Health Benefits of L-Carnitine

Can the supplement boost metabolism and brain function?

Overweight girl looking at underweight reflection
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L-carnitine is an amino acid widely available in supplement form and often marketed as a weight loss aid. Produced naturally in the body, L-carnitine transports fat to cells in order to produce energy. While supplement manufacturers often claim that L-carnitine can "speed up" your metabolism, the evidence to support these claims are generally lacking.

Among the conditions that L-carnitine is believed to treat:

It has also been suggested that L-carnitine can enhance sports performance, making it one of the more popular nutritional supplements used by athletes.

Health Benefits

To date, few clinical trials have robustly tested L-carnitine's effectiveness in treating any of the above-listed conditions. As a nutritional supplement, L-carnitine does not need to undergo the rigorous testing that pharmaceutical drugs do. Because of this, manufacturers will often make claims about the benefits of L-carnitine that are unsupported by fact.

However, there are smaller studies that suggest L-carnitine may be beneficial for certain specific conditions. None of the studies should be taken as "evidence" per se but rather the first step toward more insightful research in the future.

Weight Loss

With regards to the use of L-carnitine for weight loss, the results from studies are largely mixed to poor. Many of the studies were funded by weight loss companies with neither the proper controls nor evaluative techniques to support the often far-ranging conclusions.

The best available research includes a 2000 study from the International Journal of Sports Nutrition and Exercise Metabolism in which 36 moderately overweight premenopausal women were given either L-carnitine or a placebo twice daily.

After eight weeks, the researchers found no difference in the total body mass or fat mass between the women who took L-carnitine and those who took the placebo.

A number of animal studies have also shown that L-carnitine supplementation does little to aid in weight loss.

A 2002 study from the Annals of Nutrition and Metabolism found that rats treated with L-carnitine neither gain nor lost weight, while a 2005 study published in the International Journal for Vitamin and Nutrition Research found that rats fed an L-carnitine diet actually gained weight.

Brain Function

It has been proposed that by boosting metabolism in brain cells, the adverse effects of many brain disorders may be slowed, stopped, or even reversed.

The most ambitious claim is that a type of L-carnitine, known as acetyl-L-carnitine (ALC), can improve memory, cognition, and attention in people with Alzheimer's disease (AD).

A 2003 study involving 23 people with AD concluded that the addition of ALC to the Alzheimer's drug Aricept (donepezil) or Exelon (rivastigmine) improved all of these functions—from a baseline of 38 percent to 50 percent after the addition of ALC. The conclusions were limited by the size of the study, the lack of controls, and variations in AD therapies.

By contrast, an earlier study conducted in 1991 reported no benefits of ALC supplementation in 130 people with AD. With that said, while all participants experienced a decline in mental function after one year, the decline appeared to be slower in people prescribed ALC versus those given a placebo.

Other research suggests that ALC may help speed recovery from substance abuse by improving mental function and acuity. A 1990 study involving 55 people undergoing alcohol detox reported faster recovery in the group provided 2 grams of ALC after 30, 45, and 90 days versus those provided a placebo.

Metabolic Function

Some scientists believe that the metabolic benefits of L-carnitine may extend to the cardiovascular system as well as the regulation of insulin and blood sugar.

A 24-week study from Italy reported that, among people with insulin resistance, a 2-gram daily dose of L-carnitine improved glucose tolerance and reduced the systolic blood pressure by an average of 10 points.

L-carnitine has also been explored as a means to treat type 2 diabetes. A 2009 review of studies suggested that L-carnitine can reduce the accumulation of lipids in muscles, the condition of which inhibits the insulin response and the metabolism of blood sugar (glucose).

The findings were largely mixed. While L-carnitine was seen to lower fasting glucose levels in some studies, in others it did not. Other still reported a problematic increase in triglycerides after the long-term use of L-carnitine supplements.

Sports Performance

Even more uncertain are the benefits of L-carnitine in improving sports performance. Despite the fact that over $100 million is spent each year on L-carnitine supplements in the United States, according to research from Global Industry Analysts, Inc., there is a dearth of evidence to support its use.

While many manufacturers claim that L-carnitine can improve muscle recovery, enhance oxygen storage, reduce the buildup of lactic acid in muscles, and increase stamina, results from research range from mixed to poor.

For example, a 2018 review of studies reported that L-carnitine could improve endurance during graded treadmill running (as measured by the VO2 max) but couldn't do the same during steady-state running.

Similar findings were seen in other studies wherein supplementation increased L-carnitine concentrations in muscles, but the increase did not translate to either improved muscle function or energy metabolism.

Possible Side Effects

Generally speaking, if taken at the prescribed dose (2 to 3 grams daily), L-carnitine supplements are considered safe. Side effects tend to be minimal and may include nausea, abdominal cramps, diarrhea, and a "fishy" body odor.

More concerning is the possible impact of L-carnitine in people at risk of cardiovascular disease. Long-term use of L-carnitine supplements can raise blood levels of a substance called trimethylamine-N-oxide (TMAO). High levels of TMAO are considered an independent risk factor for atherosclerosis (the hardening of arteries).

L-carnitine may increase the effects of blood thinners like Coumadin (warfarin) and Sintrom (acenocoumarol), causing excessive bruising or bleeding. It can also decrease the effectiveness of your thyroid drugs.

To avoid interactions, always advise your doctor about any medication you are taking, whether it be pharmaceutical, over-the-counter, or herbal.

Because supplements like L-carnitine are largely unregulated, there is no evidence of their safety in pregnant women, nursing mothers, or children. Always speak to your doctor before using any nutritional, homeopathic, or herbal supplement to ensure you are fully informed about the potential risks and benefits.

Dosage and Preparation

L-carnitine supplements are available in tablet, gelcap, liquid, and powdered formulations. There is no standardized dosing schedule for L-carnitine, but manufacturers will generally recommend anywhere from 400 milligrams (mg) to 600 mg two to three times daily.

Different types of L-carnitine are marketed for different health purposes:

  • Acetyl-L-carnitine is often marketed for brain enhancement.
  • L-carnitine L-tartrate is primarily used for sports performance.
  • Propionyl-L-carnitine is commonly used for blood pressure.

Never exceed the dose recommended by the manufacturer. If you experience side effects, be sure to bring the product along with you when you visit your doctor.

What to Look For

L-carnitine is found mostly in meat and dairy products. It is considered "conditionally essential," meaning that your body can produce it as long as you're also eating foods rich in lysine and methionine, the building blocks of L-carnitine.

Foods high in lysine include cheese, yogurt, milk, tree fruits, meats, poultry, and fish. Foods rich in methionine include nuts, meat, turkey, cheese, fish, shellfish, soy, eggs, dairy, and beans. So long as you include plenty of these foods in your diet, you will usually have an ample supply of L-carnitine to help regulate metabolism.

L-carnitine supplements can be found in many health food stores and drugstores or purchased online. L-carnitine is also included in many protein powders and sports drinks marketed to athletes.

To ensure that a supplement is safe, only buy those that have been tested and approved by a recognized certifying body like the U.S. Pharmacopeia (USP), NSF International, or ConsumerLab.

Other Questions

When it comes to metabolic "boosters," the marketing hype will almost always exceed the benefits to your health. Due to the lack of quality clinical research, L-carnitine should not be considered an effective means of treating obesity or enhancing athletic performance.

If you're still considering L-carnitine for any health purposes, be sure speak with your doctor first. The more that your doctor knows about the drugs you are taking—whether pharmaceutical or nutritional—the better able you will be to avoid side effects and interactions.

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