Carnitine – A Review of Benefits, Side Effects and Dosage

L-carnitine is a nutrient and dietary supplement.

It plays a crucial role in the production of energy by transporting fatty acids into your cells’ mitochondria (1, 2, 3).

The mitochondria act as engines within your cells, burning these fats to create usable energy.

Your body can produce L-carnitine out of the amino acids lysine and methionine.

For your body to produce it in sufficient amounts, you also need plenty of vitamin C (4).

In addition to the L-carnitine produced in your body, you can also obtain small amounts by eating animal products like meat or fish (5).

Vegans or people with certain genetic issues may be unable to produce or obtain enough. This makes L-carnitine a conditionally essential nutrient (6).

Different Types

L-carnitine is the standard biologically active form of carnitine, which is found in your body, foods and most supplements.

Here are several other types of carnitine:

• D-carnitine: This inactive form may cause a carnitine deficiency in your body by inhibiting the absorption of other, more useful forms (7, 8).
• Acetyl-L-carnitine: Often called ALCAR, this is possibly the most effective form for your brain. Studies suggest that it may benefit people with neurodegenerative diseases (9).
• Propionyl-L-carnitine: This form is well-suited for circulatory issues, such as peripheral vascular disease and high blood pressure. It may boost production of nitric oxide, which improves blood flow (10, 11).
• L-carnitine L-tartrate: This is commonly added to sports supplements due to its rapid absorption rate. It may aid muscle soreness and recovery in exercise (12, 13, 14).

For most people, acetyl-L-carnitine and L-carnitine seem to be the most effective for general use. However, you should always pick the form that’s best for your personal needs and goals.

Role in Your Body

L-carnitine’s main role in your body involves mitochondrial function and energy production (3, 15, 16).

In cells, it helps transport fatty acids into the mitochondria, where they can be burned for energy.

About 98% of your L-carnitine stores are contained in your muscles, along with trace amounts in your liver and blood (17, 18).

L-carnitine may help increase mitochondrial function, which plays a key role in disease and healthy aging (19, 20, 21).

Newer research illustrates the potential benefits of carnitine’s different forms, which may be used for various conditions, including heart and brain diseases (22, 23).

Summary

L-carnitine is an amino acid derivative that transports fatty acids into your cells to be processed for energy. It is made by your body and also available as a supplement.

In theory, using L-carnitine as a weight loss supplement makes sense.

Since L-carnitine helps move more fatty acids into your cells to be burned for energy, you might think this would increase your ability to burn fat and lose weight.

However, the human body is extremely complex, and the results of both human and animal studies are mixed (24, 25, 26, 27).

In an eight-week study in 38 women who exercised four times per week, there was no difference in weight loss between those who took L-carnitine and those who didn’t (24).

What’s more, five of the participants taking L-carnitine experienced nausea or diarrhea (24).

Another human study monitored L-carnitine’s effect on fat burning during a 90-minute stationary bicycle workout. Four weeks of taking supplements did not increase fat burning (28).

However, one analysis of nine studies — mostly in obese individuals or older adults — found that people lost an average of 2.9 pounds (1.3 kg) more weight while taking L-carnitine (29).

More research is needed to confirm the benefits of L-carnitine in a younger, more active population.

While it may aid in weight loss for obese individuals or older adults, a thorough diet and exercise regimen must be in place first.

Summary

Although the cellular mechanism of L-carnitine suggests it could benefit weight loss, its effects — if present at all — are small.

L-carnitine may benefit brain function.

Some animal studies suggest that the acetyl form, acetyl-L-carnitine (ALCAR), may help prevent age-related mental decline and improve markers of learning (30, 31).

Human studies indicate that taking acetyl-L-carnitine daily helps reverse the decline in brain function associated with Alzheimer’s and other brain diseases (32, 33, 34).

This form exhibited similar benefits for general brain function in older adults who did not have Alzheimer’s or other brain conditions (35, 36, 37).

In specific cases, this form may even protect your brain from cell damage.

In a 90-day study, people with alcohol addictions who took 2 grams of acetyl-L-carnitine per day experienced significant improvements in all measures of brain function (38).

More research is needed on long-term benefits for healthy individuals.

Summary

L-carnitine — specifically acetyl-L-carnitine — can have beneficial effects on brain function in various diseases.

A few more health benefits have been linked to L-carnitine supplements.

Heart Health

Some studies demonstrate a potential for reducing blood pressure and the inflammatory process associated with heart disease (23, 39).

In one study, 2 grams of acetyl-L-carnitine per day resulted in an almost 10-point drop in systolic blood pressure — the top number of a blood pressure reading and an important indicator of heart health and disease risk (23).

L-carnitine is also linked to improvements in patients with severe heart disorders, such as coronary heart disease and chronic heart failure (40, 41).

One 12-month study observed a reduction in heart failure and deaths among participants who took L-carnitine supplements (42).

Exercise Performance

The evidence is mixed when it comes to L-carnitine’s effects on sports performance.

However, several studies note mild benefits associated with larger or more long-term doses (43, 44, 45).

L-carnitine’s benefits may be indirect and take weeks or months to appear. This differs from supplements like caffeine or creatine, which can directly enhance sports performance.

L-carnitine may benefit:

• Recovery: May improve exercise recovery (46, 47).
• Muscle oxygen supply: May increase oxygen supply to your muscles (48).
• Stamina: May increase blood flow and nitric oxide production, helping delay discomfort and reduce fatigue (48).
• Muscle soreness: May reduce muscle soreness after exercise (49).
• Red blood cell production: May increase the production of red blood cells, which transport oxygen throughout your body and muscles (50, 51).

Type 2 Diabetes

L-carnitine may also reduce symptoms of type 2 diabetes and its associated risk factors (52, 53, 54).

One study of people with type 2 diabetes taking anti-diabetic medication indicated that carnitine supplements significantly reduced blood sugar levels, compared to a placebo (55).

It may also combat diabetes by increasing a key enzyme called AMPK, which improves your body’s ability to use carbs (56).

Summary

Research suggests that L-carnitine may aid exercise performance and treat health conditions like heart disease and type 2 diabetes.

For most people, 2 grams or less per day is relatively safe and free from any serious side effects.

In one study, people who took 3 grams every day for 21 days experienced no negative effects (57).

In one review of L-carnitine’s safety, doses of approximately 2 grams per day appeared to be safe for long-term use. However, there were some mild side effects, including nausea and stomach discomfort (24, 58).

However, L-carnitine supplements may raise your blood levels of trimethylamine-N-oxide (TMAO) over time. High levels of TMAO are linked to an increased risk of atherosclerosis — a disease which clogs your arteries (59, 60).

More studies on the safety of L-carnitine supplements are needed.

Summary

Doses of 2 grams or less per day seem to be well tolerated and safe for most people. Tentative evidence suggests L-carnitine supplements might increase your risk of atherosclerosis.

You can get small amounts of L-carnitine from your diet by eating meat and fish (4, 5).

The best sources of L-carnitine are (4):

• Beef: 81 mg per 3 ounces (85 grams)
• Pork: 24 mg per 3 ounces (85 grams)
• Fish: 5 mg per 3 ounces (85 grams)
• Chicken: 3 mg per 3 ounces (85 grams)
• Milk: 8 mg per 8 ounces (227 ml)

Interestingly, food sources of L-carnitine have a greater absorption rate than supplements.

According to one study, 57–84% of L-carnitine is absorbed when it’s consumed from food, compared to only 14–18% when taken as a supplement (61).

As noted before, your body can also produce this substance naturally from the amino acids methionine and lysine if your stores are low.

For these reasons, L-carnitine supplements are only necessary in special cases, such as disease treatment.

Summary

The main dietary sources of L-carnitine are meat, fish and some other animal products, such as milk. A healthy individual can also produce sufficient amounts within the body.

Your L-carnitine levels are influenced by how much you’re eating and how much your body is producing.

For this reason, L-carnitine levels are often lower in vegetarians and vegans, since they restrict or avoid animal products (6, 62).

Therefore, vegetarians and vegans may want to consider L-carnitine supplements. However, no studies have confirmed the benefits of carnitine supplements in these specific populations.

Older adults may also benefit from L-carnitine supplements. Research shows that your levels tend to decline as you age (63, 64).

In one study, 2 grams of L-carnitine reduced fatigue and increased muscle function in older adults. Other research reveals that acetyl-L-carnitine may also help boost brain health and function as you age (64, 65).

Additionally, the risk of deficiency is higher for those with diseases like cirrhosis and kidney disease. If you have one of these conditions, a supplement may be beneficial (1, 66, 67).

As with any supplement, you should speak with your doctor before taking L-carnitine.

Summary

Specific populations may benefit from L-carnitine supplements. This includes older adults and people who rarely or never eat meat and fish.

The standard dose of L-carnitine is 500–2,000 mg per day.

Although dosage varies from study to study, here is an overview of the use and dose for each form:

• Acetyl-L-carnitine: This form is best for brain health and function. Doses vary from 600–2,500 mg per day.
• L-carnitine L-tartrate: This form is most effective for exercise performance. Doses vary from 1,000–4,000 mg per day.
• Propionyl-L-carnitine: This form is best for improving blood flow in those with high blood pressure or related health conditions. Doses vary from 400–1,000 mg per day.

Up to 2,000 mg (2 grams) per day seems safe and effective in the long term.

Summary

Although the recommended dose varies, around 500–2,000 mg (0.5–2 grams) seems to be both safe and effective.

L-carnitine is best known as a fat burner — but the overall research is mixed. It is unlikely to cause significant weight loss.

However, studies support its use for health, brain function and disease prevention. Supplements may also benefit those with lower levels, such as older adults, vegans and vegetarians.

Of the different forms, acetyl-L-carnitine and L-carnitine are the most popular and seem to be most effective.