine daily protected participants' bones and reduced other symptoms of hyperthyroidism.

Carnitine is thought to affect thyroid hormone by blocking its action in cells. ⁴⁵ This suggests a potential concern—carnitine might be harmful for people who have low or borderline thyroid levels to begin with. This possibility has not been well explored as yet.

Peyronie’s Disease

Peyronie’s disease is an inflammatory condition of the penis that develops in stages. In the first stage, penile pain occurs with erection; next, the penis becomes curved; finally, erectile dysfunction may occur. Many medications have been tried for Peyronie’s disease, with some success. One such drug is tamoxifen, which is better known as a treatment to prevent breast cancer recurrence. A 3-month, double-blind study compared the effectiveness of acetyl-L-carnitine to the drug tamoxifen in 48 men with Peyronie's disease. ⁴⁶ Acetyl-L-carnitine (at a dose of 1 g daily) reduced penile curvature while tamoxifen did not; in addition, the supplement reduced pain and slowed disease progression to a greater extent than tamoxifen.

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